Export of internal Abseil changes

--
d447fdcb801036cf08197eece193a5a706661120 by Gennadiy Rozental <rogeeff@google.com>:

Eliminate the need for static function holding help message. This decreases the cost of ABSL_FLAG abstraction by 120 bytes under clang.

PiperOrigin-RevId: 281107806

--
0aa6b91189f0e8b2381438c33465673a7ae02487 by Derek Mauro <dmauro@google.com>:

Disable the weak symbol CCTZ extension in the time test_util
on MinGW, which does not support it.

PiperOrigin-RevId: 280719769

--
67322c41c3e776eb541de90fa4526bdb49422eb6 by Abseil Team <absl-team@google.com>:

Tune PeriodicSampler implementation (for internal-use only)

PiperOrigin-RevId: 280708943

--
3a48c346340c7ed03816645cd327e1ff07729aa4 by Abseil Team <absl-team@google.com>:

Clean up public headers not to have warnings for "-Wcomma"

PiperOrigin-RevId: 280695373

--
981acd1ef3b13a83a84f04f11c8931f4ed4451c9 by Matthew Brown <matthewbr@google.com>:

Release absl::int128.

PiperOrigin-RevId: 280690817

--
d30fae9d2ec30b81322d2eb5afe7e13e45b4b422 by Derek Mauro <dmauro@google.com>:

Fix -Wundef warnings in random platform detection

PiperOrigin-RevId: 280669598
GitOrigin-RevId: d447fdcb801036cf08197eece193a5a706661120
Change-Id: Ie5e10e567c54b7de211833607689f233d4ddf734

23 files changed, 2800 insertions, 136 deletions

diff --git a/absl/base/internal/bits.h b/absl/base/internal/bits.h
index b0780f2d121c..450b923f4a4c 100644
--- a/absl/base/internal/bits.h
+++ b/absl/base/internal/bits.h
@@ -50,10 +50,22 @@ namespace base_internal {
 
 ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros64Slow(uint64_t n) {
   int zeroes = 60;
-  if (n >> 32) zeroes -= 32, n >>= 32;
-  if (n >> 16) zeroes -= 16, n >>= 16;
-  if (n >> 8) zeroes -= 8, n >>= 8;
-  if (n >> 4) zeroes -= 4, n >>= 4;
+  if (n >> 32) {
+    zeroes -= 32;
+    n >>= 32;
+  }
+  if (n >> 16) {
+    zeroes -= 16;
+    n >>= 16;
+  }
+  if (n >> 8) {
+    zeroes -= 8;
+    n >>= 8;
+  }
+  if (n >> 4) {
+    zeroes -= 4;
+    n >>= 4;
+  }
   return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0"[n] + zeroes;
 }
 
@@ -95,9 +107,18 @@ ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros64(uint64_t n) {
 
 ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros32Slow(uint64_t n) {
   int zeroes = 28;
-  if (n >> 16) zeroes -= 16, n >>= 16;
-  if (n >> 8) zeroes -= 8, n >>= 8;
-  if (n >> 4) zeroes -= 4, n >>= 4;
+  if (n >> 16) {
+    zeroes -= 16;
+    n >>= 16;
+  }
+  if (n >> 8) {
+    zeroes -= 8;
+    n >>= 8;
+  }
+  if (n >> 4) {
+    zeroes -= 4;
+    n >>= 4;
+  }
   return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0"[n] + zeroes;
 }
 
diff --git a/absl/base/internal/periodic_sampler.cc b/absl/base/internal/periodic_sampler.cc
index 439745d0c514..69656c8a0fbe 100644
--- a/absl/base/internal/periodic_sampler.cc
+++ b/absl/base/internal/periodic_sampler.cc
@@ -36,13 +36,13 @@ bool PeriodicSamplerBase::SubtleConfirmSample() noexcept {
 
   // Check if this is the first call to Sample()
   if (ABSL_PREDICT_FALSE(stride_ == 1)) {
-    stride_ = -1 - GetExponentialBiased(current_period);
-    if (stride_ < -1) {
+    stride_ = static_cast<uint64_t>(-1 - GetExponentialBiased(current_period));
+    if (static_cast<int64_t>(stride_) < -1) {
       ++stride_;
       return false;
     }
   }
-  stride_ = -1 - GetExponentialBiased(current_period);
+  stride_ = static_cast<uint64_t>(-1 - GetExponentialBiased(current_period));
   return true;
 }
 
diff --git a/absl/base/internal/periodic_sampler.h b/absl/base/internal/periodic_sampler.h
index 2c0600f05ea4..238797424fc0 100644
--- a/absl/base/internal/periodic_sampler.h
+++ b/absl/base/internal/periodic_sampler.h
@@ -111,33 +111,49 @@ class PeriodicSamplerBase {
   // Returns the current period of this sampler. Thread-safe.
   virtual int period() const noexcept = 0;
 
-  int64_t stride_ = 0;
+  // Keep and decrement stride_ as an unsigned integer, but compare the value
+  // to zero casted as a signed int. clang and msvc do not create optimum code
+  // if we use signed for the combined decrement and sign comparison.
+  //
+  // Below 3 alternative options, all compiles generate the best code
+  // using the unsigned increment <---> signed int comparison option.
+  //
+  // Option 1:
+  //   int64_t stride_;
+  //   if (ABSL_PREDICT_TRUE(++stride_ < 0)) { ... }
+  //
+  //   GCC   x64 (OK) : https://gcc.godbolt.org/z/R5MzzA
+  //   GCC   ppc (OK) : https://gcc.godbolt.org/z/z7NZAt
+  //   Clang x64 (BAD): https://gcc.godbolt.org/z/t4gPsd
+  //   ICC   x64 (OK) : https://gcc.godbolt.org/z/rE6s8W
+  //   MSVC  x64 (OK) : https://gcc.godbolt.org/z/ARMXqS
+  //
+  // Option 2:
+  //   int64_t stride_ = 0;
+  //   if (ABSL_PREDICT_TRUE(--stride_ >= 0)) { ... }
+  //
+  //   GCC   x64 (OK) : https://gcc.godbolt.org/z/jSQxYK
+  //   GCC   ppc (OK) : https://gcc.godbolt.org/z/VJdYaA
+  //   Clang x64 (BAD): https://gcc.godbolt.org/z/Xm4NjX
+  //   ICC   x64 (OK) : https://gcc.godbolt.org/z/4snaFd
+  //   MSVC  x64 (BAD): https://gcc.godbolt.org/z/BgnEKE
+  //
+  // Option 3:
+  //   uint64_t stride_;
+  //   if (ABSL_PREDICT_TRUE(static_cast<int64_t>(++stride_) < 0)) { ... }
+  //
+  //   GCC   x64 (OK) : https://gcc.godbolt.org/z/bFbfPy
+  //   GCC   ppc (OK) : https://gcc.godbolt.org/z/S9KkUE
+  //   Clang x64 (OK) : https://gcc.godbolt.org/z/UYzRb4
+  //   ICC   x64 (OK) : https://gcc.godbolt.org/z/ptTNfD
+  //   MSVC  x64 (OK) : https://gcc.godbolt.org/z/76j4-5
+  uint64_t stride_ = 0;
   ExponentialBiased rng_;
 };
 
 inline bool PeriodicSamplerBase::SubtleMaybeSample() noexcept {
-  // We explicitly count up and not down, as the compiler does not generate
-  // ideal code for counting down. See also https://gcc.godbolt.org/z/FTPDSM
-  //
-  // With `if (ABSL_PREDICT_FALSE(++stride_ < 0))`
-  //    add     QWORD PTR fs:sampler@tpoff+8, 1
-  //    jns     .L15
-  //    ret
-  //
-  // With `if (ABSL_PREDICT_FALSE(--stride_ > 0))`
-  //    mov     rax, QWORD PTR fs:sampler@tpoff+8
-  //    sub     rax, 1
-  //    mov     QWORD PTR fs:sampler@tpoff+8, rax
-  //    test    rax, rax
-  //    jle     .L15
-  //    ret
-  //    add     QWORD PTR fs:sampler@tpoff+8, 1
-  //    jns     .L15
-  //    ret
-  //
-  // --stride >= 0 does work, but makes our logic slightly harder as in that
-  // case we have less convenient zero-init and over-run values.
-  if (ABSL_PREDICT_FALSE(++stride_ < 0)) {
+  // See comments on `stride_` for the unsigned increment / signed compare.
+  if (ABSL_PREDICT_TRUE(static_cast<int64_t>(++stride_) < 0)) {
     return false;
   }
   return true;
diff --git a/absl/flags/flag.h b/absl/flags/flag.h
index 356ddb616072..09af47a06d1b 100644
--- a/absl/flags/flag.h
+++ b/absl/flags/flag.h
@@ -96,8 +96,7 @@ class Flag {
   constexpr Flag(const char* name, const flags_internal::HelpGenFunc help_gen,
                  const char* filename,
                  const flags_internal::FlagMarshallingOpFn marshalling_op,
-                 const flags_internal::InitialValGenFunc initial_value_gen,
-                 bool, void*)
+                 const flags_internal::InitialValGenFunc initial_value_gen)
       : name_(name),
         help_gen_(help_gen),
         filename_(filename),
@@ -115,8 +114,11 @@ class Flag {
         return impl_;
       }
 
-      impl_ = new flags_internal::Flag<T>(name_, help_gen_, filename_,
-                                          marshalling_op_, initial_value_gen_);
+      impl_ = new flags_internal::Flag<T>(
+          name_,
+          {flags_internal::FlagHelpSrc(help_gen_),
+           flags_internal::FlagHelpSrcKind::kGenFunc},
+          filename_, marshalling_op_, initial_value_gen_);
       inited_.store(true, std::memory_order_release);
     }
 
@@ -307,9 +309,19 @@ void SetFlag(absl::Flag<T>* flag, const V& v) {
 #define ABSL_FLAG_IMPL_FLAGHELP(txt) txt
 #endif
 
-#define ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, txt) \
-  static std::string AbslFlagsWrapHelp##name() {       \
-    return ABSL_FLAG_IMPL_FLAGHELP(txt);               \
+// AbslFlagHelpGenFor##name is used to encapsulate both immediate (method Const)
+// and lazy (method NonConst) evaluation of help message expression. We choose
+// between the two via the call to HelpArg in absl::Flag instantiation below.
+// If help message expression is constexpr evaluable compiler will optimize
+// away this whole struct.
+#define ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, txt)                     \
+  struct AbslFlagHelpGenFor##name {                                        \
+    template <typename T = void>                                           \
+    static constexpr const char* Const() {                                 \
+      return absl::flags_internal::HelpConstexprWrap(                      \
+          ABSL_FLAG_IMPL_FLAGHELP(txt));                                   \
+    }                                                                      \
+    static std::string NonConst() { return ABSL_FLAG_IMPL_FLAGHELP(txt); } \
   }
 
 #define ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value)   \
@@ -326,29 +338,28 @@ void SetFlag(absl::Flag<T>* flag, const V& v) {
 #define ABSL_FLAG_IMPL(Type, name, default_value, help)             \
   namespace absl /* block flags in namespaces */ {}                 \
   ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value) \
-  ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help)                   \
+  ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help);                  \
   ABSL_CONST_INIT absl::Flag<Type> FLAGS_##name{                    \
-      ABSL_FLAG_IMPL_FLAGNAME(#name), &AbslFlagsWrapHelp##name,     \
+      ABSL_FLAG_IMPL_FLAGNAME(#name),                               \
+      absl::flags_internal::HelpArg<AbslFlagHelpGenFor##name>(0),   \
       ABSL_FLAG_IMPL_FILENAME(),                                    \
       &absl::flags_internal::FlagMarshallingOps<Type>,              \
       &AbslFlagsInitFlag##name};                                    \
   extern bool FLAGS_no##name;                                       \
   bool FLAGS_no##name = ABSL_FLAG_IMPL_REGISTRAR(Type, FLAGS_##name)
 #else
-// MSVC version uses aggregate initialization.
-#define ABSL_FLAG_IMPL(Type, name, default_value, help)             \
-  namespace absl /* block flags in namespaces */ {}                 \
-  ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value) \
-  ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help)                   \
-  ABSL_CONST_INIT absl::Flag<Type> FLAGS_##name{                    \
-      ABSL_FLAG_IMPL_FLAGNAME(#name),                               \
-      &AbslFlagsWrapHelp##name,                                     \
-      ABSL_FLAG_IMPL_FILENAME(),                                    \
-      &absl::flags_internal::FlagMarshallingOps<Type>,              \
-      &AbslFlagsInitFlag##name,                                     \
-      false,                                                        \
-      nullptr};                                                     \
-  extern bool FLAGS_no##name;                                       \
+// MSVC version uses aggregate initialization. We also do not try to
+// optimize away help wrapper.
+#define ABSL_FLAG_IMPL(Type, name, default_value, help)                    \
+  namespace absl /* block flags in namespaces */ {}                        \
+  ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value)        \
+  ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help);                         \
+  ABSL_CONST_INIT absl::Flag<Type> FLAGS_##name{                           \
+      ABSL_FLAG_IMPL_FLAGNAME(#name), &AbslFlagHelpGenFor##name::NonConst, \
+      ABSL_FLAG_IMPL_FILENAME(),                                           \
+      &absl::flags_internal::FlagMarshallingOps<Type>,                     \
+      &AbslFlagsInitFlag##name};                                           \
+  extern bool FLAGS_no##name;                                              \
   bool FLAGS_no##name = ABSL_FLAG_IMPL_REGISTRAR(Type, FLAGS_##name)
 #endif
 
diff --git a/absl/flags/flag_test.cc b/absl/flags/flag_test.cc
index 59dc579ce029..4e08da8589a5 100644
--- a/absl/flags/flag_test.cc
+++ b/absl/flags/flag_test.cc
@@ -32,7 +32,7 @@ namespace {
 
 namespace flags = absl::flags_internal;
 
-std::string TestHelpMsg() { return "help"; }
+std::string TestHelpMsg() { return "dynamic help"; }
 template <typename T>
 void* TestMakeDflt() {
   return new T{};
@@ -41,19 +41,22 @@ void TestCallback() {}
 
 template <typename T>
 bool TestConstructionFor() {
-  constexpr flags::Flag<T> f1("f1", &TestHelpMsg, "file",
+  constexpr flags::HelpInitArg help_arg{flags::FlagHelpSrc("literal help"),
+                                        flags::FlagHelpSrcKind::kLiteral};
+  constexpr flags::Flag<T> f1("f1", help_arg, "file",
                               &flags::FlagMarshallingOps<T>, &TestMakeDflt<T>);
   EXPECT_EQ(f1.Name(), "f1");
-  EXPECT_EQ(f1.Help(), "help");
+  EXPECT_EQ(f1.Help(), "literal help");
   EXPECT_EQ(f1.Filename(), "file");
 
-  ABSL_CONST_INIT static flags::Flag<T> f2("f2", &TestHelpMsg, "file",
-                                           &flags::FlagMarshallingOps<T>,
-                                           &TestMakeDflt<T>);
+  ABSL_CONST_INIT static flags::Flag<T> f2(
+      "f2",
+      {flags::FlagHelpSrc(&TestHelpMsg), flags::FlagHelpSrcKind::kGenFunc},
+      "file", &flags::FlagMarshallingOps<T>, &TestMakeDflt<T>);
   flags::FlagRegistrar<T, false>(&f2).OnUpdate(TestCallback);
 
   EXPECT_EQ(f2.Name(), "f2");
-  EXPECT_EQ(f2.Help(), "help");
+  EXPECT_EQ(f2.Help(), "dynamic help");
   EXPECT_EQ(f2.Filename(), "file");
 
   return true;
diff --git a/absl/flags/internal/commandlineflag.cc b/absl/flags/internal/commandlineflag.cc
index caf33bc4892b..88f91e16d196 100644
--- a/absl/flags/internal/commandlineflag.cc
+++ b/absl/flags/internal/commandlineflag.cc
@@ -56,12 +56,5 @@ std::string CommandLineFlag::Filename() const {
   return flags_internal::GetUsageConfig().normalize_filename(filename_);
 }
 
-std::string HelpText::GetHelpText() const {
-  if (help_function_) return help_function_();
-  if (help_message_) return help_message_;
-
-  return {};
-}
-
 }  // namespace flags_internal
 }  // namespace absl
diff --git a/absl/flags/internal/commandlineflag.h b/absl/flags/internal/commandlineflag.h
index 13a3352e80f4..7964f1bf8c4a 100644
--- a/absl/flags/internal/commandlineflag.h
+++ b/absl/flags/internal/commandlineflag.h
@@ -60,14 +60,6 @@ enum ValueSource {
   kProgrammaticChange,
 };
 
-// Signature for the help generation function used as an argument for the
-// absl::Flag constructor.
-using HelpGenFunc = std::string (*)();
-
-// Signature for the function generating the initial flag value based (usually
-// based on default value supplied in flag's definition)
-using InitialValGenFunc = void* (*)();
-
 extern const char kStrippedFlagHelp[];
 
 // The per-type function
@@ -149,34 +141,6 @@ inline size_t Sizeof(FlagOpFn op) {
       op(flags_internal::kSizeof, nullptr, nullptr)));
 }
 
-// Holds either a pointer to help text or a function which produces it.  This is
-// needed for supporting both static initialization of Flags while supporting
-// the legacy registration framework.  We can't use absl::variant<const char*,
-// const char*(*)()> since anybody passing 0 or nullptr in to a CommandLineFlag
-// would find an ambiguity.
-class HelpText {
- public:
-  static constexpr HelpText FromFunctionPointer(const HelpGenFunc fn) {
-    return HelpText(fn, nullptr);
-  }
-  static constexpr HelpText FromStaticCString(const char* msg) {
-    return HelpText(nullptr, msg);
-  }
-
-  std::string GetHelpText() const;
-
-  HelpText() = delete;
-  HelpText(const HelpText&) = default;
-  HelpText(HelpText&&) = default;
-
- private:
-  explicit constexpr HelpText(const HelpGenFunc fn, const char* msg)
-      : help_function_(fn), help_message_(msg) {}
-
-  HelpGenFunc help_function_;
-  const char* help_message_;
-};
-
 // Handle to FlagState objects. Specific flag state objects will restore state
 // of a flag produced this flag state from method CommandLineFlag::SaveState().
 class FlagStateInterface {
@@ -190,9 +154,8 @@ class FlagStateInterface {
 // Holds all information for a flag.
 class CommandLineFlag {
  public:
-  constexpr CommandLineFlag(const char* name, HelpText help_text,
-                            const char* filename)
-      : name_(name), help_(help_text), filename_(filename) {}
+  constexpr CommandLineFlag(const char* name, const char* filename)
+      : name_(name), filename_(filename) {}
 
   // Virtual destructor
   virtual void Destroy() const = 0;
@@ -203,7 +166,6 @@ class CommandLineFlag {
 
   // Non-polymorphic access methods.
   absl::string_view Name() const { return name_; }
-  std::string Help() const { return help_.GetHelpText(); }
   absl::string_view Typename() const;
   std::string Filename() const;
 
@@ -250,6 +212,8 @@ class CommandLineFlag {
 
   // Polymorphic access methods
 
+  // Returns help message associated with this flag
+  virtual std::string Help() const = 0;
   // Returns true iff this object corresponds to retired flag
   virtual bool IsRetired() const { return false; }
   // Returns true iff this is a handle to an Abseil Flag.
@@ -285,12 +249,11 @@ class CommandLineFlag {
   // flag's value type.
   virtual void CheckDefaultValueParsingRoundtrip() const = 0;
 
-  // Constant configuration for a particular flag.
  protected:
   ~CommandLineFlag() = default;
 
+  // Constant configuration for a particular flag.
   const char* const name_;      // Flags name passed to ABSL_FLAG as second arg.
-  const HelpText help_;         // The function generating help message.
   const char* const filename_;  // The file name where ABSL_FLAG resides.
 
  private:
diff --git a/absl/flags/internal/flag.cc b/absl/flags/internal/flag.cc
index 061113d76557..d087f79e891c 100644
--- a/absl/flags/internal/flag.cc
+++ b/absl/flags/internal/flag.cc
@@ -85,6 +85,11 @@ void FlagImpl::Destroy() const {
   delete locks_;
 }
 
+std::string FlagImpl::Help() const {
+  return help_source_kind_ == FlagHelpSrcKind::kLiteral ? help_.literal
+                                                        : help_.gen_func();
+}
+
 bool FlagImpl::IsModified() const {
   absl::MutexLock l(DataGuard());
   return modified_;
diff --git a/absl/flags/internal/flag.h b/absl/flags/internal/flag.h
index ce0ccf2d31d2..d79902543a22 100644
--- a/absl/flags/internal/flag.h
+++ b/absl/flags/internal/flag.h
@@ -61,6 +61,68 @@ class FlagState : public flags_internal::FlagStateInterface {
   int64_t counter_;
 };
 
+// This is help argument for absl::Flag encapsulating the string literal pointer
+// or pointer to function generating it as well as enum descriminating two
+// cases.
+using HelpGenFunc = std::string (*)();
+
+union FlagHelpSrc {
+  constexpr explicit FlagHelpSrc(const char* help_msg) : literal(help_msg) {}
+  constexpr explicit FlagHelpSrc(HelpGenFunc help_gen) : gen_func(help_gen) {}
+
+  const char* literal;
+  HelpGenFunc gen_func;
+};
+
+enum class FlagHelpSrcKind : int8_t { kLiteral, kGenFunc };
+
+struct HelpInitArg {
+  FlagHelpSrc source;
+  FlagHelpSrcKind kind;
+};
+
+// HelpConstexprWrap is used by struct AbslFlagHelpGenFor##name generated by
+// ABSL_FLAG macro. It is only used to silence the compiler in the case where
+// help message expression is not constexpr and does not have type const char*.
+// If help message expression is indeed constexpr const char* HelpConstexprWrap
+// is just a trivial identity function.
+template <typename T>
+const char* HelpConstexprWrap(const T&) {
+  return nullptr;
+}
+constexpr const char* HelpConstexprWrap(const char* p) { return p; }
+constexpr const char* HelpConstexprWrap(char* p) { return p; }
+
+// These two HelpArg overloads allows us to select at compile time one of two
+// way to pass Help argument to absl::Flag. We'll be passing
+// AbslFlagHelpGenFor##name as T and integer 0 as a single argument to prefer
+// first overload if possible. If T::Const is evaluatable on constexpr
+// context (see non template int parameter below) we'll choose first overload.
+// In this case the help message expression is immediately evaluated and is used
+// to construct the absl::Flag. No additionl code is generated by ABSL_FLAG.
+// Otherwise SFINAE kicks in and first overload is dropped from the
+// consideration, in which case the second overload will be used. The second
+// overload does not attempt to evaluate the help message expression
+// immediately and instead delays the evaluation by returing the function
+// pointer (&T::NonConst) genering the help message when necessary. This is
+// evaluatable in constexpr context, but the cost is an extra function being
+// generated in the ABSL_FLAG code.
+template <typename T, int = (T::Const(), 1)>
+constexpr flags_internal::HelpInitArg HelpArg(int) {
+  return {flags_internal::FlagHelpSrc(T::Const()),
+          flags_internal::FlagHelpSrcKind::kLiteral};
+}
+
+template <typename T>
+constexpr flags_internal::HelpInitArg HelpArg(char) {
+  return {flags_internal::FlagHelpSrc(&T::NonConst),
+          flags_internal::FlagHelpSrcKind::kGenFunc};
+}
+
+// Signature for the function generating the initial flag value based (usually
+// based on default value supplied in flag's definition)
+using InitialValGenFunc = void* (*)();
+
 // Signature for the mutation callback used by watched Flags
 // The callback is noexcept.
 // TODO(rogeeff): add noexcept after C++17 support is added.
@@ -74,15 +136,19 @@ class FlagImpl {
  public:
   constexpr FlagImpl(const flags_internal::FlagOpFn op,
                      const flags_internal::FlagMarshallingOpFn marshalling_op,
-                     const flags_internal::InitialValGenFunc initial_value_gen)
+                     const flags_internal::InitialValGenFunc initial_value_gen,
+                     const HelpInitArg help)
       : op_(op),
         marshalling_op_(marshalling_op),
-        initial_value_gen_(initial_value_gen) {}
+        initial_value_gen_(initial_value_gen),
+        help_(help.source),
+        help_source_kind_(help.kind) {}
 
   // Forces destruction of the Flag's data.
   void Destroy() const;
 
   // Constant access methods
+  std::string Help() const;
   bool IsModified() const ABSL_LOCKS_EXCLUDED(locks_->primary_mu);
   bool IsSpecifiedOnCommandLine() const ABSL_LOCKS_EXCLUDED(locks_->primary_mu);
   std::string DefaultValue() const ABSL_LOCKS_EXCLUDED(locks_->primary_mu);
@@ -149,9 +215,12 @@ class FlagImpl {
   absl::Mutex* DataGuard() const ABSL_LOCK_RETURNED(locks_->primary_mu);
 
   // Immutable Flag's data.
-  const FlagOpFn op_;                          // Type-specific handler
-  const FlagMarshallingOpFn marshalling_op_;   // Marshalling ops handler
-  const InitialValGenFunc initial_value_gen_;  // Makes flag's initial value
+  const FlagOpFn op_;                          // Type-specific handler.
+  const FlagMarshallingOpFn marshalling_op_;   // Marshalling ops handler.
+  const InitialValGenFunc initial_value_gen_;  // Makes flag's initial value.
+  const FlagHelpSrc help_;  // Help message literal or function to generate it.
+  // Indicates if help message was supplied as literal or generator func.
+  const FlagHelpSrcKind help_source_kind_;
 
   // Mutable Flag's data. (guarded by locks_->primary_mu).
   // Indicates that locks_, cur_ and def_ fields have been lazily initialized.
@@ -191,14 +260,13 @@ class FlagImpl {
 template <typename T>
 class Flag final : public flags_internal::CommandLineFlag {
  public:
-  constexpr Flag(const char* name, const flags_internal::HelpGenFunc help_gen,
+  constexpr Flag(const char* name, const flags_internal::HelpInitArg help,
                  const char* filename,
                  const flags_internal::FlagMarshallingOpFn marshalling_op,
                  const flags_internal::InitialValGenFunc initial_value_gen)
-      : flags_internal::CommandLineFlag(
-            name, flags_internal::HelpText::FromFunctionPointer(help_gen),
-            filename),
-        impl_(&flags_internal::FlagOps<T>, marshalling_op, initial_value_gen) {}
+      : flags_internal::CommandLineFlag(name, filename),
+        impl_(&flags_internal::FlagOps<T>, marshalling_op, initial_value_gen,
+              help) {}
 
   T Get() const {
     // See implementation notes in CommandLineFlag::Get().
@@ -222,6 +290,7 @@ class Flag final : public flags_internal::CommandLineFlag {
   }
 
   // CommandLineFlag interface
+  std::string Help() const override { return impl_.Help(); }
   bool IsModified() const override { return impl_.IsModified(); }
   bool IsSpecifiedOnCommandLine() const override {
     return impl_.IsSpecifiedOnCommandLine();
diff --git a/absl/flags/internal/registry.cc b/absl/flags/internal/registry.cc
index ae7671a967e8..52d9f3c716f3 100644
--- a/absl/flags/internal/registry.cc
+++ b/absl/flags/internal/registry.cc
@@ -276,9 +276,8 @@ namespace {
 class RetiredFlagObj final : public flags_internal::CommandLineFlag {
  public:
   constexpr RetiredFlagObj(const char* name, FlagOpFn ops)
-      : flags_internal::CommandLineFlag(
-            name, flags_internal::HelpText::FromStaticCString(nullptr),
-            /*filename=*/"RETIRED"),
+      : flags_internal::CommandLineFlag(name,
+                                        /*filename=*/"RETIRED"),
         op_(ops) {}
 
  private:
@@ -288,6 +287,7 @@ class RetiredFlagObj final : public flags_internal::CommandLineFlag {
   }
 
   flags_internal::FlagOpFn TypeId() const override { return op_; }
+  std::string Help() const override { return ""; }
   bool IsRetired() const override { return true; }
   bool IsModified() const override { return false; }
   bool IsSpecifiedOnCommandLine() const override { return false; }
diff --git a/absl/numeric/int128.cc b/absl/numeric/int128.cc
index 93b62c52901e..1eba09de38c8 100644
--- a/absl/numeric/int128.cc
+++ b/absl/numeric/int128.cc
@@ -244,6 +244,111 @@ std::ostream& operator<<(std::ostream& os, uint128 v) {
   return os << rep;
 }
 
+namespace {
+
+uint128 UnsignedAbsoluteValue(int128 v) {
+  // Cast to uint128 before possibly negating because -Int128Min() is undefined.
+  return Int128High64(v) < 0 ? -uint128(v) : uint128(v);
+}
+
+}  // namespace
+
+#if !defined(ABSL_HAVE_INTRINSIC_INT128)
+namespace {
+
+template <typename T>
+int128 MakeInt128FromFloat(T v) {
+  // Conversion when v is NaN or cannot fit into int128 would be undefined
+  // behavior if using an intrinsic 128-bit integer.
+  assert(std::isfinite(v) && (std::numeric_limits<T>::max_exponent <= 127 ||
+                              (v >= -std::ldexp(static_cast<T>(1), 127) &&
+                               v < std::ldexp(static_cast<T>(1), 127))));
+
+  // We must convert the absolute value and then negate as needed, because
+  // floating point types are typically sign-magnitude. Otherwise, the
+  // difference between the high and low 64 bits when interpreted as two's
+  // complement overwhelms the precision of the mantissa.
+  uint128 result = v < 0 ? -MakeUint128FromFloat(-v) : MakeUint128FromFloat(v);
+  return MakeInt128(int128_internal::BitCastToSigned(Uint128High64(result)),
+                    Uint128Low64(result));
+}
+
+}  // namespace
+
+int128::int128(float v) : int128(MakeInt128FromFloat(v)) {}
+int128::int128(double v) : int128(MakeInt128FromFloat(v)) {}
+int128::int128(long double v) : int128(MakeInt128FromFloat(v)) {}
+
+int128 operator/(int128 lhs, int128 rhs) {
+  assert(lhs != Int128Min() || rhs != -1);  // UB on two's complement.
+
+  uint128 quotient = 0;
+  uint128 remainder = 0;
+  DivModImpl(UnsignedAbsoluteValue(lhs), UnsignedAbsoluteValue(rhs),
+             &quotient, &remainder);
+  if ((Int128High64(lhs) < 0) != (Int128High64(rhs) < 0)) quotient = -quotient;
+  return MakeInt128(int128_internal::BitCastToSigned(Uint128High64(quotient)),
+                    Uint128Low64(quotient));
+}
+
+int128 operator%(int128 lhs, int128 rhs) {
+  assert(lhs != Int128Min() || rhs != -1);  // UB on two's complement.
+
+  uint128 quotient = 0;
+  uint128 remainder = 0;
+  DivModImpl(UnsignedAbsoluteValue(lhs), UnsignedAbsoluteValue(rhs),
+             &quotient, &remainder);
+  if (Int128High64(lhs) < 0) remainder = -remainder;
+  return MakeInt128(int128_internal::BitCastToSigned(Uint128High64(remainder)),
+                    Uint128Low64(remainder));
+}
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
+std::ostream& operator<<(std::ostream& os, int128 v) {
+  std::ios_base::fmtflags flags = os.flags();
+  std::string rep;
+
+  // Add the sign if needed.
+  bool print_as_decimal =
+      (flags & std::ios::basefield) == std::ios::dec ||
+      (flags & std::ios::basefield) == std::ios_base::fmtflags();
+  if (print_as_decimal) {
+    if (Int128High64(v) < 0) {
+      rep = "-";
+    } else if (flags & std::ios::showpos) {
+      rep = "+";
+    }
+  }
+
+  rep.append(Uint128ToFormattedString(
+      print_as_decimal ? UnsignedAbsoluteValue(v) : uint128(v), os.flags()));
+
+  // Add the requisite padding.
+  std::streamsize width = os.width(0);
+  if (static_cast<size_t>(width) > rep.size()) {
+    switch (flags & std::ios::adjustfield) {
+      case std::ios::left:
+        rep.append(width - rep.size(), os.fill());
+        break;
+      case std::ios::internal:
+        if (print_as_decimal && (rep[0] == '+' || rep[0] == '-')) {
+          rep.insert(1, width - rep.size(), os.fill());
+        } else if ((flags & std::ios::basefield) == std::ios::hex &&
+                   (flags & std::ios::showbase) && v != 0) {
+          rep.insert(2, width - rep.size(), os.fill());
+        } else {
+          rep.insert(0, width - rep.size(), os.fill());
+        }
+        break;
+      default:  // std::ios::right
+        rep.insert(0, width - rep.size(), os.fill());
+        break;
+    }
+  }
+
+  return os << rep;
+}
+
 }  // namespace absl
 
 namespace std {
@@ -270,4 +375,28 @@ constexpr int numeric_limits<absl::uint128>::max_exponent;
 constexpr int numeric_limits<absl::uint128>::max_exponent10;
 constexpr bool numeric_limits<absl::uint128>::traps;
 constexpr bool numeric_limits<absl::uint128>::tinyness_before;
+
+constexpr bool numeric_limits<absl::int128>::is_specialized;
+constexpr bool numeric_limits<absl::int128>::is_signed;
+constexpr bool numeric_limits<absl::int128>::is_integer;
+constexpr bool numeric_limits<absl::int128>::is_exact;
+constexpr bool numeric_limits<absl::int128>::has_infinity;
+constexpr bool numeric_limits<absl::int128>::has_quiet_NaN;
+constexpr bool numeric_limits<absl::int128>::has_signaling_NaN;
+constexpr float_denorm_style numeric_limits<absl::int128>::has_denorm;
+constexpr bool numeric_limits<absl::int128>::has_denorm_loss;
+constexpr float_round_style numeric_limits<absl::int128>::round_style;
+constexpr bool numeric_limits<absl::int128>::is_iec559;
+constexpr bool numeric_limits<absl::int128>::is_bounded;
+constexpr bool numeric_limits<absl::int128>::is_modulo;
+constexpr int numeric_limits<absl::int128>::digits;
+constexpr int numeric_limits<absl::int128>::digits10;
+constexpr int numeric_limits<absl::int128>::max_digits10;
+constexpr int numeric_limits<absl::int128>::radix;
+constexpr int numeric_limits<absl::int128>::min_exponent;
+constexpr int numeric_limits<absl::int128>::min_exponent10;
+constexpr int numeric_limits<absl::int128>::max_exponent;
+constexpr int numeric_limits<absl::int128>::max_exponent10;
+constexpr bool numeric_limits<absl::int128>::traps;
+constexpr bool numeric_limits<absl::int128>::tinyness_before;
 }  // namespace std
diff --git a/absl/numeric/int128.h b/absl/numeric/int128.h
index 4f965686c393..506176127fe6 100644
--- a/absl/numeric/int128.h
+++ b/absl/numeric/int128.h
@@ -17,10 +17,7 @@
 // File: int128.h
 // -----------------------------------------------------------------------------
 //
-// This header file defines 128-bit integer types.
-//
-// Currently, this file defines `uint128`, an unsigned 128-bit integer;
-// a signed 128-bit integer is forthcoming.
+// This header file defines 128-bit integer types, `uint128` and `int128`.
 
 #ifndef ABSL_NUMERIC_INT128_H_
 #define ABSL_NUMERIC_INT128_H_
@@ -53,6 +50,8 @@
 
 namespace absl {
 
+class int128;
+
 // uint128
 //
 // An unsigned 128-bit integer type. The API is meant to mimic an intrinsic type
@@ -116,6 +115,7 @@ class
   constexpr uint128(__int128 v);           // NOLINT(runtime/explicit)
   constexpr uint128(unsigned __int128 v);  // NOLINT(runtime/explicit)
 #endif  // ABSL_HAVE_INTRINSIC_INT128
+  constexpr uint128(int128 v);  // NOLINT(runtime/explicit)
   explicit uint128(float v);
   explicit uint128(double v);
   explicit uint128(long double v);
@@ -131,6 +131,7 @@ class
   uint128& operator=(__int128 v);
   uint128& operator=(unsigned __int128 v);
 #endif  // ABSL_HAVE_INTRINSIC_INT128
+  uint128& operator=(int128 v);
 
   // Conversion operators to other arithmetic types
   constexpr explicit operator bool() const;
@@ -291,7 +292,238 @@ class numeric_limits<absl::uint128> {
 };
 }  // namespace std
 
-// TODO(absl-team): Implement signed 128-bit type
+namespace absl {
+
+// int128
+//
+// A signed 128-bit integer type. The API is meant to mimic an intrinsic
+// integral type as closely as is practical, including exhibiting undefined
+// behavior in analogous cases (e.g. division by zero).
+//
+// An `int128` supports the following:
+//
+//   * Implicit construction from integral types
+//   * Explicit conversion to integral types
+//
+// However, an `int128` differs from intrinsic integral types in the following
+// ways:
+//
+//   * It is not implicitly convertible to other integral types.
+//   * Requires explicit construction from and conversion to floating point
+//     types.
+
+// Additionally, if your compiler supports `__int128`, `int128` is
+// interoperable with that type. (Abseil checks for this compatibility through
+// the `ABSL_HAVE_INTRINSIC_INT128` macro.)
+//
+// The design goal for `int128` is that it will be compatible with a future
+// `int128_t`, if that type becomes a part of the standard.
+//
+// Example:
+//
+//     float y = absl::int128(17);  // Error. int128 cannot be implicitly
+//                                  // converted to float.
+//
+//     absl::int128 v;
+//     int64_t i = v;                        // Error
+//     int64_t i = static_cast<int64_t>(v);  // OK
+//
+class int128 {
+ public:
+  int128() = default;
+
+  // Constructors from arithmetic types
+  constexpr int128(int v);                 // NOLINT(runtime/explicit)
+  constexpr int128(unsigned int v);        // NOLINT(runtime/explicit)
+  constexpr int128(long v);                // NOLINT(runtime/int)
+  constexpr int128(unsigned long v);       // NOLINT(runtime/int)
+  constexpr int128(long long v);           // NOLINT(runtime/int)
+  constexpr int128(unsigned long long v);  // NOLINT(runtime/int)
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  constexpr int128(__int128 v);  // NOLINT(runtime/explicit)
+  constexpr explicit int128(unsigned __int128 v);
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+  constexpr explicit int128(uint128 v);
+  explicit int128(float v);
+  explicit int128(double v);
+  explicit int128(long double v);
+
+  // Assignment operators from arithmetic types
+  int128& operator=(int v);
+  int128& operator=(unsigned int v);
+  int128& operator=(long v);                // NOLINT(runtime/int)
+  int128& operator=(unsigned long v);       // NOLINT(runtime/int)
+  int128& operator=(long long v);           // NOLINT(runtime/int)
+  int128& operator=(unsigned long long v);  // NOLINT(runtime/int)
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  int128& operator=(__int128 v);
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
+  // Conversion operators to other arithmetic types
+  constexpr explicit operator bool() const;
+  constexpr explicit operator char() const;
+  constexpr explicit operator signed char() const;
+  constexpr explicit operator unsigned char() const;
+  constexpr explicit operator char16_t() const;
+  constexpr explicit operator char32_t() const;
+  constexpr explicit operator ABSL_INTERNAL_WCHAR_T() const;
+  constexpr explicit operator short() const;  // NOLINT(runtime/int)
+  // NOLINTNEXTLINE(runtime/int)
+  constexpr explicit operator unsigned short() const;
+  constexpr explicit operator int() const;
+  constexpr explicit operator unsigned int() const;
+  constexpr explicit operator long() const;  // NOLINT(runtime/int)
+  // NOLINTNEXTLINE(runtime/int)
+  constexpr explicit operator unsigned long() const;
+  // NOLINTNEXTLINE(runtime/int)
+  constexpr explicit operator long long() const;
+  // NOLINTNEXTLINE(runtime/int)
+  constexpr explicit operator unsigned long long() const;
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  constexpr explicit operator __int128() const;
+  constexpr explicit operator unsigned __int128() const;
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+  explicit operator float() const;
+  explicit operator double() const;
+  explicit operator long double() const;
+
+  // Trivial copy constructor, assignment operator and destructor.
+
+  // Arithmetic operators
+  int128& operator+=(int128 other);
+  int128& operator-=(int128 other);
+  int128& operator*=(int128 other);
+  int128& operator/=(int128 other);
+  int128& operator%=(int128 other);
+  int128 operator++(int);  // postfix increment: i++
+  int128 operator--(int);  // postfix decrement: i--
+  int128& operator++();    // prefix increment:  ++i
+  int128& operator--();    // prefix decrement:  --i
+  int128& operator&=(int128 other);
+  int128& operator|=(int128 other);
+  int128& operator^=(int128 other);
+  int128& operator<<=(int amount);
+  int128& operator>>=(int amount);
+
+  // Int128Low64()
+  //
+  // Returns the lower 64-bit value of a `int128` value.
+  friend constexpr uint64_t Int128Low64(int128 v);
+
+  // Int128High64()
+  //
+  // Returns the higher 64-bit value of a `int128` value.
+  friend constexpr int64_t Int128High64(int128 v);
+
+  // MakeInt128()
+  //
+  // Constructs a `int128` numeric value from two 64-bit integers. Note that
+  // signedness is conveyed in the upper `high` value.
+  //
+  //   (absl::int128(1) << 64) * high + low
+  //
+  // Note that this factory function is the only way to construct a `int128`
+  // from integer values greater than 2^64 or less than -2^64.
+  //
+  // Example:
+  //
+  //   absl::int128 big = absl::MakeInt128(1, 0);
+  //   absl::int128 big_n = absl::MakeInt128(-1, 0);
+  friend constexpr int128 MakeInt128(int64_t high, uint64_t low);
+
+  // Int128Max()
+  //
+  // Returns the maximum value for a 128-bit signed integer.
+  friend constexpr int128 Int128Max();
+
+  // Int128Min()
+  //
+  // Returns the minimum value for a 128-bit signed integer.
+  friend constexpr int128 Int128Min();
+
+  // Support for absl::Hash.
+  template <typename H>
+  friend H AbslHashValue(H h, int128 v) {
+    return H::combine(std::move(h), Int128High64(v), Int128Low64(v));
+  }
+
+ private:
+  constexpr int128(int64_t high, uint64_t low);
+
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  __int128 v_;
+#else  // ABSL_HAVE_INTRINSIC_INT128
+#if defined(ABSL_IS_LITTLE_ENDIAN)
+  uint64_t lo_;
+  int64_t hi_;
+#elif defined(ABSL_IS_BIG_ENDIAN)
+  int64_t hi_;
+  uint64_t lo_;
+#else  // byte order
+#error "Unsupported byte order: must be little-endian or big-endian."
+#endif  // byte order
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+};
+
+std::ostream& operator<<(std::ostream& os, int128 v);
+
+// TODO(absl-team) add operator>>(std::istream&, int128)
+
+constexpr int128 Int128Max() {
+  return int128((std::numeric_limits<int64_t>::max)(),
+                (std::numeric_limits<uint64_t>::max)());
+}
+
+constexpr int128 Int128Min() {
+  return int128((std::numeric_limits<int64_t>::min)(), 0);
+}
+
+}  // namespace absl
+
+// Specialized numeric_limits for int128.
+namespace std {
+template <>
+class numeric_limits<absl::int128> {
+ public:
+  static constexpr bool is_specialized = true;
+  static constexpr bool is_signed = true;
+  static constexpr bool is_integer = true;
+  static constexpr bool is_exact = true;
+  static constexpr bool has_infinity = false;
+  static constexpr bool has_quiet_NaN = false;
+  static constexpr bool has_signaling_NaN = false;
+  static constexpr float_denorm_style has_denorm = denorm_absent;
+  static constexpr bool has_denorm_loss = false;
+  static constexpr float_round_style round_style = round_toward_zero;
+  static constexpr bool is_iec559 = false;
+  static constexpr bool is_bounded = true;
+  static constexpr bool is_modulo = false;
+  static constexpr int digits = 127;
+  static constexpr int digits10 = 38;
+  static constexpr int max_digits10 = 0;
+  static constexpr int radix = 2;
+  static constexpr int min_exponent = 0;
+  static constexpr int min_exponent10 = 0;
+  static constexpr int max_exponent = 0;
+  static constexpr int max_exponent10 = 0;
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  static constexpr bool traps = numeric_limits<__int128>::traps;
+#else   // ABSL_HAVE_INTRINSIC_INT128
+  static constexpr bool traps = numeric_limits<uint64_t>::traps;
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+  static constexpr bool tinyness_before = false;
+
+  static constexpr absl::int128 (min)() { return absl::Int128Min(); }
+  static constexpr absl::int128 lowest() { return absl::Int128Min(); }
+  static constexpr absl::int128 (max)() { return absl::Int128Max(); }
+  static constexpr absl::int128 epsilon() { return 0; }
+  static constexpr absl::int128 round_error() { return 0; }
+  static constexpr absl::int128 infinity() { return 0; }
+  static constexpr absl::int128 quiet_NaN() { return 0; }
+  static constexpr absl::int128 signaling_NaN() { return 0; }
+  static constexpr absl::int128 denorm_min() { return 0; }
+};
+}  // namespace std
 
 // --------------------------------------------------------------------------
 //                      Implementation details follow
@@ -339,6 +571,10 @@ inline uint128& uint128::operator=(unsigned __int128 v) {
 }
 #endif  // ABSL_HAVE_INTRINSIC_INT128
 
+inline uint128& uint128::operator=(int128 v) {
+  return *this = uint128(v);
+}
+
 // Arithmetic operators.
 
 uint128 operator<<(uint128 lhs, int amount);
@@ -420,6 +656,9 @@ constexpr uint128::uint128(unsigned __int128 v)
       hi_{static_cast<uint64_t>(v >> 64)} {}
 #endif  // ABSL_HAVE_INTRINSIC_INT128
 
+constexpr uint128::uint128(int128 v)
+    : lo_{Int128Low64(v)}, hi_{static_cast<uint64_t>(Int128High64(v))} {}
+
 #elif defined(ABSL_IS_BIG_ENDIAN)
 
 constexpr uint128::uint128(uint64_t high, uint64_t low)
@@ -450,6 +689,9 @@ constexpr uint128::uint128(unsigned __int128 v)
       lo_{static_cast<uint64_t>(v & ~uint64_t{0})} {}
 #endif  // ABSL_HAVE_INTRINSIC_INT128
 
+constexpr uint128::uint128(int128 v)
+    : hi_{static_cast<uint64_t>(Int128High64(v))}, lo_{Int128Low64(v)} {}
+
 #else  // byte order
 #error "Unsupported byte order: must be little-endian or big-endian."
 #endif  // byte order
@@ -719,6 +961,117 @@ inline uint128& uint128::operator--() {
   return *this;
 }
 
+constexpr int128 MakeInt128(int64_t high, uint64_t low) {
+  return int128(high, low);
+}
+
+// Assignment from integer types.
+inline int128& int128::operator=(int v) {
+  return *this = int128(v);
+}
+
+inline int128& int128::operator=(unsigned int v) {
+  return *this = int128(v);
+}
+
+inline int128& int128::operator=(long v) {  // NOLINT(runtime/int)
+  return *this = int128(v);
+}
+
+// NOLINTNEXTLINE(runtime/int)
+inline int128& int128::operator=(unsigned long v) {
+  return *this = int128(v);
+}
+
+// NOLINTNEXTLINE(runtime/int)
+inline int128& int128::operator=(long long v) {
+  return *this = int128(v);
+}
+
+// NOLINTNEXTLINE(runtime/int)
+inline int128& int128::operator=(unsigned long long v) {
+  return *this = int128(v);
+}
+
+// Arithmetic operators.
+
+int128 operator+(int128 lhs, int128 rhs);
+int128 operator-(int128 lhs, int128 rhs);
+int128 operator*(int128 lhs, int128 rhs);
+int128 operator/(int128 lhs, int128 rhs);
+int128 operator%(int128 lhs, int128 rhs);
+int128 operator|(int128 lhs, int128 rhs);
+int128 operator&(int128 lhs, int128 rhs);
+int128 operator^(int128 lhs, int128 rhs);
+int128 operator<<(int128 lhs, int amount);
+int128 operator>>(int128 lhs, int amount);
+
+inline int128& int128::operator+=(int128 other) {
+  *this = *this + other;
+  return *this;
+}
+
+inline int128& int128::operator-=(int128 other) {
+  *this = *this - other;
+  return *this;
+}
+
+inline int128& int128::operator*=(int128 other) {
+  *this = *this * other;
+  return *this;
+}
+
+inline int128& int128::operator/=(int128 other) {
+  *this = *this / other;
+  return *this;
+}
+
+inline int128& int128::operator%=(int128 other) {
+  *this = *this % other;
+  return *this;
+}
+
+inline int128& int128::operator|=(int128 other) {
+  *this = *this | other;
+  return *this;
+}
+
+inline int128& int128::operator&=(int128 other) {
+  *this = *this & other;
+  return *this;
+}
+
+inline int128& int128::operator^=(int128 other) {
+  *this = *this ^ other;
+  return *this;
+}
+
+inline int128& int128::operator<<=(int amount) {
+  *this = *this << amount;
+  return *this;
+}
+
+inline int128& int128::operator>>=(int amount) {
+  *this = *this >> amount;
+  return *this;
+}
+
+namespace int128_internal {
+
+// Casts from unsigned to signed while preserving the underlying binary
+// representation.
+constexpr int64_t BitCastToSigned(uint64_t v) {
+  // Casting an unsigned integer to a signed integer of the same
+  // width is implementation defined behavior if the source value would not fit
+  // in the destination type. We step around it with a roundtrip bitwise not
+  // operation to make sure this function remains constexpr. Clang, GCC, and
+  // MSVC optimize this to a no-op on x86-64.
+  return v & (uint64_t{1} << 63) ? ~static_cast<int64_t>(~v)
+                                 : static_cast<int64_t>(v);
+}
+
+}  // namespace int128_internal
+
 #if defined(ABSL_HAVE_INTRINSIC_INT128)
 #include "absl/numeric/int128_have_intrinsic.inc"  // IWYU pragma: export
 #else  // ABSL_HAVE_INTRINSIC_INT128
diff --git a/absl/numeric/int128_have_intrinsic.inc b/absl/numeric/int128_have_intrinsic.inc
index c7ea68345307..d6c76dd320ce 100644
--- a/absl/numeric/int128_have_intrinsic.inc
+++ b/absl/numeric/int128_have_intrinsic.inc
@@ -16,3 +16,287 @@
 // This file contains :int128 implementation details that depend on internal
 // representation when ABSL_HAVE_INTRINSIC_INT128 is defined. This file is
 // included by int128.h and relies on ABSL_INTERNAL_WCHAR_T being defined.
+
+namespace int128_internal {
+
+// Casts from unsigned to signed while preserving the underlying binary
+// representation.
+constexpr __int128 BitCastToSigned(unsigned __int128 v) {
+  // Casting an unsigned integer to a signed integer of the same
+  // width is implementation defined behavior if the source value would not fit
+  // in the destination type. We step around it with a roundtrip bitwise not
+  // operation to make sure this function remains constexpr. Clang and GCC
+  // optimize this to a no-op on x86-64.
+  return v & (static_cast<unsigned __int128>(1) << 127)
+             ? ~static_cast<__int128>(~v)
+             : static_cast<__int128>(v);
+}
+
+}  // namespace int128_internal
+
+inline int128& int128::operator=(__int128 v) {
+  v_ = v;
+  return *this;
+}
+
+constexpr uint64_t Int128Low64(int128 v) {
+  return static_cast<uint64_t>(v.v_ & ~uint64_t{0});
+}
+
+constexpr int64_t Int128High64(int128 v) {
+  // Initially cast to unsigned to prevent a right shift on a negative value.
+  return int128_internal::BitCastToSigned(
+      static_cast<uint64_t>(static_cast<unsigned __int128>(v.v_) >> 64));
+}
+
+constexpr int128::int128(int64_t high, uint64_t low)
+    // Initially cast to unsigned to prevent a left shift that overflows.
+    : v_(int128_internal::BitCastToSigned(static_cast<unsigned __int128>(high)
+                                           << 64) |
+         low) {}
+
+
+constexpr int128::int128(int v) : v_{v} {}
+
+constexpr int128::int128(long v) : v_{v} {}       // NOLINT(runtime/int)
+
+constexpr int128::int128(long long v) : v_{v} {}  // NOLINT(runtime/int)
+
+constexpr int128::int128(__int128 v) : v_{v} {}
+
+constexpr int128::int128(unsigned int v) : v_{v} {}
+
+constexpr int128::int128(unsigned long v) : v_{v} {}  // NOLINT(runtime/int)
+
+// NOLINTNEXTLINE(runtime/int)
+constexpr int128::int128(unsigned long long v) : v_{v} {}
+
+constexpr int128::int128(unsigned __int128 v) : v_{static_cast<__int128>(v)} {}
+
+inline int128::int128(float v) {
+  v_ = static_cast<__int128>(v);
+}
+
+inline int128::int128(double v) {
+  v_ = static_cast<__int128>(v);
+}
+
+inline int128::int128(long double v) {
+  v_ = static_cast<__int128>(v);
+}
+
+constexpr int128::int128(uint128 v) : v_{static_cast<__int128>(v)} {}
+
+constexpr int128::operator bool() const { return static_cast<bool>(v_); }
+
+constexpr int128::operator char() const { return static_cast<char>(v_); }
+
+constexpr int128::operator signed char() const {
+  return static_cast<signed char>(v_);
+}
+
+constexpr int128::operator unsigned char() const {
+  return static_cast<unsigned char>(v_);
+}
+
+constexpr int128::operator char16_t() const {
+  return static_cast<char16_t>(v_);
+}
+
+constexpr int128::operator char32_t() const {
+  return static_cast<char32_t>(v_);
+}
+
+constexpr int128::operator ABSL_INTERNAL_WCHAR_T() const {
+  return static_cast<ABSL_INTERNAL_WCHAR_T>(v_);
+}
+
+constexpr int128::operator short() const {  // NOLINT(runtime/int)
+  return static_cast<short>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator unsigned short() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned short>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator int() const {
+  return static_cast<int>(v_);
+}
+
+constexpr int128::operator unsigned int() const {
+  return static_cast<unsigned int>(v_);
+}
+
+constexpr int128::operator long() const {  // NOLINT(runtime/int)
+  return static_cast<long>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator unsigned long() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned long>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator long long() const {  // NOLINT(runtime/int)
+  return static_cast<long long>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator unsigned long long() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned long long>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator __int128() const { return v_; }
+
+constexpr int128::operator unsigned __int128() const {
+  return static_cast<unsigned __int128>(v_);
+}
+
+// Clang on PowerPC sometimes produces incorrect __int128 to floating point
+// conversions. In that case, we do the conversion with a similar implementation
+// to the conversion operators in int128_no_intrinsic.inc.
+#if defined(__clang__) && !defined(__ppc64__)
+inline int128::operator float() const { return static_cast<float>(v_); }
+
+inline int128::operator double () const { return static_cast<double>(v_); }
+
+inline int128::operator long double() const {
+  return static_cast<long double>(v_);
+}
+
+#else  // Clang on PowerPC
+// Forward declaration for conversion operators to floating point types.
+int128 operator-(int128 v);
+bool operator!=(int128 lhs, int128 rhs);
+
+inline int128::operator float() const {
+  // We must convert the absolute value and then negate as needed, because
+  // floating point types are typically sign-magnitude. Otherwise, the
+  // difference between the high and low 64 bits when interpreted as two's
+  // complement overwhelms the precision of the mantissa.
+  //
+  // Also check to make sure we don't negate Int128Min()
+  return v_ < 0 && *this != Int128Min()
+             ? -static_cast<float>(-*this)
+             : static_cast<float>(Int128Low64(*this)) +
+                   std::ldexp(static_cast<float>(Int128High64(*this)), 64);
+}
+
+inline int128::operator double() const {
+  // See comment in int128::operator float() above.
+  return v_ < 0 && *this != Int128Min()
+             ? -static_cast<double>(-*this)
+             : static_cast<double>(Int128Low64(*this)) +
+                   std::ldexp(static_cast<double>(Int128High64(*this)), 64);
+}
+
+inline int128::operator long double() const {
+  // See comment in int128::operator float() above.
+  return v_ < 0 && *this != Int128Min()
+             ? -static_cast<long double>(-*this)
+             : static_cast<long double>(Int128Low64(*this)) +
+                   std::ldexp(static_cast<long double>(Int128High64(*this)),
+                              64);
+}
+#endif  // Clang on PowerPC
+
+// Comparison operators.
+
+inline bool operator==(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) == static_cast<__int128>(rhs);
+}
+
+inline bool operator!=(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) != static_cast<__int128>(rhs);
+}
+
+inline bool operator<(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) < static_cast<__int128>(rhs);
+}
+
+inline bool operator>(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) > static_cast<__int128>(rhs);
+}
+
+inline bool operator<=(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) <= static_cast<__int128>(rhs);
+}
+
+inline bool operator>=(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) >= static_cast<__int128>(rhs);
+}
+
+// Unary operators.
+
+inline int128 operator-(int128 v) {
+  return -static_cast<__int128>(v);
+}
+
+inline bool operator!(int128 v) {
+  return !static_cast<__int128>(v);
+}
+
+inline int128 operator~(int128 val) {
+  return ~static_cast<__int128>(val);
+}
+
+// Arithmetic operators.
+
+inline int128 operator+(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) + static_cast<__int128>(rhs);
+}
+
+inline int128 operator-(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) - static_cast<__int128>(rhs);
+}
+
+inline int128 operator*(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) * static_cast<__int128>(rhs);
+}
+
+inline int128 operator/(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) / static_cast<__int128>(rhs);
+}
+
+inline int128 operator%(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) % static_cast<__int128>(rhs);
+}
+
+inline int128 int128::operator++(int) {
+  int128 tmp(*this);
+  ++v_;
+  return tmp;
+}
+
+inline int128 int128::operator--(int) {
+  int128 tmp(*this);
+  --v_;
+  return tmp;
+}
+
+inline int128& int128::operator++() {
+  ++v_;
+  return *this;
+}
+
+inline int128& int128::operator--() {
+  --v_;
+  return *this;
+}
+
+inline int128 operator|(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) | static_cast<__int128>(rhs);
+}
+
+inline int128 operator&(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) & static_cast<__int128>(rhs);
+}
+
+inline int128 operator^(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) ^ static_cast<__int128>(rhs);
+}
+
+inline int128 operator<<(int128 lhs, int amount) {
+  return static_cast<__int128>(lhs) << amount;
+}
+
+inline int128 operator>>(int128 lhs, int amount) {
+  return static_cast<__int128>(lhs) >> amount;
+}
diff --git a/absl/numeric/int128_no_intrinsic.inc b/absl/numeric/int128_no_intrinsic.inc
index 046cb9b3a407..c753771ae73a 100644
--- a/absl/numeric/int128_no_intrinsic.inc
+++ b/absl/numeric/int128_no_intrinsic.inc
@@ -16,3 +16,293 @@
 // This file contains :int128 implementation details that depend on internal
 // representation when ABSL_HAVE_INTRINSIC_INT128 is *not* defined. This file
 // is included by int128.h and relies on ABSL_INTERNAL_WCHAR_T being defined.
+
+constexpr uint64_t Int128Low64(int128 v) { return v.lo_; }
+
+constexpr int64_t Int128High64(int128 v) { return v.hi_; }
+
+#if defined(ABSL_IS_LITTLE_ENDIAN)
+
+constexpr int128::int128(int64_t high, uint64_t low) :
+    lo_(low), hi_(high) {}
+
+constexpr int128::int128(int v)
+    : lo_{static_cast<uint64_t>(v)}, hi_{v < 0 ? ~int64_t{0} : 0} {}
+constexpr int128::int128(long v)  // NOLINT(runtime/int)
+    : lo_{static_cast<uint64_t>(v)}, hi_{v < 0 ? ~int64_t{0} : 0} {}
+constexpr int128::int128(long long v)  // NOLINT(runtime/int)
+    : lo_{static_cast<uint64_t>(v)}, hi_{v < 0 ? ~int64_t{0} : 0} {}
+
+constexpr int128::int128(unsigned int v) : lo_{v}, hi_{0} {}
+// NOLINTNEXTLINE(runtime/int)
+constexpr int128::int128(unsigned long v) : lo_{v}, hi_{0} {}
+// NOLINTNEXTLINE(runtime/int)
+constexpr int128::int128(unsigned long long v) : lo_{v}, hi_{0} {}
+
+constexpr int128::int128(uint128 v)
+    : lo_{Uint128Low64(v)}, hi_{static_cast<int64_t>(Uint128High64(v))} {}
+
+#elif defined(ABSL_IS_BIG_ENDIAN)
+
+constexpr int128::int128(int64_t high, uint64_t low) :
+    hi_{high}, lo_{low} {}
+
+constexpr int128::int128(int v)
+    : hi_{v < 0 ? ~int64_t{0} : 0}, lo_{static_cast<uint64_t>(v)} {}
+constexpr int128::int128(long v)  // NOLINT(runtime/int)
+    : hi_{v < 0 ? ~int64_t{0} : 0}, lo_{static_cast<uint64_t>(v)} {}
+constexpr int128::int128(long long v)  // NOLINT(runtime/int)
+    : hi_{v < 0 ? ~int64_t{0} : 0}, lo_{static_cast<uint64_t>(v)} {}
+
+constexpr int128::int128(unsigned int v) : hi_{0}, lo_{v} {}
+// NOLINTNEXTLINE(runtime/int)
+constexpr int128::int128(unsigned long v) : hi_{0}, lo_{v} {}
+// NOLINTNEXTLINE(runtime/int)
+constexpr int128::int128(unsigned long long v) : hi_{0}, lo_{v} {}
+
+constexpr int128::int128(uint128 v)
+    : hi_{static_cast<int64_t>(Uint128High64(v))}, lo_{Uint128Low64(v)} {}
+
+#else  // byte order
+#error "Unsupported byte order: must be little-endian or big-endian."
+#endif  // byte order
+
+constexpr int128::operator bool() const { return lo_ || hi_; }
+
+constexpr int128::operator char() const {
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<char>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator signed char() const {
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<signed char>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator unsigned char() const {
+  return static_cast<unsigned char>(lo_);
+}
+
+constexpr int128::operator char16_t() const {
+  return static_cast<char16_t>(lo_);
+}
+
+constexpr int128::operator char32_t() const {
+  return static_cast<char32_t>(lo_);
+}
+
+constexpr int128::operator ABSL_INTERNAL_WCHAR_T() const {
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<ABSL_INTERNAL_WCHAR_T>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator short() const {  // NOLINT(runtime/int)
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<short>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator unsigned short() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned short>(lo_);           // NOLINT(runtime/int)
+}
+
+constexpr int128::operator int() const {
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<int>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator unsigned int() const {
+  return static_cast<unsigned int>(lo_);
+}
+
+constexpr int128::operator long() const {  // NOLINT(runtime/int)
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<long>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator unsigned long() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned long>(lo_);           // NOLINT(runtime/int)
+}
+
+constexpr int128::operator long long() const {  // NOLINT(runtime/int)
+  // We don't bother checking the value of hi_. If *this < 0, lo_'s high bit
+  // must be set in order for the value to fit into a long long. Conversely, if
+  // lo_'s high bit is set, *this must be < 0 for the value to fit.
+  return int128_internal::BitCastToSigned(lo_);
+}
+
+constexpr int128::operator unsigned long long() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned long long>(lo_);           // NOLINT(runtime/int)
+}
+
+// Forward declaration for conversion operators to floating point types.
+int128 operator-(int128 v);
+bool operator!=(int128 lhs, int128 rhs);
+
+inline int128::operator float() const {
+  // We must convert the absolute value and then negate as needed, because
+  // floating point types are typically sign-magnitude. Otherwise, the
+  // difference between the high and low 64 bits when interpreted as two's
+  // complement overwhelms the precision of the mantissa.
+  //
+  // Also check to make sure we don't negate Int128Min()
+  return hi_ < 0 && *this != Int128Min()
+             ? -static_cast<float>(-*this)
+             : static_cast<float>(lo_) +
+                   std::ldexp(static_cast<float>(hi_), 64);
+}
+
+inline int128::operator double() const {
+  // See comment in int128::operator float() above.
+  return hi_ < 0 && *this != Int128Min()
+             ? -static_cast<double>(-*this)
+             : static_cast<double>(lo_) +
+                   std::ldexp(static_cast<double>(hi_), 64);
+}
+
+inline int128::operator long double() const {
+  // See comment in int128::operator float() above.
+  return hi_ < 0 && *this != Int128Min()
+             ? -static_cast<long double>(-*this)
+             : static_cast<long double>(lo_) +
+                   std::ldexp(static_cast<long double>(hi_), 64);
+}
+
+// Comparison operators.
+
+inline bool operator==(int128 lhs, int128 rhs) {
+  return (Int128Low64(lhs) == Int128Low64(rhs) &&
+          Int128High64(lhs) == Int128High64(rhs));
+}
+
+inline bool operator!=(int128 lhs, int128 rhs) {
+  return !(lhs == rhs);
+}
+
+inline bool operator<(int128 lhs, int128 rhs) {
+  return (Int128High64(lhs) == Int128High64(rhs))
+             ? (Int128Low64(lhs) < Int128Low64(rhs))
+             : (Int128High64(lhs) < Int128High64(rhs));
+}
+
+inline bool operator>(int128 lhs, int128 rhs) {
+  return (Int128High64(lhs) == Int128High64(rhs))
+             ? (Int128Low64(lhs) > Int128Low64(rhs))
+             : (Int128High64(lhs) > Int128High64(rhs));
+}
+
+inline bool operator<=(int128 lhs, int128 rhs) {
+  return !(lhs > rhs);
+}
+
+inline bool operator>=(int128 lhs, int128 rhs) {
+  return !(lhs < rhs);
+}
+
+// Unary operators.
+
+inline int128 operator-(int128 v) {
+  int64_t hi = ~Int128High64(v);
+  uint64_t lo = ~Int128Low64(v) + 1;
+  if (lo == 0) ++hi;  // carry
+  return MakeInt128(hi, lo);
+}
+
+inline bool operator!(int128 v) {
+  return !Int128Low64(v) && !Int128High64(v);
+}
+
+inline int128 operator~(int128 val) {
+  return MakeInt128(~Int128High64(val), ~Int128Low64(val));
+}
+
+// Arithmetic operators.
+
+inline int128 operator+(int128 lhs, int128 rhs) {
+  int128 result = MakeInt128(Int128High64(lhs) + Int128High64(rhs),
+                             Int128Low64(lhs) + Int128Low64(rhs));
+  if (Int128Low64(result) < Int128Low64(lhs)) {  // check for carry
+    return MakeInt128(Int128High64(result) + 1, Int128Low64(result));
+  }
+  return result;
+}
+
+inline int128 operator-(int128 lhs, int128 rhs) {
+  int128 result = MakeInt128(Int128High64(lhs) - Int128High64(rhs),
+                             Int128Low64(lhs) - Int128Low64(rhs));
+  if (Int128Low64(lhs) < Int128Low64(rhs)) {  // check for carry
+    return MakeInt128(Int128High64(result) - 1, Int128Low64(result));
+  }
+  return result;
+}
+
+inline int128 operator*(int128 lhs, int128 rhs) {
+  uint128 result = uint128(lhs) * rhs;
+  return MakeInt128(int128_internal::BitCastToSigned(Uint128High64(result)),
+                    Uint128Low64(result));
+}
+
+inline int128 int128::operator++(int) {
+  int128 tmp(*this);
+  *this += 1;
+  return tmp;
+}
+
+inline int128 int128::operator--(int) {
+  int128 tmp(*this);
+  *this -= 1;
+  return tmp;
+}
+
+inline int128& int128::operator++() {
+  *this += 1;
+  return *this;
+}
+
+inline int128& int128::operator--() {
+  *this -= 1;
+  return *this;
+}
+
+inline int128 operator|(int128 lhs, int128 rhs) {
+  return MakeInt128(Int128High64(lhs) | Int128High64(rhs),
+                    Int128Low64(lhs) | Int128Low64(rhs));
+}
+
+inline int128 operator&(int128 lhs, int128 rhs) {
+  return MakeInt128(Int128High64(lhs) & Int128High64(rhs),
+                    Int128Low64(lhs) & Int128Low64(rhs));
+}
+
+inline int128 operator^(int128 lhs, int128 rhs) {
+  return MakeInt128(Int128High64(lhs) ^ Int128High64(rhs),
+                    Int128Low64(lhs) ^ Int128Low64(rhs));
+}
+
+inline int128 operator<<(int128 lhs, int amount) {
+  // uint64_t shifts of >= 64 are undefined, so we need some special-casing.
+  if (amount < 64) {
+    if (amount != 0) {
+      return MakeInt128(
+          (Int128High64(lhs) << amount) |
+              static_cast<int64_t>(Int128Low64(lhs) >> (64 - amount)),
+          Int128Low64(lhs) << amount);
+    }
+    return lhs;
+  }
+  return MakeInt128(static_cast<int64_t>(Int128Low64(lhs) << (amount - 64)), 0);
+}
+
+inline int128 operator>>(int128 lhs, int amount) {
+  // uint64_t shifts of >= 64 are undefined, so we need some special-casing.
+  if (amount < 64) {
+    if (amount != 0) {
+      return MakeInt128(
+          Int128High64(lhs) >> amount,
+          (Int128Low64(lhs) >> amount) |
+              (static_cast<uint64_t>(Int128High64(lhs)) << (64 - amount)));
+    }
+    return lhs;
+  }
+  return MakeInt128(0,
+                    static_cast<uint64_t>(Int128High64(lhs) >> (amount - 64)));
+}
diff --git a/absl/numeric/int128_stream_test.cc b/absl/numeric/int128_stream_test.cc
index 3cfa9dc1835a..479ad66cf4d5 100644
--- a/absl/numeric/int128_stream_test.cc
+++ b/absl/numeric/int128_stream_test.cc
@@ -147,6 +147,735 @@ TEST(Uint128, OStreamFormatTest) {
   }
 }
 
+struct Int128TestCase {
+  absl::int128 value;
+  std::ios_base::fmtflags flags;
+  std::streamsize width;
+  const char* expected;
+};
+
+void CheckInt128Case(const Int128TestCase& test_case) {
+  std::ostringstream os;
+  os.flags(test_case.flags);
+  os.width(test_case.width);
+  os.fill(kFill);
+  os << test_case.value;
+  SCOPED_TRACE(StreamFormatToString(test_case.flags, test_case.width));
+  EXPECT_EQ(test_case.expected, os.str());
+}
+
+TEST(Int128, OStreamValueTest) {
+  CheckInt128Case({1, kDec, /*width = */ 0, "1"});
+  CheckInt128Case({1, kOct, /*width = */ 0, "1"});
+  CheckInt128Case({1, kHex, /*width = */ 0, "1"});
+  CheckInt128Case({9, kDec, /*width = */ 0, "9"});
+  CheckInt128Case({9, kOct, /*width = */ 0, "11"});
+  CheckInt128Case({9, kHex, /*width = */ 0, "9"});
+  CheckInt128Case({12345, kDec, /*width = */ 0, "12345"});
+  CheckInt128Case({12345, kOct, /*width = */ 0, "30071"});
+  CheckInt128Case({12345, kHex, /*width = */ 0, "3039"});
+  CheckInt128Case(
+      {0x8000000000000000, kDec, /*width = */ 0, "9223372036854775808"});
+  CheckInt128Case(
+      {0x8000000000000000, kOct, /*width = */ 0, "1000000000000000000000"});
+  CheckInt128Case(
+      {0x8000000000000000, kHex, /*width = */ 0, "8000000000000000"});
+  CheckInt128Case({std::numeric_limits<uint64_t>::max(), kDec,
+                   /*width = */ 0, "18446744073709551615"});
+  CheckInt128Case({std::numeric_limits<uint64_t>::max(), kOct,
+                   /*width = */ 0, "1777777777777777777777"});
+  CheckInt128Case({std::numeric_limits<uint64_t>::max(), kHex,
+                   /*width = */ 0, "ffffffffffffffff"});
+  CheckInt128Case(
+      {absl::MakeInt128(1, 0), kDec, /*width = */ 0, "18446744073709551616"});
+  CheckInt128Case(
+      {absl::MakeInt128(1, 0), kOct, /*width = */ 0, "2000000000000000000000"});
+  CheckInt128Case(
+      {absl::MakeInt128(1, 0), kHex, /*width = */ 0, "10000000000000000"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::max(),
+                                    std::numeric_limits<uint64_t>::max()),
+                   std::ios::dec, /*width = */ 0,
+                   "170141183460469231731687303715884105727"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::max(),
+                                    std::numeric_limits<uint64_t>::max()),
+                   std::ios::oct, /*width = */ 0,
+                   "1777777777777777777777777777777777777777777"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::max(),
+                                    std::numeric_limits<uint64_t>::max()),
+                   std::ios::hex, /*width = */ 0,
+                   "7fffffffffffffffffffffffffffffff"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::min(), 0),
+                   std::ios::dec, /*width = */ 0,
+                   "-170141183460469231731687303715884105728"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::min(), 0),
+                   std::ios::oct, /*width = */ 0,
+                   "2000000000000000000000000000000000000000000"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::min(), 0),
+                   std::ios::hex, /*width = */ 0,
+                   "80000000000000000000000000000000"});
+  CheckInt128Case({-1, std::ios::dec, /*width = */ 0, "-1"});
+  CheckInt128Case({-1, std::ios::oct, /*width = */ 0,
+                   "3777777777777777777777777777777777777777777"});
+  CheckInt128Case(
+      {-1, std::ios::hex, /*width = */ 0, "ffffffffffffffffffffffffffffffff"});
+  CheckInt128Case({-12345, std::ios::dec, /*width = */ 0, "-12345"});
+  CheckInt128Case({-12345, std::ios::oct, /*width = */ 0,
+                   "3777777777777777777777777777777777777747707"});
+  CheckInt128Case({-12345, std::ios::hex, /*width = */ 0,
+                   "ffffffffffffffffffffffffffffcfc7"});
+}
+
+std::vector<Int128TestCase> GetInt128FormatCases();
+TEST(Int128, OStreamFormatTest) {
+  for (const Int128TestCase& test_case : GetInt128FormatCases()) {
+    CheckInt128Case(test_case);
+  }
+}
+
+std::vector<Int128TestCase> GetInt128FormatCases() {
+  return {
+      {0, std::ios_base::fmtflags(), /*width = */ 0, "0"},
+      {0, std::ios_base::fmtflags(), /*width = */ 6, "_____0"},
+      {0, kPos, /*width = */ 0, "+0"},
+      {0, kPos, /*width = */ 6, "____+0"},
+      {0, kBase, /*width = */ 0, "0"},
+      {0, kBase, /*width = */ 6, "_____0"},
+      {0, kBase | kPos, /*width = */ 0, "+0"},
+      {0, kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kUpper, /*width = */ 0, "0"},
+      {0, kUpper, /*width = */ 6, "_____0"},
+      {0, kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kUpper | kPos, /*width = */ 6, "____+0"},
+      {0, kUpper | kBase, /*width = */ 0, "0"},
+      {0, kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kUpper | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kLeft, /*width = */ 0, "0"},
+      {0, kLeft, /*width = */ 6, "0_____"},
+      {0, kLeft | kPos, /*width = */ 0, "+0"},
+      {0, kLeft | kPos, /*width = */ 6, "+0____"},
+      {0, kLeft | kBase, /*width = */ 0, "0"},
+      {0, kLeft | kBase, /*width = */ 6, "0_____"},
+      {0, kLeft | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kLeft | kBase | kPos, /*width = */ 6, "+0____"},
+      {0, kLeft | kUpper, /*width = */ 0, "0"},
+      {0, kLeft | kUpper, /*width = */ 6, "0_____"},
+      {0, kLeft | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kLeft | kUpper | kPos, /*width = */ 6, "+0____"},
+      {0, kLeft | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kLeft | kUpper | kBase, /*width = */ 6, "0_____"},
+      {0, kLeft | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kLeft | kUpper | kBase | kPos, /*width = */ 6, "+0____"},
+      {0, kInt, /*width = */ 0, "0"},
+      {0, kInt, /*width = */ 6, "_____0"},
+      {0, kInt | kPos, /*width = */ 0, "+0"},
+      {0, kInt | kPos, /*width = */ 6, "+____0"},
+      {0, kInt | kBase, /*width = */ 0, "0"},
+      {0, kInt | kBase, /*width = */ 6, "_____0"},
+      {0, kInt | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kInt | kBase | kPos, /*width = */ 6, "+____0"},
+      {0, kInt | kUpper, /*width = */ 0, "0"},
+      {0, kInt | kUpper, /*width = */ 6, "_____0"},
+      {0, kInt | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kInt | kUpper | kPos, /*width = */ 6, "+____0"},
+      {0, kInt | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kInt | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kInt | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kInt | kUpper | kBase | kPos, /*width = */ 6, "+____0"},
+      {0, kRight, /*width = */ 0, "0"},
+      {0, kRight, /*width = */ 6, "_____0"},
+      {0, kRight | kPos, /*width = */ 0, "+0"},
+      {0, kRight | kPos, /*width = */ 6, "____+0"},
+      {0, kRight | kBase, /*width = */ 0, "0"},
+      {0, kRight | kBase, /*width = */ 6, "_____0"},
+      {0, kRight | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kRight | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kRight | kUpper, /*width = */ 0, "0"},
+      {0, kRight | kUpper, /*width = */ 6, "_____0"},
+      {0, kRight | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kRight | kUpper | kPos, /*width = */ 6, "____+0"},
+      {0, kRight | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kRight | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kRight | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kRight | kUpper | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kDec, /*width = */ 0, "0"},
+      {0, kDec, /*width = */ 6, "_____0"},
+      {0, kDec | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kBase, /*width = */ 0, "0"},
+      {0, kDec | kBase, /*width = */ 6, "_____0"},
+      {0, kDec | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kUpper, /*width = */ 0, "0"},
+      {0, kDec | kUpper, /*width = */ 6, "_____0"},
+      {0, kDec | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kUpper | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kDec | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kDec | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kUpper | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kLeft, /*width = */ 0, "0"},
+      {0, kDec | kLeft, /*width = */ 6, "0_____"},
+      {0, kDec | kLeft | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kLeft | kPos, /*width = */ 6, "+0____"},
+      {0, kDec | kLeft | kBase, /*width = */ 0, "0"},
+      {0, kDec | kLeft | kBase, /*width = */ 6, "0_____"},
+      {0, kDec | kLeft | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kLeft | kBase | kPos, /*width = */ 6, "+0____"},
+      {0, kDec | kLeft | kUpper, /*width = */ 0, "0"},
+      {0, kDec | kLeft | kUpper, /*width = */ 6, "0_____"},
+      {0, kDec | kLeft | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kLeft | kUpper | kPos, /*width = */ 6, "+0____"},
+      {0, kDec | kLeft | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kDec | kLeft | kUpper | kBase, /*width = */ 6, "0_____"},
+      {0, kDec | kLeft | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kLeft | kUpper | kBase | kPos, /*width = */ 6, "+0____"},
+      {0, kDec | kInt, /*width = */ 0, "0"},
+      {0, kDec | kInt, /*width = */ 6, "_____0"},
+      {0, kDec | kInt | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kInt | kPos, /*width = */ 6, "+____0"},
+      {0, kDec | kInt | kBase, /*width = */ 0, "0"},
+      {0, kDec | kInt | kBase, /*width = */ 6, "_____0"},
+      {0, kDec | kInt | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kInt | kBase | kPos, /*width = */ 6, "+____0"},
+      {0, kDec | kInt | kUpper, /*width = */ 0, "0"},
+      {0, kDec | kInt | kUpper, /*width = */ 6, "_____0"},
+      {0, kDec | kInt | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kInt | kUpper | kPos, /*width = */ 6, "+____0"},
+      {0, kDec | kInt | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kDec | kInt | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kDec | kInt | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kInt | kUpper | kBase | kPos, /*width = */ 6, "+____0"},
+      {0, kDec | kRight, /*width = */ 0, "0"},
+      {0, kDec | kRight, /*width = */ 6, "_____0"},
+      {0, kDec | kRight | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kRight | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kRight | kBase, /*width = */ 0, "0"},
+      {0, kDec | kRight | kBase, /*width = */ 6, "_____0"},
+      {0, kDec | kRight | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kRight | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kRight | kUpper, /*width = */ 0, "0"},
+      {0, kDec | kRight | kUpper, /*width = */ 6, "_____0"},
+      {0, kDec | kRight | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kRight | kUpper | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kRight | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kDec | kRight | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kDec | kRight | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kRight | kUpper | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kOct, /*width = */ 0, "0"},
+      {0, kOct, /*width = */ 6, "_____0"},
+      {0, kOct | kPos, /*width = */ 0, "0"},
+      {0, kOct | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kBase, /*width = */ 0, "0"},
+      {0, kOct | kBase, /*width = */ 6, "_____0"},
+      {0, kOct | kBase | kPos, /*width = */ 0, "0"},
+      {0, kOct | kBase | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kUpper, /*width = */ 0, "0"},
+      {0, kOct | kUpper, /*width = */ 6, "_____0"},
+      {0, kOct | kUpper | kPos, /*width = */ 0, "0"},
+      {0, kOct | kUpper | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kOct | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kOct | kUpper | kBase | kPos, /*width = */ 0, "0"},
+      {0, kOct | kUpper | kBase | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kLeft, /*width = */ 0, "0"},
+      {0, kOct | kLeft, /*width = */ 6, "0_____"},
+      {0, kOct | kLeft | kPos, /*width = */ 0, "0"},
+      {0, kOct | kLeft | kPos, /*width = */ 6, "0_____"},
+      {0, kOct | kLeft | kBase, /*width = */ 0, "0"},
+      {0, kOct | kLeft | kBase, /*width = */ 6, "0_____"},
+      {0, kOct | kLeft | kBase | kPos, /*width = */ 0, "0"},
+      {0, kOct | kLeft | kBase | kPos, /*width = */ 6, "0_____"},
+      {0, kOct | kLeft | kUpper, /*width = */ 0, "0"},
+      {0, kOct | kLeft | kUpper, /*width = */ 6, "0_____"},
+      {0, kOct | kLeft | kUpper | kPos, /*width = */ 0, "0"},
+      {0, kOct | kLeft | kUpper | kPos, /*width = */ 6, "0_____"},
+      {0, kOct | kLeft | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kOct | kLeft | kUpper | kBase, /*width = */ 6, "0_____"},
+      {0, kOct | kLeft | kUpper | kBase | kPos, /*width = */ 0, "0"},
+      {0, kOct | kLeft | kUpper | kBase | kPos, /*width = */ 6, "0_____"},
+      {0, kOct | kInt, /*width = */ 0, "0"},
+      {0, kOct | kInt, /*width = */ 6, "_____0"},
+      {0, kOct | kInt | kPos, /*width = */ 0, "0"},
+      {0, kOct | kInt | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kInt | kBase, /*width = */ 0, "0"},
+      {0, kOct | kInt | kBase, /*width = */ 6, "_____0"},
+      {0, kOct | kInt | kBase | kPos, /*width = */ 0, "0"},
+      {0, kOct | kInt | kBase | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kInt | kUpper, /*width = */ 0, "0"},
+      {0, kOct | kInt | kUpper, /*width = */ 6, "_____0"},
+      {0, kOct | kInt | kUpper | kPos, /*width = */ 0, "0"},
+      {0, kOct | kInt | kUpper | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kInt | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kOct | kInt | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kOct | kInt | kUpper | kBase | kPos, /*width = */ 0, "0"},
+      {0, kOct | kInt | kUpper | kBase | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kRight, /*width = */ 0, "0"},
+      {0, kOct | kRight, /*width = */ 6, "_____0"},
+      {0, kOct | kRight | kPos, /*width = */ 0, "0"},
+      {0, kOct | kRight | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kRight | kBase, /*width = */ 0, "0"},
+      {0, kOct | kRight | kBase, /*width = */ 6, "_____0"},
+      {0, kOct | kRight | kBase | kPos, /*width = */ 0, "0"},
+      {0, kOct | kRight | kBase | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kRight | kUpper, /*width = */ 0, "0"},
+      {0, kOct | kRight | kUpper, /*width = */ 6, "_____0"},
+      {0, kOct | kRight | kUpper | kPos, /*width = */ 0, "0"},
+      {0, kOct | kRight | kUpper | kPos, /*width = */ 6, "_____0"},
+      {0, kOct | kRight | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kOct | kRight | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kOct | kRight | kUpper | kBase | kPos, /*width = */ 0, "0"},
+      {0, kOct | kRight | kUpper | kBase | kPos, /*width = */ 6, "_____0"},
+      {0, kHex, /*width = */ 0, "0"},
+      {0, kHex, /*width = */ 6, "_____0"},
+      {0, kHex | kPos, /*width = */ 0, "0"},
+      {0, kHex | kPos, /*width = */ 6, "_____0"},
+      {0, kHex | kBase, /*width = */ 0, "0"},
+      {0, kHex | kBase, /*width = */ 6, "_____0"},
+      {0, kHex | kBase | kPos, /*width = */ 0, "0"},
+      {0, kHex | kBase | kPos, /*width = */ 6, "_____0"},
+      {0, kHex | kUpper, /*width = */ 0, "0"},
+      {0, kHex | kUpper, /*width = */ 6, "_____0"},
+      {0, kHex | kUpper | kPos, /*width = */ 0, "0"},
+      {0, kHex | kUpper | kPos, /*width = */ 6, "_____0"},
+      {0, kHex | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kHex | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kHex | kUpper | kBase | kPos, /*width = */ 0, "0"},
+      {0, kHex | kUpper | kBase | kPos, /*width = */ 6, "_____0"},
+      {0, kHex | kLeft, /*width = */ 0, "0"},
+      {0, kHex | kLeft, /*width = */ 6, "0_____"},
+      {0, kHex | kLeft | kPos, /*width = */ 0, "0"},
+      {0, kHex | kLeft | kPos, /*width = */ 6, "0_____"},
+      {0, kHex | kLeft | kBase, /*width = */ 0, "0"},
+      {0, kHex | kLeft | kBase, /*width = */ 6, "0_____"},
+      {0, kHex | kLeft | kBase | kPos, /*width = */ 0, "0"},
+      {0, kHex | kLeft | kBase | kPos, /*width = */ 6, "0_____"},
+      {0, kHex | kLeft | kUpper, /*width = */ 0, "0"},
+      {0, kHex | kLeft | kUpper, /*width = */ 6, "0_____"},
+      {0, kHex | kLeft | kUpper | kPos, /*width = */ 0, "0"},
+      {0, kHex | kLeft | kUpper | kPos, /*width = */ 6, "0_____"},
+      {0, kHex | kLeft | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kHex | kLeft | kUpper | kBase, /*width = */ 6, "0_____"},
+      {0, kHex | kLeft | kUpper | kBase | kPos, /*width = */ 0, "0"},
+      {0, kHex | kLeft | kUpper | kBase | kPos, /*width = */ 6, "0_____"},
+      {0, kHex | kInt, /*width = */ 0, "0"},
+      {0, kHex | kInt, /*width = */ 6, "_____0"},
+      {0, kHex | kInt | kPos, /*width = */ 0, "0"},
+      {0, kHex | kInt | kPos, /*width = */ 6, "_____0"},
+      {0, kHex | kInt | kBase, /*width = */ 0, "0"},
+      {0, kHex | kInt | kBase, /*width = */ 6, "_____0"},
+      {0, kHex | kInt | kBase | kPos, /*width = */ 0, "0"},
+      {0, kHex | kInt | kBase | kPos, /*width = */ 6, "_____0"},
+      {0, kHex | kInt | kUpper, /*width = */ 0, "0"},
+      {0, kHex | kInt | kUpper, /*width = */ 6, "_____0"},
+      {0, kHex | kInt | kUpper | kPos, /*width = */ 0, "0"},
+      {0, kHex | kInt | kUpper | kPos, /*width = */ 6, "_____0"},
+      {0, kHex | kInt | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kHex | kInt | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kHex | kInt | kUpper | kBase | kPos, /*width = */ 0, "0"},
+      {0, kHex | kInt | kUpper | kBase | kPos, /*width = */ 6, "_____0"},
+      {0, kHex | kRight, /*width = */ 0, "0"},
+      {0, kHex | kRight, /*width = */ 6, "_____0"},
+      {0, kHex | kRight | kPos, /*width = */ 0, "0"},
+      {0, kHex | kRight | kPos, /*width = */ 6, "_____0"},
+      {0, kHex | kRight | kBase, /*width = */ 0, "0"},
+      {0, kHex | kRight | kBase, /*width = */ 6, "_____0"},
+      {0, kHex | kRight | kBase | kPos, /*width = */ 0, "0"},
+      {0, kHex | kRight | kBase | kPos, /*width = */ 6, "_____0"},
+      {0, kHex | kRight | kUpper, /*width = */ 0, "0"},
+      {0, kHex | kRight | kUpper, /*width = */ 6, "_____0"},
+      {0, kHex | kRight | kUpper | kPos, /*width = */ 0, "0"},
+      {0, kHex | kRight | kUpper | kPos, /*width = */ 6, "_____0"},
+      {0, kHex | kRight | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kHex | kRight | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kHex | kRight | kUpper | kBase | kPos, /*width = */ 0, "0"},
+      {0, kHex | kRight | kUpper | kBase | kPos, /*width = */ 6, "_____0"},
+      {42, std::ios_base::fmtflags(), /*width = */ 0, "42"},
+      {42, std::ios_base::fmtflags(), /*width = */ 6, "____42"},
+      {42, kPos, /*width = */ 0, "+42"},
+      {42, kPos, /*width = */ 6, "___+42"},
+      {42, kBase, /*width = */ 0, "42"},
+      {42, kBase, /*width = */ 6, "____42"},
+      {42, kBase | kPos, /*width = */ 0, "+42"},
+      {42, kBase | kPos, /*width = */ 6, "___+42"},
+      {42, kUpper, /*width = */ 0, "42"},
+      {42, kUpper, /*width = */ 6, "____42"},
+      {42, kUpper | kPos, /*width = */ 0, "+42"},
+      {42, kUpper | kPos, /*width = */ 6, "___+42"},
+      {42, kUpper | kBase, /*width = */ 0, "42"},
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+      {-321, kDec | kBase | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kUpper, /*width = */ 0, "-321"},
+      {-321, kDec | kUpper, /*width = */ 6, "__-321"},
+      {-321, kDec | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kUpper | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kUpper | kBase, /*width = */ 6, "__-321"},
+      {-321, kDec | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kUpper | kBase | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kLeft, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kPos, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kBase, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kBase | kPos, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kUpper, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kUpper, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kUpper | kPos, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kUpper | kBase, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kUpper | kBase | kPos, /*width = */ 6, "-321__"},
+      {-321, kDec | kInt, /*width = */ 0, "-321"},
+      {-321, kDec | kInt, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kPos, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kBase, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kBase | kPos, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kUpper, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kUpper, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kUpper | kPos, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kUpper | kBase, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kUpper | kBase | kPos, /*width = */ 6, "-__321"},
+      {-321, kDec | kRight, /*width = */ 0, "-321"},
+      {-321, kDec | kRight, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kBase, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kBase | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kUpper, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kUpper, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kUpper | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kUpper | kBase, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kUpper | kBase | kPos, /*width = */ 6, "__-321"}};
+}
+
 std::vector<Uint128TestCase> GetUint128FormatCases() {
   return {
       {0, std::ios_base::fmtflags(), /*width = */ 0, "0"},
diff --git a/absl/numeric/int128_test.cc b/absl/numeric/int128_test.cc
index 5e1b5ec352b0..bc86c714acce 100644
--- a/absl/numeric/int128_test.cc
+++ b/absl/numeric/int128_test.cc
@@ -479,4 +479,747 @@ TEST(Uint128, Hash) {
   }));
 }
 
+
+TEST(Int128Uint128, ConversionTest) {
+  absl::int128 nonnegative_signed_values[] = {
+      0,
+      1,
+      0xffeeddccbbaa9988,
+      absl::MakeInt128(0x7766554433221100, 0),
+      absl::MakeInt128(0x1234567890abcdef, 0xfedcba0987654321),
+      absl::Int128Max()};
+  for (absl::int128 value : nonnegative_signed_values) {
+    EXPECT_EQ(value, absl::int128(absl::uint128(value)));
+
+    absl::uint128 assigned_value;
+    assigned_value = value;
+    EXPECT_EQ(value, absl::int128(assigned_value));
+  }
+
+  absl::int128 negative_values[] = {
+      -1, -0x1234567890abcdef,
+      absl::MakeInt128(-0x5544332211ffeedd, 0),
+      -absl::MakeInt128(0x76543210fedcba98, 0xabcdef0123456789)};
+  for (absl::int128 value : negative_values) {
+    EXPECT_EQ(absl::uint128(-value), -absl::uint128(value));
+
+    absl::uint128 assigned_value;
+    assigned_value = value;
+    EXPECT_EQ(absl::uint128(-value), -assigned_value);
+  }
+}
+
+template <typename T>
+class Int128IntegerTraitsTest : public ::testing::Test {};
+
+TYPED_TEST_SUITE(Int128IntegerTraitsTest, IntegerTypes);
+
+TYPED_TEST(Int128IntegerTraitsTest, ConstructAssignTest) {
+  static_assert(std::is_constructible<absl::int128, TypeParam>::value,
+                "absl::int128 must be constructible from TypeParam");
+  static_assert(std::is_assignable<absl::int128&, TypeParam>::value,
+                "absl::int128 must be assignable from TypeParam");
+  static_assert(!std::is_assignable<TypeParam&, absl::int128>::value,
+                "TypeParam must not be assignable from absl::int128");
+}
+
+template <typename T>
+class Int128FloatTraitsTest : public ::testing::Test {};
+
+TYPED_TEST_SUITE(Int128FloatTraitsTest, FloatingPointTypes);
+
+TYPED_TEST(Int128FloatTraitsTest, ConstructAssignTest) {
+  static_assert(std::is_constructible<absl::int128, TypeParam>::value,
+                "absl::int128 must be constructible from TypeParam");
+  static_assert(!std::is_assignable<absl::int128&, TypeParam>::value,
+                "absl::int128 must not be assignable from TypeParam");
+  static_assert(!std::is_assignable<TypeParam&, absl::int128>::value,
+                "TypeParam must not be assignable from absl::int128");
+}
+
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+// These type traits done separately as TYPED_TEST requires typeinfo, and not
+// all platforms have this for __int128 even though they define the type.
+TEST(Int128, IntrinsicTypeTraitsTest) {
+  static_assert(std::is_constructible<absl::int128, __int128>::value,
+                "absl::int128 must be constructible from __int128");
+  static_assert(std::is_assignable<absl::int128&, __int128>::value,
+                "absl::int128 must be assignable from __int128");
+  static_assert(!std::is_assignable<__int128&, absl::int128>::value,
+                "__int128 must not be assignable from absl::int128");
+
+  static_assert(std::is_constructible<absl::int128, unsigned __int128>::value,
+                "absl::int128 must be constructible from unsigned __int128");
+  static_assert(!std::is_assignable<absl::int128&, unsigned __int128>::value,
+                "absl::int128 must be assignable from unsigned __int128");
+  static_assert(!std::is_assignable<unsigned __int128&, absl::int128>::value,
+                "unsigned __int128 must not be assignable from absl::int128");
+}
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
+TEST(Int128, TrivialTraitsTest) {
+  static_assert(absl::is_trivially_default_constructible<absl::int128>::value,
+                "");
+  static_assert(absl::is_trivially_copy_constructible<absl::int128>::value, "");
+  static_assert(absl::is_trivially_copy_assignable<absl::int128>::value, "");
+  static_assert(std::is_trivially_destructible<absl::int128>::value, "");
+}
+
+TEST(Int128, BoolConversionTest) {
+  EXPECT_FALSE(absl::int128(0));
+  for (int i = 0; i < 64; ++i) {
+    EXPECT_TRUE(absl::MakeInt128(0, uint64_t{1} << i));
+  }
+  for (int i = 0; i < 63; ++i) {
+    EXPECT_TRUE(absl::MakeInt128(int64_t{1} << i, 0));
+  }
+  EXPECT_TRUE(absl::Int128Min());
+
+  EXPECT_EQ(absl::int128(1), absl::int128(true));
+  EXPECT_EQ(absl::int128(0), absl::int128(false));
+}
+
+template <typename T>
+class Int128IntegerConversionTest : public ::testing::Test {};
+
+TYPED_TEST_SUITE(Int128IntegerConversionTest, IntegerTypes);
+
+TYPED_TEST(Int128IntegerConversionTest, RoundTripTest) {
+  EXPECT_EQ(TypeParam{0}, static_cast<TypeParam>(absl::int128(0)));
+  EXPECT_EQ(std::numeric_limits<TypeParam>::min(),
+            static_cast<TypeParam>(
+                absl::int128(std::numeric_limits<TypeParam>::min())));
+  EXPECT_EQ(std::numeric_limits<TypeParam>::max(),
+            static_cast<TypeParam>(
+                absl::int128(std::numeric_limits<TypeParam>::max())));
+}
+
+template <typename T>
+class Int128FloatConversionTest : public ::testing::Test {};
+
+TYPED_TEST_SUITE(Int128FloatConversionTest, FloatingPointTypes);
+
+TYPED_TEST(Int128FloatConversionTest, ConstructAndCastTest) {
+  // Conversions where the floating point values should be exactly the same.
+  // 0x9f5b is a randomly chosen small value.
+  for (int i = 0; i < 110; ++i) {  // 110 = 126 - #bits in 0x9f5b
+    SCOPED_TRACE(::testing::Message() << "i = " << i);
+
+    TypeParam float_value = std::ldexp(static_cast<TypeParam>(0x9f5b), i);
+    absl::int128 int_value = absl::int128(0x9f5b) << i;
+
+    EXPECT_EQ(float_value, static_cast<TypeParam>(int_value));
+    EXPECT_EQ(-float_value, static_cast<TypeParam>(-int_value));
+    EXPECT_EQ(int_value, absl::int128(float_value));
+    EXPECT_EQ(-int_value, absl::int128(-float_value));
+  }
+
+  // Round trip conversions with a small sample of randomly generated uint64_t
+  // values (less than int64_t max so that value * 2^64 fits into int128).
+  uint64_t values[] = {0x6d4492c24fb86199, 0x26ead65e4cb359b5,
+                       0x2c43407433ba3fd1, 0x3b574ec668df6b55,
+                       0x1c750e55a29f4f0f};
+  for (uint64_t value : values) {
+    for (int i = 0; i <= 64; ++i) {
+      SCOPED_TRACE(::testing::Message()
+                   << "value = " << value << "; i = " << i);
+
+      TypeParam fvalue = std::ldexp(static_cast<TypeParam>(value), i);
+      EXPECT_DOUBLE_EQ(fvalue, static_cast<TypeParam>(absl::int128(fvalue)));
+      EXPECT_DOUBLE_EQ(-fvalue, static_cast<TypeParam>(-absl::int128(fvalue)));
+      EXPECT_DOUBLE_EQ(-fvalue, static_cast<TypeParam>(absl::int128(-fvalue)));
+      EXPECT_DOUBLE_EQ(fvalue, static_cast<TypeParam>(-absl::int128(-fvalue)));
+    }
+  }
+
+  // Round trip conversions with a small sample of random large positive values.
+  absl::int128 large_values[] = {
+      absl::MakeInt128(0x5b0640d96c7b3d9f, 0xb7a7189e51d18622),
+      absl::MakeInt128(0x34bed042c6f65270, 0x73b236570669a089),
+      absl::MakeInt128(0x43deba9e6da12724, 0xf7f0f83da686797d),
+      absl::MakeInt128(0x71e8d383be4e5589, 0x75c3f96fb00752b6)};
+  for (absl::int128 value : large_values) {
+    // Make value have as many significant bits as can be represented by
+    // the mantissa, also making sure the highest and lowest bit in the range
+    // are set.
+    value >>= (127 - std::numeric_limits<TypeParam>::digits);
+    value |= absl::int128(1) << (std::numeric_limits<TypeParam>::digits - 1);
+    value |= 1;
+    for (int i = 0; i < 127 - std::numeric_limits<TypeParam>::digits; ++i) {
+      absl::int128 int_value = value << i;
+      EXPECT_EQ(int_value,
+                static_cast<absl::int128>(static_cast<TypeParam>(int_value)));
+      EXPECT_EQ(-int_value,
+                static_cast<absl::int128>(static_cast<TypeParam>(-int_value)));
+    }
+  }
+
+  // Small sample of checks that rounding is toward zero
+  EXPECT_EQ(0, absl::int128(TypeParam(0.1)));
+  EXPECT_EQ(17, absl::int128(TypeParam(17.8)));
+  EXPECT_EQ(0, absl::int128(TypeParam(-0.8)));
+  EXPECT_EQ(-53, absl::int128(TypeParam(-53.1)));
+  EXPECT_EQ(0, absl::int128(TypeParam(0.5)));
+  EXPECT_EQ(0, absl::int128(TypeParam(-0.5)));
+  TypeParam just_lt_one = std::nexttoward(TypeParam(1), TypeParam(0));
+  EXPECT_EQ(0, absl::int128(just_lt_one));
+  TypeParam just_gt_minus_one = std::nexttoward(TypeParam(-1), TypeParam(0));
+  EXPECT_EQ(0, absl::int128(just_gt_minus_one));
+
+  // Check limits
+  EXPECT_DOUBLE_EQ(std::ldexp(static_cast<TypeParam>(1), 127),
+                   static_cast<TypeParam>(absl::Int128Max()));
+  EXPECT_DOUBLE_EQ(-std::ldexp(static_cast<TypeParam>(1), 127),
+                   static_cast<TypeParam>(absl::Int128Min()));
+}
+
+TEST(Int128, FactoryTest) {
+  EXPECT_EQ(absl::int128(-1), absl::MakeInt128(-1, -1));
+  EXPECT_EQ(absl::int128(-31), absl::MakeInt128(-1, -31));
+  EXPECT_EQ(absl::int128(std::numeric_limits<int64_t>::min()),
+            absl::MakeInt128(-1, std::numeric_limits<int64_t>::min()));
+  EXPECT_EQ(absl::int128(0), absl::MakeInt128(0, 0));
+  EXPECT_EQ(absl::int128(1), absl::MakeInt128(0, 1));
+  EXPECT_EQ(absl::int128(std::numeric_limits<int64_t>::max()),
+            absl::MakeInt128(0, std::numeric_limits<int64_t>::max()));
+}
+
+TEST(Int128, HighLowTest) {
+  struct HighLowPair {
+    int64_t high;
+    uint64_t low;
+  };
+  HighLowPair values[]{{0, 0}, {0, 1}, {1, 0}, {123, 456}, {-654, 321}};
+  for (const HighLowPair& pair : values) {
+    absl::int128 value = absl::MakeInt128(pair.high, pair.low);
+    EXPECT_EQ(pair.low, absl::Int128Low64(value));
+    EXPECT_EQ(pair.high, absl::Int128High64(value));
+  }
+}
+
+TEST(Int128, LimitsTest) {
+  EXPECT_EQ(absl::MakeInt128(0x7fffffffffffffff, 0xffffffffffffffff),
+            absl::Int128Max());
+  EXPECT_EQ(absl::Int128Max(), ~absl::Int128Min());
+}
+
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+TEST(Int128, IntrinsicConversionTest) {
+  __int128 intrinsic =
+      (static_cast<__int128>(0x3a5b76c209de76f6) << 64) + 0x1f25e1d63a2b46c5;
+  absl::int128 custom =
+      absl::MakeInt128(0x3a5b76c209de76f6, 0x1f25e1d63a2b46c5);
+
+  EXPECT_EQ(custom, absl::int128(intrinsic));
+  EXPECT_EQ(intrinsic, static_cast<__int128>(custom));
+}
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
+TEST(Int128, ConstexprTest) {
+  constexpr absl::int128 zero = absl::int128();
+  constexpr absl::int128 one = 1;
+  constexpr absl::int128 minus_two = -2;
+  constexpr absl::int128 min = absl::Int128Min();
+  constexpr absl::int128 max = absl::Int128Max();
+  EXPECT_EQ(zero, absl::int128(0));
+  EXPECT_EQ(one, absl::int128(1));
+  EXPECT_EQ(minus_two, absl::MakeInt128(-1, -2));
+  EXPECT_GT(max, one);
+  EXPECT_LT(min, minus_two);
+}
+
+TEST(Int128, ComparisonTest) {
+  struct TestCase {
+    absl::int128 smaller;
+    absl::int128 larger;
+  };
+  TestCase cases[] = {
+      {absl::int128(0), absl::int128(123)},
+      {absl::MakeInt128(-12, 34), absl::MakeInt128(12, 34)},
+      {absl::MakeInt128(1, 1000), absl::MakeInt128(1000, 1)},
+      {absl::MakeInt128(-1000, 1000), absl::MakeInt128(-1, 1)},
+  };
+  for (const TestCase& pair : cases) {
+    SCOPED_TRACE(::testing::Message() << "pair.smaller = " << pair.smaller
+                                      << "; pair.larger = " << pair.larger);
+
+    EXPECT_TRUE(pair.smaller == pair.smaller);  // NOLINT(readability/check)
+    EXPECT_TRUE(pair.larger == pair.larger);    // NOLINT(readability/check)
+    EXPECT_FALSE(pair.smaller == pair.larger);  // NOLINT(readability/check)
+
+    EXPECT_TRUE(pair.smaller != pair.larger);    // NOLINT(readability/check)
+    EXPECT_FALSE(pair.smaller != pair.smaller);  // NOLINT(readability/check)
+    EXPECT_FALSE(pair.larger != pair.larger);    // NOLINT(readability/check)
+
+    EXPECT_TRUE(pair.smaller < pair.larger);   // NOLINT(readability/check)
+    EXPECT_FALSE(pair.larger < pair.smaller);  // NOLINT(readability/check)
+
+    EXPECT_TRUE(pair.larger > pair.smaller);   // NOLINT(readability/check)
+    EXPECT_FALSE(pair.smaller > pair.larger);  // NOLINT(readability/check)
+
+    EXPECT_TRUE(pair.smaller <= pair.larger);   // NOLINT(readability/check)
+    EXPECT_FALSE(pair.larger <= pair.smaller);  // NOLINT(readability/check)
+    EXPECT_TRUE(pair.smaller <= pair.smaller);  // NOLINT(readability/check)
+    EXPECT_TRUE(pair.larger <= pair.larger);    // NOLINT(readability/check)
+
+    EXPECT_TRUE(pair.larger >= pair.smaller);   // NOLINT(readability/check)
+    EXPECT_FALSE(pair.smaller >= pair.larger);  // NOLINT(readability/check)
+    EXPECT_TRUE(pair.smaller >= pair.smaller);  // NOLINT(readability/check)
+    EXPECT_TRUE(pair.larger >= pair.larger);    // NOLINT(readability/check)
+  }
+}
+
+TEST(Int128, UnaryNegationTest) {
+  int64_t values64[] = {0, 1, 12345, 0x4000000000000000,
+                        std::numeric_limits<int64_t>::max()};
+  for (int64_t value : values64) {
+    SCOPED_TRACE(::testing::Message() << "value = " << value);
+
+    EXPECT_EQ(absl::int128(-value), -absl::int128(value));
+    EXPECT_EQ(absl::int128(value), -absl::int128(-value));
+    EXPECT_EQ(absl::MakeInt128(-value, 0), -absl::MakeInt128(value, 0));
+    EXPECT_EQ(absl::MakeInt128(value, 0), -absl::MakeInt128(-value, 0));
+  }
+}
+
+TEST(Int128, LogicalNotTest) {
+  EXPECT_TRUE(!absl::int128(0));
+  for (int i = 0; i < 64; ++i) {
+    EXPECT_FALSE(!absl::MakeInt128(0, uint64_t{1} << i));
+  }
+  for (int i = 0; i < 63; ++i) {
+    EXPECT_FALSE(!absl::MakeInt128(int64_t{1} << i, 0));
+  }
+}
+
+TEST(Int128, AdditionSubtractionTest) {
+  // 64 bit pairs that will not cause overflow / underflow. These test negative
+  // carry; positive carry must be checked separately.
+  std::pair<int64_t, int64_t> cases[]{
+      {0, 0},                              // 0, 0
+      {0, 2945781290834},                  // 0, +
+      {1908357619234, 0},                  // +, 0
+      {0, -1204895918245},                 // 0, -
+      {-2957928523560, 0},                 // -, 0
+      {89023982312461, 98346012567134},    // +, +
+      {-63454234568239, -23456235230773},  // -, -
+      {98263457263502, -21428561935925},   // +, -
+      {-88235237438467, 15923659234573},   // -, +
+  };
+  for (const auto& pair : cases) {
+    SCOPED_TRACE(::testing::Message()
+                 << "pair = {" << pair.first << ", " << pair.second << '}');
+
+    EXPECT_EQ(absl::int128(pair.first + pair.second),
+              absl::int128(pair.first) + absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.second + pair.first),
+              absl::int128(pair.second) += absl::int128(pair.first));
+
+    EXPECT_EQ(absl::int128(pair.first - pair.second),
+              absl::int128(pair.first) - absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.second - pair.first),
+              absl::int128(pair.second) -= absl::int128(pair.first));
+
+    EXPECT_EQ(
+        absl::MakeInt128(pair.second + pair.first, 0),
+        absl::MakeInt128(pair.second, 0) + absl::MakeInt128(pair.first, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first + pair.second, 0),
+        absl::MakeInt128(pair.first, 0) += absl::MakeInt128(pair.second, 0));
+
+    EXPECT_EQ(
+        absl::MakeInt128(pair.second - pair.first, 0),
+        absl::MakeInt128(pair.second, 0) - absl::MakeInt128(pair.first, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first - pair.second, 0),
+        absl::MakeInt128(pair.first, 0) -= absl::MakeInt128(pair.second, 0));
+  }
+
+  // check positive carry
+  EXPECT_EQ(absl::MakeInt128(31, 0),
+            absl::MakeInt128(20, 1) +
+                absl::MakeInt128(10, std::numeric_limits<uint64_t>::max()));
+}
+
+TEST(Int128, IncrementDecrementTest) {
+  absl::int128 value = 0;
+  EXPECT_EQ(0, value++);
+  EXPECT_EQ(1, value);
+  EXPECT_EQ(1, value--);
+  EXPECT_EQ(0, value);
+  EXPECT_EQ(-1, --value);
+  EXPECT_EQ(-1, value);
+  EXPECT_EQ(0, ++value);
+  EXPECT_EQ(0, value);
+}
+
+TEST(Int128, MultiplicationTest) {
+  // 1 bit x 1 bit, and negative combinations
+  for (int i = 0; i < 64; ++i) {
+    for (int j = 0; j < 127 - i; ++j) {
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      absl::int128 a = absl::int128(1) << i;
+      absl::int128 b = absl::int128(1) << j;
+      absl::int128 c = absl::int128(1) << (i + j);
+
+      EXPECT_EQ(c, a * b);
+      EXPECT_EQ(-c, -a * b);
+      EXPECT_EQ(-c, a * -b);
+      EXPECT_EQ(c, -a * -b);
+
+      EXPECT_EQ(c, absl::int128(a) *= b);
+      EXPECT_EQ(-c, absl::int128(-a) *= b);
+      EXPECT_EQ(-c, absl::int128(a) *= -b);
+      EXPECT_EQ(c, absl::int128(-a) *= -b);
+    }
+  }
+
+  // Pairs of random values that will not overflow signed 64-bit multiplication
+  std::pair<int64_t, int64_t> small_values[] = {
+      {0x5e61, 0xf29f79ca14b4},    // +, +
+      {0x3e033b, -0x612c0ee549},   // +, -
+      {-0x052ce7e8, 0x7c728f0f},   // -, +
+      {-0x3af7054626, -0xfb1e1d},  // -, -
+  };
+  for (const std::pair<int64_t, int64_t>& pair : small_values) {
+    SCOPED_TRACE(::testing::Message()
+                 << "pair = {" << pair.first << ", " << pair.second << '}');
+
+    EXPECT_EQ(absl::int128(pair.first * pair.second),
+              absl::int128(pair.first) * absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.first * pair.second),
+              absl::int128(pair.first) *= absl::int128(pair.second));
+
+    EXPECT_EQ(absl::MakeInt128(pair.first * pair.second, 0),
+              absl::MakeInt128(pair.first, 0) * absl::int128(pair.second));
+    EXPECT_EQ(absl::MakeInt128(pair.first * pair.second, 0),
+              absl::MakeInt128(pair.first, 0) *= absl::int128(pair.second));
+  }
+
+  // Pairs of positive random values that will not overflow 64-bit
+  // multiplication and can be left shifted by 32 without overflow
+  std::pair<int64_t, int64_t> small_values2[] = {
+      {0x1bb0a110, 0x31487671},
+      {0x4792784e, 0x28add7d7},
+      {0x7b66553a, 0x11dff8ef},
+  };
+  for (const std::pair<int64_t, int64_t>& pair : small_values2) {
+    SCOPED_TRACE(::testing::Message()
+                 << "pair = {" << pair.first << ", " << pair.second << '}');
+
+    absl::int128 a = absl::int128(pair.first << 32);
+    absl::int128 b = absl::int128(pair.second << 32);
+    absl::int128 c = absl::MakeInt128(pair.first * pair.second, 0);
+
+    EXPECT_EQ(c, a * b);
+    EXPECT_EQ(-c, -a * b);
+    EXPECT_EQ(-c, a * -b);
+    EXPECT_EQ(c, -a * -b);
+
+    EXPECT_EQ(c, absl::int128(a) *= b);
+    EXPECT_EQ(-c, absl::int128(-a) *= b);
+    EXPECT_EQ(-c, absl::int128(a) *= -b);
+    EXPECT_EQ(c, absl::int128(-a) *= -b);
+  }
+
+  // check 0, 1, and -1 behavior with large values
+  absl::int128 large_values[] = {
+      {absl::MakeInt128(0xd66f061af02d0408, 0x727d2846cb475b53)},
+      {absl::MakeInt128(0x27b8d5ed6104452d, 0x03f8a33b0ee1df4f)},
+      {-absl::MakeInt128(0x621b6626b9e8d042, 0x27311ac99df00938)},
+      {-absl::MakeInt128(0x34e0656f1e95fb60, 0x4281cfd731257a47)},
+  };
+  for (absl::int128 value : large_values) {
+    EXPECT_EQ(0, 0 * value);
+    EXPECT_EQ(0, value * 0);
+    EXPECT_EQ(0, absl::int128(0) *= value);
+    EXPECT_EQ(0, value *= 0);
+
+    EXPECT_EQ(value, 1 * value);
+    EXPECT_EQ(value, value * 1);
+    EXPECT_EQ(value, absl::int128(1) *= value);
+    EXPECT_EQ(value, value *= 1);
+
+    EXPECT_EQ(-value, -1 * value);
+    EXPECT_EQ(-value, value * -1);
+    EXPECT_EQ(-value, absl::int128(-1) *= value);
+    EXPECT_EQ(-value, value *= -1);
+  }
+
+  // Manually calculated random large value cases
+  EXPECT_EQ(absl::MakeInt128(0xcd0efd3442219bb, 0xde47c05bcd9df6e1),
+            absl::MakeInt128(0x7c6448, 0x3bc4285c47a9d253) * 0x1a6037537b);
+  EXPECT_EQ(-absl::MakeInt128(0x1f8f149850b1e5e6, 0x1e50d6b52d272c3e),
+            -absl::MakeInt128(0x23, 0x2e68a513ca1b8859) * 0xe5a434cd14866e);
+  EXPECT_EQ(-absl::MakeInt128(0x55cae732029d1fce, 0xca6474b6423263e4),
+            0xa9b98a8ddf66bc * -absl::MakeInt128(0x81, 0x672e58231e2469d7));
+  EXPECT_EQ(absl::MakeInt128(0x19c8b7620b507dc4, 0xfec042b71a5f29a4),
+            -0x3e39341147 * -absl::MakeInt128(0x6a14b2, 0x5ed34cca42327b3c));
+
+  EXPECT_EQ(absl::MakeInt128(0xcd0efd3442219bb, 0xde47c05bcd9df6e1),
+            absl::MakeInt128(0x7c6448, 0x3bc4285c47a9d253) *= 0x1a6037537b);
+  EXPECT_EQ(-absl::MakeInt128(0x1f8f149850b1e5e6, 0x1e50d6b52d272c3e),
+            -absl::MakeInt128(0x23, 0x2e68a513ca1b8859) *= 0xe5a434cd14866e);
+  EXPECT_EQ(-absl::MakeInt128(0x55cae732029d1fce, 0xca6474b6423263e4),
+            absl::int128(0xa9b98a8ddf66bc) *=
+            -absl::MakeInt128(0x81, 0x672e58231e2469d7));
+  EXPECT_EQ(absl::MakeInt128(0x19c8b7620b507dc4, 0xfec042b71a5f29a4),
+            absl::int128(-0x3e39341147) *=
+            -absl::MakeInt128(0x6a14b2, 0x5ed34cca42327b3c));
+}
+
+TEST(Int128, DivisionAndModuloTest) {
+  // Check against 64 bit division and modulo operators with a sample of
+  // randomly generated pairs.
+  std::pair<int64_t, int64_t> small_pairs[] = {
+      {0x15f2a64138, 0x67da05},    {0x5e56d194af43045f, 0xcf1543fb99},
+      {0x15e61ed052036a, -0xc8e6}, {0x88125a341e85, -0xd23fb77683},
+      {-0xc06e20, 0x5a},           {-0x4f100219aea3e85d, 0xdcc56cb4efe993},
+      {-0x168d629105, -0xa7},      {-0x7b44e92f03ab2375, -0x6516},
+  };
+  for (const std::pair<int64_t, int64_t>& pair : small_pairs) {
+    SCOPED_TRACE(::testing::Message()
+                 << "pair = {" << pair.first << ", " << pair.second << '}');
+
+    absl::int128 dividend = pair.first;
+    absl::int128 divisor = pair.second;
+    int64_t quotient = pair.first / pair.second;
+    int64_t remainder = pair.first % pair.second;
+
+    EXPECT_EQ(quotient, dividend / divisor);
+    EXPECT_EQ(quotient, absl::int128(dividend) /= divisor);
+    EXPECT_EQ(remainder, dividend % divisor);
+    EXPECT_EQ(remainder, absl::int128(dividend) %= divisor);
+  }
+
+  // Test behavior with 0, 1, and -1 with a sample of randomly generated large
+  // values.
+  absl::int128 values[] = {
+      absl::MakeInt128(0x63d26ee688a962b2, 0x9e1411abda5c1d70),
+      absl::MakeInt128(0x152f385159d6f986, 0xbf8d48ef63da395d),
+      -absl::MakeInt128(0x3098d7567030038c, 0x14e7a8a098dc2164),
+      -absl::MakeInt128(0x49a037aca35c809f, 0xa6a87525480ef330),
+  };
+  for (absl::int128 value : values) {
+    SCOPED_TRACE(::testing::Message() << "value = " << value);
+
+    EXPECT_EQ(0, 0 / value);
+    EXPECT_EQ(0, absl::int128(0) /= value);
+    EXPECT_EQ(0, 0 % value);
+    EXPECT_EQ(0, absl::int128(0) %= value);
+
+    EXPECT_EQ(value, value / 1);
+    EXPECT_EQ(value, absl::int128(value) /= 1);
+    EXPECT_EQ(0, value % 1);
+    EXPECT_EQ(0, absl::int128(value) %= 1);
+
+    EXPECT_EQ(-value, value / -1);
+    EXPECT_EQ(-value, absl::int128(value) /= -1);
+    EXPECT_EQ(0, value % -1);
+    EXPECT_EQ(0, absl::int128(value) %= -1);
+  }
+
+  // Min and max values
+  EXPECT_EQ(0, absl::Int128Max() / absl::Int128Min());
+  EXPECT_EQ(absl::Int128Max(), absl::Int128Max() % absl::Int128Min());
+  EXPECT_EQ(-1, absl::Int128Min() / absl::Int128Max());
+  EXPECT_EQ(-1, absl::Int128Min() % absl::Int128Max());
+
+  // Power of two division and modulo of random large dividends
+  absl::int128 positive_values[] = {
+      absl::MakeInt128(0x21e1a1cc69574620, 0xe7ac447fab2fc869),
+      absl::MakeInt128(0x32c2ff3ab89e66e8, 0x03379a613fd1ce74),
+      absl::MakeInt128(0x6f32ca786184dcaf, 0x046f9c9ecb3a9ce1),
+      absl::MakeInt128(0x1aeb469dd990e0ee, 0xda2740f243cd37eb),
+  };
+  for (absl::int128 value : positive_values) {
+    for (int i = 0; i < 127; ++i) {
+      SCOPED_TRACE(::testing::Message()
+                   << "value = " << value << "; i = " << i);
+      absl::int128 power_of_two = absl::int128(1) << i;
+
+      EXPECT_EQ(value >> i, value / power_of_two);
+      EXPECT_EQ(value >> i, absl::int128(value) /= power_of_two);
+      EXPECT_EQ(value & (power_of_two - 1), value % power_of_two);
+      EXPECT_EQ(value & (power_of_two - 1),
+                absl::int128(value) %= power_of_two);
+    }
+  }
+
+  // Manually calculated cases with random large dividends
+  struct DivisionModCase {
+    absl::int128 dividend;
+    absl::int128 divisor;
+    absl::int128 quotient;
+    absl::int128 remainder;
+  };
+  DivisionModCase manual_cases[] = {
+      {absl::MakeInt128(0x6ada48d489007966, 0x3c9c5c98150d5d69),
+       absl::MakeInt128(0x8bc308fb, 0x8cb9cc9a3b803344), 0xc3b87e08,
+       absl::MakeInt128(0x1b7db5e1, 0xd9eca34b7af04b49)},
+      {absl::MakeInt128(0xd6946511b5b, 0x4886c5c96546bf5f),
+       -absl::MakeInt128(0x263b, 0xfd516279efcfe2dc), -0x59cbabf0,
+       absl::MakeInt128(0x622, 0xf462909155651d1f)},
+      {-absl::MakeInt128(0x33db734f9e8d1399, 0x8447ac92482bca4d), 0x37495078240,
+       -absl::MakeInt128(0xf01f1, 0xbc0368bf9a77eae8), -0x21a508f404d},
+      {-absl::MakeInt128(0x13f837b409a07e7d, 0x7fc8e248a7d73560), -0x1b9f,
+       absl::MakeInt128(0xb9157556d724, 0xb14f635714d7563e), -0x1ade},
+  };
+  for (const DivisionModCase test_case : manual_cases) {
+    EXPECT_EQ(test_case.quotient, test_case.dividend / test_case.divisor);
+    EXPECT_EQ(test_case.quotient,
+              absl::int128(test_case.dividend) /= test_case.divisor);
+    EXPECT_EQ(test_case.remainder, test_case.dividend % test_case.divisor);
+    EXPECT_EQ(test_case.remainder,
+              absl::int128(test_case.dividend) %= test_case.divisor);
+  }
+}
+
+TEST(Int128, BitwiseLogicTest) {
+  EXPECT_EQ(absl::int128(-1), ~absl::int128(0));
+
+  absl::int128 values[]{
+      0, -1, 0xde400bee05c3ff6b, absl::MakeInt128(0x7f32178dd81d634a, 0),
+      absl::MakeInt128(0xaf539057055613a9, 0x7d104d7d946c2e4d)};
+  for (absl::int128 value : values) {
+    EXPECT_EQ(value, ~~value);
+
+    EXPECT_EQ(value, value | value);
+    EXPECT_EQ(value, value & value);
+    EXPECT_EQ(0, value ^ value);
+
+    EXPECT_EQ(value, absl::int128(value) |= value);
+    EXPECT_EQ(value, absl::int128(value) &= value);
+    EXPECT_EQ(0, absl::int128(value) ^= value);
+
+    EXPECT_EQ(value, value | 0);
+    EXPECT_EQ(0, value & 0);
+    EXPECT_EQ(value, value ^ 0);
+
+    EXPECT_EQ(absl::int128(-1), value | absl::int128(-1));
+    EXPECT_EQ(value, value & absl::int128(-1));
+    EXPECT_EQ(~value, value ^ absl::int128(-1));
+  }
+
+  // small sample of randomly generated int64_t's
+  std::pair<int64_t, int64_t> pairs64[]{
+      {0x7f86797f5e991af4, 0x1ee30494fb007c97},
+      {0x0b278282bacf01af, 0x58780e0a57a49e86},
+      {0x059f266ccb93a666, 0x3d5b731bae9286f5},
+      {0x63c0c4820f12108c, 0x58166713c12e1c3a},
+      {0x381488bb2ed2a66e, 0x2220a3eb76a3698c},
+      {0x2a0a0dfb81e06f21, 0x4b60585927f5523c},
+      {0x555b1c3a03698537, 0x25478cd19d8e53cb},
+      {0x4750f6f27d779225, 0x16397553c6ff05fc},
+  };
+  for (const std::pair<int64_t, int64_t>& pair : pairs64) {
+    SCOPED_TRACE(::testing::Message()
+                 << "pair = {" << pair.first << ", " << pair.second << '}');
+
+    EXPECT_EQ(absl::MakeInt128(~pair.first, ~pair.second),
+              ~absl::MakeInt128(pair.first, pair.second));
+
+    EXPECT_EQ(absl::int128(pair.first & pair.second),
+              absl::int128(pair.first) & absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.first | pair.second),
+              absl::int128(pair.first) | absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.first ^ pair.second),
+              absl::int128(pair.first) ^ absl::int128(pair.second));
+
+    EXPECT_EQ(absl::int128(pair.first & pair.second),
+              absl::int128(pair.first) &= absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.first | pair.second),
+              absl::int128(pair.first) |= absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.first ^ pair.second),
+              absl::int128(pair.first) ^= absl::int128(pair.second));
+
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first & pair.second, 0),
+        absl::MakeInt128(pair.first, 0) & absl::MakeInt128(pair.second, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first | pair.second, 0),
+        absl::MakeInt128(pair.first, 0) | absl::MakeInt128(pair.second, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first ^ pair.second, 0),
+        absl::MakeInt128(pair.first, 0) ^ absl::MakeInt128(pair.second, 0));
+
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first & pair.second, 0),
+        absl::MakeInt128(pair.first, 0) &= absl::MakeInt128(pair.second, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first | pair.second, 0),
+        absl::MakeInt128(pair.first, 0) |= absl::MakeInt128(pair.second, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first ^ pair.second, 0),
+        absl::MakeInt128(pair.first, 0) ^= absl::MakeInt128(pair.second, 0));
+  }
+}
+
+TEST(Int128, BitwiseShiftTest) {
+  for (int i = 0; i < 64; ++i) {
+    for (int j = 0; j <= i; ++j) {
+      // Left shift from j-th bit to i-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(uint64_t{1} << i, absl::int128(uint64_t{1} << j) << (i - j));
+      EXPECT_EQ(uint64_t{1} << i, absl::int128(uint64_t{1} << j) <<= (i - j));
+    }
+  }
+  for (int i = 0; i < 63; ++i) {
+    for (int j = 0; j < 64; ++j) {
+      // Left shift from j-th bit to (i + 64)-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::int128(uint64_t{1} << j) << (i + 64 - j));
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::int128(uint64_t{1} << j) <<= (i + 64 - j));
+    }
+    for (int j = 0; j <= i; ++j) {
+      // Left shift from (j + 64)-th bit to (i + 64)-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::MakeInt128(uint64_t{1} << j, 0) << (i - j));
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::MakeInt128(uint64_t{1} << j, 0) <<= (i - j));
+    }
+  }
+
+  for (int i = 0; i < 64; ++i) {
+    for (int j = i; j < 64; ++j) {
+      // Right shift from j-th bit to i-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(uint64_t{1} << i, absl::int128(uint64_t{1} << j) >> (j - i));
+      EXPECT_EQ(uint64_t{1} << i, absl::int128(uint64_t{1} << j) >>= (j - i));
+    }
+    for (int j = 0; j < 63; ++j) {
+      // Right shift from (j + 64)-th bit to i-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(uint64_t{1} << i,
+                absl::MakeInt128(uint64_t{1} << j, 0) >> (j + 64 - i));
+      EXPECT_EQ(uint64_t{1} << i,
+                absl::MakeInt128(uint64_t{1} << j, 0) >>= (j + 64 - i));
+    }
+  }
+  for (int i = 0; i < 63; ++i) {
+    for (int j = i; j < 63; ++j) {
+      // Right shift from (j + 64)-th bit to (i + 64)-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::MakeInt128(uint64_t{1} << j, 0) >> (j - i));
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::MakeInt128(uint64_t{1} << j, 0) >>= (j - i));
+    }
+  }
+}
+
+TEST(Int128, NumericLimitsTest) {
+  static_assert(std::numeric_limits<absl::int128>::is_specialized, "");
+  static_assert(std::numeric_limits<absl::int128>::is_signed, "");
+  static_assert(std::numeric_limits<absl::int128>::is_integer, "");
+  EXPECT_EQ(static_cast<int>(127 * std::log10(2)),
+            std::numeric_limits<absl::int128>::digits10);
+  EXPECT_EQ(absl::Int128Min(), std::numeric_limits<absl::int128>::min());
+  EXPECT_EQ(absl::Int128Min(), std::numeric_limits<absl::int128>::lowest());
+  EXPECT_EQ(absl::Int128Max(), std::numeric_limits<absl::int128>::max());
+}
+
 }  // namespace
diff --git a/absl/random/internal/platform.h b/absl/random/internal/platform.h
index a5a42cbb80ea..bbdb4e620816 100644
--- a/absl/random/internal/platform.h
+++ b/absl/random/internal/platform.h
@@ -162,7 +162,8 @@
 // iOS does not support dispatch, even on x86, since applications
 // should be bundled as fat binaries, with a different build tailored for
 // each specific supported platform/architecture.
-#if defined(__APPLE__) && (TARGET_OS_IPHONE || TARGET_OS_IPHONE_SIMULATOR)
+#if (defined(TARGET_OS_IPHONE) && TARGET_OS_IPHONE) || \
+    (defined(TARGET_OS_IPHONE_SIMULATOR) && TARGET_OS_IPHONE_SIMULATOR)
 #undef ABSL_RANDOM_INTERNAL_AES_DISPATCH
 #define ABSL_RANDOM_INTERNAL_AES_DISPATCH 0
 #endif
diff --git a/absl/strings/internal/str_format/arg.cc b/absl/strings/internal/str_format/arg.cc
index b40be8ff3824..b567a5c5bcf0 100644
--- a/absl/strings/internal/str_format/arg.cc
+++ b/absl/strings/internal/str_format/arg.cc
@@ -32,6 +32,10 @@ void ReducePadding(size_t n, size_t *capacity) {
 template <typename T>
 struct MakeUnsigned : std::make_unsigned<T> {};
 template <>
+struct MakeUnsigned<absl::int128> {
+  using type = absl::uint128;
+};
+template <>
 struct MakeUnsigned<absl::uint128> {
   using type = absl::uint128;
 };
@@ -39,6 +43,8 @@ struct MakeUnsigned<absl::uint128> {
 template <typename T>
 struct IsSigned : std::is_signed<T> {};
 template <>
+struct IsSigned<absl::int128> : std::true_type {};
+template <>
 struct IsSigned<absl::uint128> : std::false_type {};
 
 class ConvertedIntInfo {
@@ -363,6 +369,11 @@ IntegralConvertResult FormatConvertImpl(unsigned long long v,  // NOLINT
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
+IntegralConvertResult FormatConvertImpl(absl::int128 v,
+                                        const ConversionSpec conv,
+                                        FormatSinkImpl *sink) {
+  return {ConvertIntArg(v, conv, sink)};
+}
 IntegralConvertResult FormatConvertImpl(absl::uint128 v,
                                         const ConversionSpec conv,
                                         FormatSinkImpl *sink) {
@@ -372,6 +383,7 @@ IntegralConvertResult FormatConvertImpl(absl::uint128 v,
 ABSL_INTERNAL_FORMAT_DISPATCH_OVERLOADS_EXPAND_();
 
 
+
 }  // namespace str_format_internal
 
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/arg.h b/absl/strings/internal/str_format/arg.h
index 5cb3a14d6833..2119ca0036e5 100644
--- a/absl/strings/internal/str_format/arg.h
+++ b/absl/strings/internal/str_format/arg.h
@@ -144,6 +144,8 @@ IntegralConvertResult FormatConvertImpl(long long v,  // NOLINT
 IntegralConvertResult FormatConvertImpl(unsigned long long v,  // NOLINT
                                         ConversionSpec conv,
                                         FormatSinkImpl* sink);
+IntegralConvertResult FormatConvertImpl(int128 v, ConversionSpec conv,
+                                        FormatSinkImpl* sink);
 IntegralConvertResult FormatConvertImpl(uint128 v, ConversionSpec conv,
                                         FormatSinkImpl* sink);
 template <typename T, enable_if_t<std::is_same<T, bool>::value, int> = 0>
@@ -408,6 +410,7 @@ class FormatArgImpl {
                                              __VA_ARGS__);                     \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(unsigned long long, /* NOLINT */  \
                                              __VA_ARGS__);                     \
+  ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(int128, __VA_ARGS__);             \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(uint128, __VA_ARGS__);            \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(float, __VA_ARGS__);              \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(double, __VA_ARGS__);             \
@@ -418,6 +421,7 @@ class FormatArgImpl {
 
 ABSL_INTERNAL_FORMAT_DISPATCH_OVERLOADS_EXPAND_(extern);
 
+
 }  // namespace str_format_internal
 }  // namespace absl
 
diff --git a/absl/strings/internal/str_format/convert_test.cc b/absl/strings/internal/str_format/convert_test.cc
index ab8d53914119..5f198059fe5f 100644
--- a/absl/strings/internal/str_format/convert_test.cc
+++ b/absl/strings/internal/str_format/convert_test.cc
@@ -390,7 +390,6 @@ typedef ::testing::Types<
     AllIntTypes;
 INSTANTIATE_TYPED_TEST_CASE_P(TypedFormatConvertTestWithAllIntTypes,
                               TypedFormatConvertTest, AllIntTypes);
-
 TEST_F(FormatConvertTest, VectorBool) {
   // Make sure vector<bool>'s values behave as bools.
   std::vector<bool> v = {true, false};
@@ -402,6 +401,42 @@ TEST_F(FormatConvertTest, VectorBool) {
                             FormatArgImpl(cv[0]), FormatArgImpl(cv[1])})));
 }
 
+
+TEST_F(FormatConvertTest, Int128) {
+  absl::int128 positive = static_cast<absl::int128>(0x1234567890abcdef) * 1979;
+  absl::int128 negative = -positive;
+  absl::int128 max = absl::Int128Max(), min = absl::Int128Min();
+  const FormatArgImpl args[] = {FormatArgImpl(positive),
+                                FormatArgImpl(negative), FormatArgImpl(max),
+                                FormatArgImpl(min)};
+
+  struct Case {
+    const char* format;
+    const char* expected;
+  } cases[] = {
+      {"%1$d", "2595989796776606496405"},
+      {"%1$30d", "        2595989796776606496405"},
+      {"%1$-30d", "2595989796776606496405        "},
+      {"%1$u", "2595989796776606496405"},
+      {"%1$x", "8cba9876066020f695"},
+      {"%2$d", "-2595989796776606496405"},
+      {"%2$30d", "       -2595989796776606496405"},
+      {"%2$-30d", "-2595989796776606496405       "},
+      {"%2$u", "340282366920938460867384810655161715051"},
+      {"%2$x", "ffffffffffffff73456789f99fdf096b"},
+      {"%3$d", "170141183460469231731687303715884105727"},
+      {"%3$u", "170141183460469231731687303715884105727"},
+      {"%3$x", "7fffffffffffffffffffffffffffffff"},
+      {"%4$d", "-170141183460469231731687303715884105728"},
+      {"%4$x", "80000000000000000000000000000000"},
+  };
+
+  for (auto c : cases) {
+    UntypedFormatSpecImpl format(c.format);
+    EXPECT_EQ(c.expected, FormatPack(format, absl::MakeSpan(args)));
+  }
+}
+
 TEST_F(FormatConvertTest, Uint128) {
   absl::uint128 v = static_cast<absl::uint128>(0x1234567890abcdef) * 1979;
   absl::uint128 max = absl::Uint128Max();
diff --git a/absl/strings/string_view.h b/absl/strings/string_view.h
index 07075e80ec64..6a650874aa27 100644
--- a/absl/strings/string_view.h
+++ b/absl/strings/string_view.h
@@ -290,7 +290,8 @@ class string_view {
   constexpr const_reference at(size_type i) const {
     return ABSL_PREDICT_TRUE(i < size())
                ? ptr_[i]
-               : (base_internal::ThrowStdOutOfRange("absl::string_view::at"),
+               : ((void)base_internal::ThrowStdOutOfRange(
+                      "absl::string_view::at"),
                   ptr_[i]);
   }
 
@@ -511,7 +512,7 @@ class string_view {
       (std::numeric_limits<difference_type>::max)();
 
   static constexpr size_type CheckLengthInternal(size_type len) {
-    return ABSL_ASSERT(len <= kMaxSize), len;
+    return (void)ABSL_ASSERT(len <= kMaxSize), len;
   }
 
   static constexpr size_type StrlenInternal(const char* str) {
diff --git a/absl/time/internal/test_util.cc b/absl/time/internal/test_util.cc
index 59166a7c82dd..fbddbb733698 100644
--- a/absl/time/internal/test_util.cc
+++ b/absl/time/internal/test_util.cc
@@ -116,7 +116,10 @@ std::unique_ptr<cctz::ZoneInfoSource> TestFactory(
 
 }  // namespace
 
+#if !defined(__MINGW32__)
+// MinGW does not support the weak symbol extension mechanism.
 ZoneInfoSourceFactory zone_info_source_factory = TestFactory;
+#endif
 
 }  // namespace cctz_extension
 }  // namespace time_internal
diff --git a/absl/time/time.h b/absl/time/time.h
index 46ac26b3c0d2..be0648134efd 100644
--- a/absl/time/time.h
+++ b/absl/time/time.h
@@ -1412,8 +1412,7 @@ constexpr Duration FromInt64(int64_t v, std::ratio<3600>) {
 // IsValidRep64<T>(0) is true if the expression `int64_t{std::declval<T>()}` is
 // valid. That is, if a T can be assigned to an int64_t without narrowing.
 template <typename T>
-constexpr auto IsValidRep64(int)
-    -> decltype(int64_t{std::declval<T>()}, bool()) {
+constexpr auto IsValidRep64(int) -> decltype(int64_t{std::declval<T>()} == 0) {
   return true;
 }
 template <typename T>