Export of internal Abseil changes.

--
ed3a3431eee9e48e6553b0320e0308d2dde6725c by Derek Mauro <dmauro@google.com>:

Project import generated by Copybara.

PiperOrigin-RevId: 258631680
GitOrigin-RevId: ed3a3431eee9e48e6553b0320e0308d2dde6725c
Change-Id: I1d7ae86a79783842092d29504605ba039c369603

32 files changed, 1160 insertions, 649 deletions

diff --git a/absl/base/spinlock_test_common.cc b/absl/base/spinlock_test_common.cc
index e62b2eaeba7b..84fc4dac0be7 100644
--- a/absl/base/spinlock_test_common.cc
+++ b/absl/base/spinlock_test_common.cc
@@ -62,7 +62,6 @@ static SpinLock static_cooperative_spinlock(
 static SpinLock static_noncooperative_spinlock(
     base_internal::kLinkerInitialized, base_internal::SCHEDULE_KERNEL_ONLY);
 
-
 // Simple integer hash function based on the public domain lookup2 hash.
 // http://burtleburtle.net/bob/c/lookup2.c
 static uint32_t Hash32(uint32_t a, uint32_t c) {
@@ -194,6 +193,7 @@ TEST(SpinLock, WaitCyclesEncoding) {
 TEST(SpinLockWithThreads, StaticSpinLock) {
   ThreadedTest(&static_spinlock);
 }
+
 TEST(SpinLockWithThreads, StackSpinLock) {
   SpinLock spinlock;
   ThreadedTest(&spinlock);
diff --git a/absl/container/BUILD.bazel b/absl/container/BUILD.bazel
index 17d725c18ba9..b8cf17fc6072 100644
--- a/absl/container/BUILD.bazel
+++ b/absl/container/BUILD.bazel
@@ -44,7 +44,10 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":compressed_tuple",
+        ":test_instance_tracker",
         "//absl/memory",
+        "//absl/types:any",
+        "//absl/types:optional",
         "//absl/utility",
         "@com_google_googletest//:gtest_main",
     ],
diff --git a/absl/container/CMakeLists.txt b/absl/container/CMakeLists.txt
index 6df331e17165..5d7c75dd59cd 100644
--- a/absl/container/CMakeLists.txt
+++ b/absl/container/CMakeLists.txt
@@ -43,8 +43,11 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::any
     absl::compressed_tuple
     absl::memory
+    absl::optional
+    absl::test_instance_tracker
     absl::utility
     gmock_main
 )
diff --git a/absl/container/internal/common.h b/absl/container/internal/common.h
index b06e71131801..591d3ea11fa2 100644
--- a/absl/container/internal/common.h
+++ b/absl/container/internal/common.h
@@ -77,10 +77,18 @@ class node_handle_base {
  protected:
   friend struct CommonAccess;
 
-  node_handle_base(const allocator_type& a, slot_type* s) : alloc_(a) {
+  struct transfer_tag_t {};
+  node_handle_base(transfer_tag_t, const allocator_type& a, slot_type* s)
+      : alloc_(a) {
     PolicyTraits::transfer(alloc(), slot(), s);
   }
 
+  struct move_tag_t {};
+  node_handle_base(move_tag_t, const allocator_type& a, slot_type* s)
+      : alloc_(a) {
+    PolicyTraits::construct(alloc(), slot(), s);
+  }
+
   void destroy() {
     if (!empty()) {
       PolicyTraits::destroy(alloc(), slot());
@@ -121,7 +129,7 @@ class node_handle : public node_handle_base<PolicyTraits, Alloc> {
  private:
   friend struct CommonAccess;
 
-  node_handle(const Alloc& a, typename Base::slot_type* s) : Base(a, s) {}
+  using Base::Base;
 };
 
 // For maps.
@@ -148,7 +156,7 @@ class node_handle<Policy, PolicyTraits, Alloc,
  private:
   friend struct CommonAccess;
 
-  node_handle(const Alloc& a, typename Base::slot_type* s) : Base(a, s) {}
+  using Base::Base;
 };
 
 // Provide access to non-public node-handle functions.
@@ -164,8 +172,13 @@ struct CommonAccess {
   }
 
   template <typename T, typename... Args>
-  static T Make(Args&&... args) {
-    return T(std::forward<Args>(args)...);
+  static T Transfer(Args&&... args) {
+    return T(typename T::transfer_tag_t{}, std::forward<Args>(args)...);
+  }
+
+  template <typename T, typename... Args>
+  static T Move(Args&&... args) {
+    return T(typename T::move_tag_t{}, std::forward<Args>(args)...);
   }
 };
 
diff --git a/absl/container/internal/compressed_tuple.h b/absl/container/internal/compressed_tuple.h
index c29ab41eb9fd..7d08e370dec1 100644
--- a/absl/container/internal/compressed_tuple.h
+++ b/absl/container/internal/compressed_tuple.h
@@ -102,7 +102,9 @@ template <typename T, size_t I,
 struct Storage {
   T value;
   constexpr Storage() = default;
-  explicit constexpr Storage(T&& v) : value(absl::forward<T>(v)) {}
+  template <typename V>
+  explicit constexpr Storage(absl::in_place_t, V&& v)
+      : value(absl::forward<V>(v)) {}
   constexpr const T& get() const& { return value; }
   T& get() & { return value; }
   constexpr const T&& get() const&& { return absl::move(*this).value; }
@@ -112,7 +114,11 @@ struct Storage {
 template <typename T, size_t I>
 struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC Storage<T, I, true> : T {
   constexpr Storage() = default;
-  explicit constexpr Storage(T&& v) : T(absl::forward<T>(v)) {}
+
+  template <typename V>
+  explicit constexpr Storage(absl::in_place_t, V&& v)
+      : T(absl::forward<V>(v)) {}
+
   constexpr const T& get() const& { return *this; }
   T& get() & { return *this; }
   constexpr const T&& get() const&& { return absl::move(*this); }
@@ -132,8 +138,9 @@ struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTupleImpl<
     : uses_inheritance,
       Storage<Ts, std::integral_constant<size_t, I>::value>... {
   constexpr CompressedTupleImpl() = default;
-  explicit constexpr CompressedTupleImpl(Ts&&... args)
-      : Storage<Ts, I>(absl::forward<Ts>(args))... {}
+  template <typename... Vs>
+  explicit constexpr CompressedTupleImpl(absl::in_place_t, Vs&&... args)
+      : Storage<Ts, I>(absl::in_place, absl::forward<Vs>(args))... {}
   friend CompressedTuple<Ts...>;
 };
 
@@ -143,8 +150,9 @@ struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTupleImpl<
     // We use the dummy identity function as above...
     : Storage<Ts, std::integral_constant<size_t, I>::value, false>... {
   constexpr CompressedTupleImpl() = default;
-  explicit constexpr CompressedTupleImpl(Ts&&... args)
-      : Storage<Ts, I, false>(absl::forward<Ts>(args))... {}
+  template <typename... Vs>
+  explicit constexpr CompressedTupleImpl(absl::in_place_t, Vs&&... args)
+      : Storage<Ts, I, false>(absl::in_place, absl::forward<Vs>(args))... {}
   friend CompressedTuple<Ts...>;
 };
 
@@ -159,6 +167,11 @@ constexpr bool ShouldAnyUseBase() {
       Or({std::integral_constant<bool, ShouldUseBase<Ts>()>()...})){};
 }
 
+template <typename T, typename V>
+using TupleMoveConstructible = typename std::conditional<
+      std::is_reference<T>::value, std::is_convertible<V, T>,
+      std::is_constructible<T, V&&>>::type;
+
 }  // namespace internal_compressed_tuple
 
 // Helper class to perform the Empty Base Class Optimization.
@@ -192,9 +205,29 @@ class ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTuple
   using StorageT = internal_compressed_tuple::Storage<ElemT<I>, I>;
 
  public:
+  // There seems to be a bug in MSVC dealing in which using '=default' here will
+  // cause the compiler to ignore the body of other constructors. The work-
+  // around is to explicitly implement the default constructor.
+#if defined(_MSC_VER)
+  constexpr CompressedTuple() : CompressedTuple::CompressedTupleImpl() {}
+#else
   constexpr CompressedTuple() = default;
-  explicit constexpr CompressedTuple(Ts... base)
-      : CompressedTuple::CompressedTupleImpl(absl::forward<Ts>(base)...) {}
+#endif
+  explicit constexpr CompressedTuple(const Ts&... base)
+      : CompressedTuple::CompressedTupleImpl(absl::in_place, base...) {}
+
+  template <typename... Vs,
+            absl::enable_if_t<
+                absl::conjunction<
+                    // Ensure we are not hiding default copy/move constructors.
+                    absl::negation<std::is_same<void(CompressedTuple),
+                                                void(absl::decay_t<Vs>...)>>,
+                    internal_compressed_tuple::TupleMoveConstructible<
+                        Ts, Vs&&>...>::value,
+                bool> = true>
+  explicit constexpr CompressedTuple(Vs&&... base)
+      : CompressedTuple::CompressedTupleImpl(absl::in_place,
+                                             absl::forward<Vs>(base)...) {}
 
   template <int I>
   ElemT<I>& get() & {
diff --git a/absl/container/internal/compressed_tuple_test.cc b/absl/container/internal/compressed_tuple_test.cc
index 3b0ec4555abf..19af8f10aa6c 100644
--- a/absl/container/internal/compressed_tuple_test.cc
+++ b/absl/container/internal/compressed_tuple_test.cc
@@ -19,7 +19,10 @@
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/container/internal/test_instance_tracker.h"
 #include "absl/memory/memory.h"
+#include "absl/types/any.h"
+#include "absl/types/optional.h"
 #include "absl/utility/utility.h"
 
 // These are declared at global scope purely so that error messages
@@ -43,10 +46,14 @@ struct TwoValues {
   U value2;
 };
 
+
 namespace absl {
 namespace container_internal {
 namespace {
 
+using absl::test_internal::CopyableMovableInstance;
+using absl::test_internal::InstanceTracker;
+
 TEST(CompressedTupleTest, Sizeof) {
   EXPECT_EQ(sizeof(int), sizeof(CompressedTuple<int>));
   EXPECT_EQ(sizeof(int), sizeof(CompressedTuple<int, Empty<0>>));
@@ -62,6 +69,141 @@ TEST(CompressedTupleTest, Sizeof) {
             sizeof(CompressedTuple<int, Empty<0>, NotEmpty<double>, Empty<1>>));
 }
 
+TEST(CompressedTupleTest, OneMoveOnRValueConstructionTemp) {
+  InstanceTracker tracker;
+  CompressedTuple<CopyableMovableInstance> x1(CopyableMovableInstance(1));
+  EXPECT_EQ(tracker.instances(), 1);
+  EXPECT_EQ(tracker.copies(), 0);
+  EXPECT_LE(tracker.moves(), 1);
+  EXPECT_EQ(x1.get<0>().value(), 1);
+}
+
+TEST(CompressedTupleTest, OneMoveOnRValueConstructionMove) {
+  InstanceTracker tracker;
+
+  CopyableMovableInstance i1(1);
+  CompressedTuple<CopyableMovableInstance> x1(std::move(i1));
+  EXPECT_EQ(tracker.instances(), 2);
+  EXPECT_EQ(tracker.copies(), 0);
+  EXPECT_LE(tracker.moves(), 1);
+  EXPECT_EQ(x1.get<0>().value(), 1);
+}
+
+TEST(CompressedTupleTest, OneMoveOnRValueConstructionMixedTypes) {
+  InstanceTracker tracker;
+  CopyableMovableInstance i1(1);
+  CopyableMovableInstance i2(2);
+  Empty<0> empty;
+  CompressedTuple<CopyableMovableInstance, CopyableMovableInstance&, Empty<0>>
+      x1(std::move(i1), i2, empty);
+  EXPECT_EQ(x1.get<0>().value(), 1);
+  EXPECT_EQ(x1.get<1>().value(), 2);
+  EXPECT_EQ(tracker.copies(), 0);
+  EXPECT_EQ(tracker.moves(), 1);
+}
+
+struct IncompleteType;
+CompressedTuple<CopyableMovableInstance, IncompleteType&, Empty<0>>
+MakeWithIncomplete(CopyableMovableInstance i1,
+                   IncompleteType& t,  // NOLINT
+                   Empty<0> empty) {
+  return CompressedTuple<CopyableMovableInstance, IncompleteType&, Empty<0>>{
+      std::move(i1), t, empty};
+}
+
+struct IncompleteType {};
+TEST(CompressedTupleTest, OneMoveOnRValueConstructionWithIncompleteType) {
+  InstanceTracker tracker;
+  CopyableMovableInstance i1(1);
+  Empty<0> empty;
+  struct DerivedType : IncompleteType {int value = 0;};
+  DerivedType fd;
+  fd.value = 7;
+
+  CompressedTuple<CopyableMovableInstance, IncompleteType&, Empty<0>> x1 =
+      MakeWithIncomplete(std::move(i1), fd, empty);
+
+  EXPECT_EQ(x1.get<0>().value(), 1);
+  EXPECT_EQ(static_cast<DerivedType&>(x1.get<1>()).value, 7);
+
+  EXPECT_EQ(tracker.copies(), 0);
+  EXPECT_EQ(tracker.moves(), 2);
+}
+
+TEST(CompressedTupleTest,
+     OneMoveOnRValueConstructionMixedTypes_BraceInitPoisonPillExpected) {
+  InstanceTracker tracker;
+  CopyableMovableInstance i1(1);
+  CopyableMovableInstance i2(2);
+  CompressedTuple<CopyableMovableInstance, CopyableMovableInstance&, Empty<0>>
+      x1(std::move(i1), i2, {});  // NOLINT
+  EXPECT_EQ(x1.get<0>().value(), 1);
+  EXPECT_EQ(x1.get<1>().value(), 2);
+  EXPECT_EQ(tracker.instances(), 3);
+  // We are forced into the `const Ts&...` constructor (invoking copies)
+  // because we need it to deduce the type of `{}`.
+  // std::tuple also has this behavior.
+  // Note, this test is proof that this is expected behavior, but it is not
+  // _desired_ behavior.
+  EXPECT_EQ(tracker.copies(), 1);
+  EXPECT_EQ(tracker.moves(), 0);
+}
+
+TEST(CompressedTupleTest, OneCopyOnLValueConstruction) {
+  InstanceTracker tracker;
+  CopyableMovableInstance i1(1);
+
+  CompressedTuple<CopyableMovableInstance> x1(i1);
+  EXPECT_EQ(tracker.copies(), 1);
+  EXPECT_EQ(tracker.moves(), 0);
+
+  tracker.ResetCopiesMovesSwaps();
+
+  CopyableMovableInstance i2(2);
+  const CopyableMovableInstance& i2_ref = i2;
+  CompressedTuple<CopyableMovableInstance> x2(i2_ref);
+  EXPECT_EQ(tracker.copies(), 1);
+  EXPECT_EQ(tracker.moves(), 0);
+}
+
+TEST(CompressedTupleTest, OneMoveOnRValueAccess) {
+  InstanceTracker tracker;
+  CopyableMovableInstance i1(1);
+  CompressedTuple<CopyableMovableInstance> x(std::move(i1));
+  tracker.ResetCopiesMovesSwaps();
+
+  CopyableMovableInstance i2 = std::move(x).get<0>();
+  EXPECT_EQ(tracker.copies(), 0);
+  EXPECT_EQ(tracker.moves(), 1);
+}
+
+TEST(CompressedTupleTest, OneCopyOnLValueAccess) {
+  InstanceTracker tracker;
+
+  CompressedTuple<CopyableMovableInstance> x(CopyableMovableInstance(0));
+  EXPECT_EQ(tracker.copies(), 0);
+  EXPECT_EQ(tracker.moves(), 1);
+
+  CopyableMovableInstance t = x.get<0>();
+  EXPECT_EQ(tracker.copies(), 1);
+  EXPECT_EQ(tracker.moves(), 1);
+}
+
+TEST(CompressedTupleTest, ZeroCopyOnRefAccess) {
+  InstanceTracker tracker;
+
+  CompressedTuple<CopyableMovableInstance> x(CopyableMovableInstance(0));
+  EXPECT_EQ(tracker.copies(), 0);
+  EXPECT_EQ(tracker.moves(), 1);
+
+  CopyableMovableInstance& t1 = x.get<0>();
+  const CopyableMovableInstance& t2 = x.get<0>();
+  EXPECT_EQ(tracker.copies(), 0);
+  EXPECT_EQ(tracker.moves(), 1);
+  EXPECT_EQ(t1.value(), 0);
+  EXPECT_EQ(t2.value(), 0);
+}
+
 TEST(CompressedTupleTest, Access) {
   struct S {
     std::string x;
@@ -173,7 +315,40 @@ TEST(CompressedTupleTest, MoveOnlyElements) {
   EXPECT_EQ(*x1, 5);
 }
 
+TEST(CompressedTupleTest, MoveConstructionMoveOnlyElements) {
+  CompressedTuple<std::unique_ptr<std::string>> base(
+      absl::make_unique<std::string>("str"));
+  EXPECT_EQ(*base.get<0>(), "str");
+
+  CompressedTuple<std::unique_ptr<std::string>> copy(std::move(base));
+  EXPECT_EQ(*copy.get<0>(), "str");
+}
+
+TEST(CompressedTupleTest, AnyElements) {
+  any a(std::string("str"));
+  CompressedTuple<any, any&> x(any(5), a);
+  EXPECT_EQ(absl::any_cast<int>(x.get<0>()), 5);
+  EXPECT_EQ(absl::any_cast<std::string>(x.get<1>()), "str");
+
+  a = 0.5f;
+  EXPECT_EQ(absl::any_cast<float>(x.get<1>()), 0.5);
+
+  // Ensure copy construction work in the face of a type with a universal
+  // implicit constructor;
+  CompressedTuple<absl::any> c{}, d(c);  // NOLINT
+}
+
 TEST(CompressedTupleTest, Constexpr) {
+  struct NonTrivialStruct {
+    constexpr NonTrivialStruct() = default;
+    constexpr int value() const { return v; }
+    int v = 5;
+  };
+  struct TrivialStruct {
+    TrivialStruct() = default;
+    constexpr int value() const { return v; }
+    int v;
+  };
   constexpr CompressedTuple<int, double, CompressedTuple<int>, Empty<0>> x(
       7, 1.25, CompressedTuple<int>(5), {});
   constexpr int x0 = x.get<0>();
@@ -186,6 +361,32 @@ TEST(CompressedTupleTest, Constexpr) {
   EXPECT_EQ(x2, 5);
   EXPECT_EQ(x3, CallType::kConstRef);
 
+#if !defined(__GNUC__) || defined(__clang__) || __GNUC__ > 4
+  constexpr CompressedTuple<Empty<0>, TrivialStruct, int> trivial = {};
+  constexpr CallType trivial0 = trivial.get<0>().value();
+  constexpr int trivial1 = trivial.get<1>().value();
+  constexpr int trivial2 = trivial.get<2>();
+
+  EXPECT_EQ(trivial0, CallType::kConstRef);
+  EXPECT_EQ(trivial1, 0);
+  EXPECT_EQ(trivial2, 0);
+#endif
+
+  constexpr CompressedTuple<Empty<0>, NonTrivialStruct, absl::optional<int>>
+      non_trivial = {};
+  constexpr CallType non_trivial0 = non_trivial.get<0>().value();
+  constexpr int non_trivial1 = non_trivial.get<1>().value();
+  constexpr absl::optional<int> non_trivial2 = non_trivial.get<2>();
+
+  EXPECT_EQ(non_trivial0, CallType::kConstRef);
+  EXPECT_EQ(non_trivial1, 5);
+  EXPECT_EQ(non_trivial2, absl::nullopt);
+
+  static constexpr char data[] = "DEF";
+  constexpr CompressedTuple<const char*> z(data);
+  constexpr const char* z1 = z.get<0>();
+  EXPECT_EQ(std::string(z1), std::string(data));
+
 #if defined(__clang__)
   // An apparent bug in earlier versions of gcc claims these are ambiguous.
   constexpr int x2m = absl::move(x.get<2>()).get<0>();
diff --git a/absl/container/internal/hashtablez_sampler.cc b/absl/container/internal/hashtablez_sampler.cc
index 6667d3ad847a..d03dd82e7fd5 100644
--- a/absl/container/internal/hashtablez_sampler.cc
+++ b/absl/container/internal/hashtablez_sampler.cc
@@ -32,7 +32,7 @@ constexpr int HashtablezInfo::kMaxStackDepth;
 
 namespace {
 ABSL_CONST_INIT std::atomic<bool> g_hashtablez_enabled{
-   false
+    false
 };
 ABSL_CONST_INIT std::atomic<int32_t> g_hashtablez_sample_parameter{1 << 10};
 ABSL_CONST_INIT std::atomic<int32_t> g_hashtablez_max_samples{1 << 20};
diff --git a/absl/container/internal/raw_hash_set.h b/absl/container/internal/raw_hash_set.h
index c4889cd47506..656e9806d899 100644
--- a/absl/container/internal/raw_hash_set.h
+++ b/absl/container/internal/raw_hash_set.h
@@ -1182,7 +1182,7 @@ class raw_hash_set {
 
   node_type extract(const_iterator position) {
     auto node =
-        CommonAccess::Make<node_type>(alloc_ref(), position.inner_.slot_);
+        CommonAccess::Transfer<node_type>(alloc_ref(), position.inner_.slot_);
     erase_meta_only(position);
     return node;
   }
diff --git a/absl/container/internal/test_instance_tracker.h b/absl/container/internal/test_instance_tracker.h
index 3d4b2980ea8b..c4731dbe3d1d 100644
--- a/absl/container/internal/test_instance_tracker.h
+++ b/absl/container/internal/test_instance_tracker.h
@@ -23,7 +23,7 @@
 namespace absl {
 namespace test_internal {
 
-// A type that counts number of occurences of the type, the live occurrences of
+// A type that counts number of occurrences of the type, the live occurrences of
 // the type, as well as the number of copies, moves, swaps, and comparisons that
 // have occurred on the type. This is used as a base class for the copyable,
 // copyable+movable, and movable types below that are used in actual tests. Use
diff --git a/absl/flags/BUILD.bazel b/absl/flags/BUILD.bazel
index 82e6ffcfcef3..2fe61eaa8a8b 100644
--- a/absl/flags/BUILD.bazel
+++ b/absl/flags/BUILD.bazel
@@ -155,14 +155,12 @@ cc_library(
 )
 
 cc_library(
-    name = "usage",
+    name = "usage_internal",
     srcs = [
         "internal/usage.cc",
-        "usage.cc",
     ],
     hdrs = [
         "internal/usage.h",
-        "usage.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
@@ -180,6 +178,23 @@ cc_library(
 )
 
 cc_library(
+    name = "usage",
+    srcs = [
+        "usage.cc",
+    ],
+    hdrs = [
+        "usage.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":usage_internal",
+        "//absl/strings",
+        "//absl/synchronization",
+    ],
+)
+
+cc_library(
     name = "parse",
     srcs = ["parse.cc"],
     hdrs = [
@@ -195,6 +210,7 @@ cc_library(
         ":internal",
         ":registry",
         ":usage",
+        ":usage_internal",
         "//absl/strings",
         "//absl/synchronization",
     ],
@@ -360,6 +376,7 @@ cc_test(
         ":internal",
         ":parse",
         ":usage",
+        ":usage_internal",
         "//absl/memory",
         "//absl/strings",
         "@com_google_googletest//:gtest",
diff --git a/absl/flags/CMakeLists.txt b/absl/flags/CMakeLists.txt
index 9c936eccc8e3..fa1d4e177b3e 100644
--- a/absl/flags/CMakeLists.txt
+++ b/absl/flags/CMakeLists.txt
@@ -141,13 +141,11 @@ absl_cc_library(
 # Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
-    flags_usage
+    flags_usage_internal
   SRCS
     "internal/usage.cc"
-    "usage.cc"
   HDRS
     "internal/usage.h"
-    "usage.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
@@ -163,6 +161,23 @@ absl_cc_library(
 
 absl_cc_library(
   NAME
+    flags_usage
+  SRCS
+    "usage.cc"
+  HDRS
+    "usage.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::flags_usage_internal
+    absl::strings
+    absl::synchronization
+)
+
+absl_cc_library(
+  NAME
     flags_parse
   SRCS
     "parse.cc"
diff --git a/absl/flags/flag.cc b/absl/flags/flag.cc
index ba002edd24f1..f16cc75e3843 100644
--- a/absl/flags/flag.cc
+++ b/absl/flags/flag.cc
@@ -24,24 +24,16 @@ namespace absl {
 // so in debug builds we always use the slower implementation, which always
 // validates the type.
 #ifndef NDEBUG
-#define ABSL_FLAGS_ATOMIC_GET(T)                              \
-  T GetFlag(const absl::Flag<T>& flag) {                      \
-    T result;                                                 \
-    flag.internal.Read(&result, &flags_internal::FlagOps<T>); \
-    return result;                                            \
-  }
+#define ABSL_FLAGS_ATOMIC_GET(T) \
+  T GetFlag(const absl::Flag<T>& flag) { return flag.Get(); }
 #else
-#define ABSL_FLAGS_ATOMIC_GET(T)                                            \
-  T GetFlag(const absl::Flag<T>& flag) {                                    \
-    const int64_t r = flag.internal.atomic.load(std::memory_order_acquire); \
-    if (r != flags_internal::CommandLineFlag::kAtomicInit) {                \
-      T t;                                                                  \
-      memcpy(&t, &r, sizeof(T));                                            \
-      return t;                                                             \
-    }                                                                       \
-    T result;                                                               \
-    flag.internal.Read(&result, &flags_internal::FlagOps<T>);               \
-    return result;                                                          \
+#define ABSL_FLAGS_ATOMIC_GET(T)         \
+  T GetFlag(const absl::Flag<T>& flag) { \
+    T result;                            \
+    if (flag.AtomicGet(&result)) {       \
+      return result;                     \
+    }                                    \
+    return flag.Get();                   \
   }
 #endif
 
diff --git a/absl/flags/flag.h b/absl/flags/flag.h
index 193755e56aa6..c70023aba8c7 100644
--- a/absl/flags/flag.h
+++ b/absl/flags/flag.h
@@ -91,30 +91,7 @@ T GetFlag(const absl::Flag<T>& flag) {
   ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(ABSL_FLAGS_INTERNAL_LOCK_FREE_VALIDATE)
 #undef ABSL_FLAGS_INTERNAL_LOCK_FREE_VALIDATE
 
-  // Implementation notes:
-  //
-  // We are wrapping a union around the value of `T` to serve three purposes:
-  //
-  //  1. `U.value` has correct size and alignment for a value of type `T`
-  //  2. The `U.value` constructor is not invoked since U's constructor does not
-  //     do it explicitly.
-  //  3. The `U.value` destructor is invoked since U's destructor does it
-  //     explicitly. This makes `U` a kind of RAII wrapper around non default
-  //     constructible value of T, which is destructed when we leave the scope.
-  //     We do need to destroy U.value, which is constructed by
-  //     CommandLineFlag::Read even though we left it in a moved-from state
-  //     after std::move.
-  //
-  // All of this serves to avoid requiring `T` being default constructible.
-  union U {
-    T value;
-    U() {}
-    ~U() { value.~T(); }
-  };
-  U u;
-
-  flag.internal.Read(&u.value, &flags_internal::FlagOps<T>);
-  return std::move(u.value);
+  return flag.Get();
 }
 
 // Overload for `GetFlag()` for types that support lock-free reads.
@@ -132,7 +109,7 @@ ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT)
 // but especially within performance-critical code.
 template <typename T>
 void SetFlag(absl::Flag<T>* flag, const T& v) {
-  flag->internal.Write(&v, &flags_internal::FlagOps<T>);
+  flag->Set(v);
 }
 
 // Overload of `SetFlag()` to allow callers to pass in a value that is
@@ -141,7 +118,7 @@ void SetFlag(absl::Flag<T>* flag, const T& v) {
 template <typename T, typename V>
 void SetFlag(absl::Flag<T>* flag, const V& v) {
   T value(v);
-  SetFlag<T>(flag, value);
+  flag->Set(value);
 }
 
 }  // namespace absl
@@ -239,17 +216,17 @@ void SetFlag(absl::Flag<T>* flag, const V& v) {
 // Note: Name of registrar object is not arbitrary. It is used to "grab"
 // global name for FLAGS_no<flag_name> symbol, thus preventing the possibility
 // of defining two flags with names foo and nofoo.
-#define ABSL_FLAG_IMPL(Type, name, default_value, help)                   \
-  namespace absl {}                                                       \
-  ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value)       \
-  ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help)                         \
-  absl::Flag<Type> FLAGS_##name(                                          \
-      ABSL_FLAG_IMPL_FLAGNAME(#name),                                     \
-      &AbslFlagsWrapHelp##name,                                           \
-      ABSL_FLAG_IMPL_FILENAME(),                                          \
-      &absl::flags_internal::FlagMarshallingOps<Type>,                    \
-      &AbslFlagsInitFlag##name);                                          \
-  extern bool FLAGS_no##name;                                             \
+#define ABSL_FLAG_IMPL(Type, name, default_value, help)              \
+  namespace absl {}                                                  \
+  ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value)  \
+  ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help)                    \
+  absl::Flag<Type> FLAGS_##name(                                     \
+      ABSL_FLAG_IMPL_FLAGNAME(#name),                                \
+      &AbslFlagsWrapHelp##name,                                      \
+      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)
 
 // ABSL_RETIRED_FLAG
diff --git a/absl/flags/internal/commandlineflag.cc b/absl/flags/internal/commandlineflag.cc
index 744447219c71..98a46695304b 100644
--- a/absl/flags/internal/commandlineflag.cc
+++ b/absl/flags/internal/commandlineflag.cc
@@ -37,44 +37,6 @@ const char kStrippedFlagHelp[] = "\001\002\003\004 (unknown) \004\003\002\001";
 
 namespace {
 
-void StoreAtomic(CommandLineFlag* flag, const void* data, size_t size) {
-  int64_t t = 0;
-  assert(size <= sizeof(int64_t));
-  memcpy(&t, data, size);
-  flag->atomic.store(t, std::memory_order_release);
-}
-
-// If the flag has a mutation callback this function invokes it. While the
-// callback is being invoked the primary flag's mutex is unlocked and it is
-// re-locked back after call to callback is completed. Callback invocation is
-// guarded by flag's secondary mutex instead which prevents concurrent callback
-// invocation. Note that it is possible for other thread to grab the primary
-// lock and update flag's value at any time during the callback invocation.
-// This is by design. Callback can get a value of the flag if necessary, but it
-// might be different from the value initiated the callback and it also can be
-// different by the time the callback invocation is completed.
-// Requires that *primary_lock be held in exclusive mode; it may be released
-// and reacquired by the implementation.
-void InvokeCallback(CommandLineFlag* flag, absl::Mutex* primary_lock)
-    EXCLUSIVE_LOCKS_REQUIRED(primary_lock) {
-  if (!flag->callback) return;
-
-  // The callback lock is guaranteed initialized, because *primary_lock exists.
-  absl::Mutex* callback_mu = &flag->locks->callback_mu;
-
-  // When executing the callback we need the primary flag's mutex to be unlocked
-  // so that callback can retrieve the flag's value.
-  primary_lock->Unlock();
-
-  {
-    absl::MutexLock lock(callback_mu);
-
-    flag->callback();
-  }
-
-  primary_lock->Lock();
-}
-
 // Currently we only validate flag values for user-defined flag types.
 bool ShouldValidateFlagValue(const CommandLineFlag& flag) {
 #define DONT_VALIDATE(T) \
@@ -89,145 +51,72 @@ bool ShouldValidateFlagValue(const CommandLineFlag& flag) {
 
 }  // namespace
 
-// Update any copy of the flag value that is stored in an atomic word.
-// In addition if flag has a mutation callback this function invokes it.
-void UpdateCopy(CommandLineFlag* flag, absl::Mutex* primary_lock)
-    EXCLUSIVE_LOCKS_REQUIRED(primary_lock) {
-#define STORE_ATOMIC(T)                      \
-  else if (flag->IsOfType<T>()) {            \
-    StoreAtomic(flag, flag->cur, sizeof(T)); \
-  }
-
-  if (false) {
-  }
-  ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(STORE_ATOMIC)
-#undef STORE_ATOMIC
+absl::Mutex* InitFlag(CommandLineFlag* flag) {
+  ABSL_CONST_INIT static absl::Mutex init_lock(absl::kConstInit);
+  absl::Mutex* mu;
 
-  InvokeCallback(flag, primary_lock);
-}
+  {
+    absl::MutexLock lock(&init_lock);
 
-// Ensure that the lazily initialized fields of *flag have been initialized,
-// and return &flag->locks->primary_mu.
-absl::Mutex* InitFlagIfNecessary(CommandLineFlag* flag)
-    LOCK_RETURNED(flag->locks->primary_mu) {
-  absl::Mutex* mu;
-  if (!flag->inited.load(std::memory_order_acquire)) {
-    // Need to initialize lazily initialized fields.
-    ABSL_CONST_INIT static absl::Mutex init_lock(absl::kConstInit);
-    init_lock.Lock();
     if (flag->locks == nullptr) {  // Must initialize Mutexes for this flag.
       flag->locks = new flags_internal::CommandLineFlagLocks;
     }
+
     mu = &flag->locks->primary_mu;
-    init_lock.Unlock();
-    mu->Lock();
-    if (!flag->retired &&
-        flag->def == nullptr) {  // Need to initialize def and cur fields.
+  }
+
+  {
+    absl::MutexLock lock(mu);
+
+    if (!flag->retired && flag->def == nullptr) {
+      // Need to initialize def and cur fields.
       flag->def = (*flag->make_init_value)();
       flag->cur = Clone(flag->op, flag->def);
-      UpdateCopy(flag, mu);
+      UpdateCopy(flag);
+      flag->inited.store(true, std::memory_order_release);
+      flag->InvokeCallback();
     }
-    mu->Unlock();
-    flag->inited.store(true, std::memory_order_release);
-  } else {  // All fields initialized; flag->locks is therefore safe to read.
-    mu = &flag->locks->primary_mu;
   }
+
+  flag->inited.store(true, std::memory_order_release);
   return mu;
 }
 
-// Return true iff flag value was changed via direct-access.
-bool ChangedDirectly(CommandLineFlag* flag, const void* a, const void* b) {
-  if (!flag->IsAbseilFlag()) {
-    // Need to compare values for direct-access flags.
-#define CHANGED_FOR_TYPE(T)                                                  \
-  if (flag->IsOfType<T>()) {                                                 \
-    return *reinterpret_cast<const T*>(a) != *reinterpret_cast<const T*>(b); \
+// Ensure that the lazily initialized fields of *flag have been initialized,
+// and return &flag->locks->primary_mu.
+absl::Mutex* CommandLineFlag::InitFlagIfNecessary() const
+    LOCK_RETURNED(locks->primary_mu) {
+  if (!this->inited.load(std::memory_order_acquire)) {
+    return InitFlag(const_cast<CommandLineFlag*>(this));
   }
 
-    CHANGED_FOR_TYPE(bool);
-    CHANGED_FOR_TYPE(int32_t);
-    CHANGED_FOR_TYPE(int64_t);
-    CHANGED_FOR_TYPE(uint64_t);
-    CHANGED_FOR_TYPE(double);
-    CHANGED_FOR_TYPE(std::string);
-#undef CHANGED_FOR_TYPE
-  }
-  return false;
+  // All fields initialized; this->locks is therefore safe to read.
+  return &this->locks->primary_mu;
 }
 
-// Direct-access flags can be modified without going through the
-// flag API. Detect such changes and updated the modified bit.
-void UpdateModifiedBit(CommandLineFlag* flag) {
-  if (!flag->IsAbseilFlag()) {
-    absl::MutexLock l(InitFlagIfNecessary(flag));
-    if (!flag->modified && ChangedDirectly(flag, flag->cur, flag->def)) {
-      flag->modified = true;
-    }
+void CommandLineFlag::Destroy() const {
+  // Values are heap allocated for retired and Abseil Flags.
+  if (IsRetired() || IsAbseilFlag()) {
+    if (this->cur) Delete(this->op, this->cur);
+    if (this->def) Delete(this->op, this->def);
   }
-}
 
-bool Validate(CommandLineFlag*, const void*) {
-  return true;
+  delete this->locks;
 }
 
-std::string HelpText::GetHelpText() const {
-  if (help_function_) return help_function_();
-  if (help_message_) return help_message_;
-
-  return {};
+bool CommandLineFlag::IsModified() const {
+  absl::MutexLock l(InitFlagIfNecessary());
+  return modified;
 }
 
-const int64_t CommandLineFlag::kAtomicInit;
-
-void CommandLineFlag::Read(void* dst,
-                           const flags_internal::FlagOpFn dst_op) const {
-  absl::ReaderMutexLock l(
-      InitFlagIfNecessary(const_cast<CommandLineFlag*>(this)));
-
-  // `dst_op` is the unmarshaling operation corresponding to the declaration
-  // visibile at the call site. `op` is the Flag's defined unmarshalling
-  // operation. They must match for this operation to be well-defined.
-  if (ABSL_PREDICT_FALSE(dst_op != op)) {
-    ABSL_INTERNAL_LOG(
-        ERROR,
-        absl::StrCat("Flag '", name,
-                     "' is defined as one type and declared as another"));
-  }
-  CopyConstruct(op, cur, dst);
+void CommandLineFlag::SetModified(bool is_modified) {
+  absl::MutexLock l(InitFlagIfNecessary());
+  modified = is_modified;
 }
 
-void CommandLineFlag::Write(const void* src,
-                            const flags_internal::FlagOpFn src_op) {
-  absl::Mutex* mu = InitFlagIfNecessary(this);
-  absl::MutexLock l(mu);
-
-  // `src_op` is the marshalling operation corresponding to the declaration
-  // visible at the call site. `op` is the Flag's defined marshalling operation.
-  // They must match for this operation to be well-defined.
-  if (ABSL_PREDICT_FALSE(src_op != op)) {
-    ABSL_INTERNAL_LOG(
-        ERROR,
-        absl::StrCat("Flag '", name,
-                     "' is defined as one type and declared as another"));
-  }
-
-  if (ShouldValidateFlagValue(*this)) {
-    void* obj = Clone(op, src);
-    std::string ignored_error;
-    std::string src_as_str = Unparse(marshalling_op, src);
-    if (!Parse(marshalling_op, src_as_str, obj, &ignored_error) ||
-        !Validate(this, obj)) {
-      ABSL_INTERNAL_LOG(ERROR, absl::StrCat("Attempt to set flag '", name,
-                                            "' to invalid value ", src_as_str));
-    }
-    Delete(op, obj);
-  }
-
-  modified = true;
-  counter++;
-  Copy(op, src, cur);
-
-  UpdateCopy(this, mu);
+bool CommandLineFlag::IsSpecifiedOnCommandLine() const {
+  absl::MutexLock l(InitFlagIfNecessary());
+  return on_command_line;
 }
 
 absl::string_view CommandLineFlag::Typename() const {
@@ -259,21 +148,96 @@ std::string CommandLineFlag::Filename() const {
 }
 
 std::string CommandLineFlag::DefaultValue() const {
+  absl::MutexLock l(InitFlagIfNecessary());
+
   return Unparse(this->marshalling_op, this->def);
 }
 
 std::string CommandLineFlag::CurrentValue() const {
+  absl::MutexLock l(InitFlagIfNecessary());
+
   return Unparse(this->marshalling_op, this->cur);
 }
 
+bool CommandLineFlag::HasValidatorFn() const {
+  absl::MutexLock l(InitFlagIfNecessary());
+
+  return this->validator != nullptr;
+}
+
+bool CommandLineFlag::SetValidatorFn(FlagValidator fn) {
+  absl::MutexLock l(InitFlagIfNecessary());
+
+  // ok to register the same function over and over again
+  if (fn == this->validator) return true;
+
+  // Can't set validator to a different function, unless reset first.
+  if (fn != nullptr && this->validator != nullptr) {
+    ABSL_INTERNAL_LOG(
+        WARNING, absl::StrCat("Ignoring SetValidatorFn() for flag '", Name(),
+                              "': validate-fn already registered"));
+
+    return false;
+  }
+
+  this->validator = fn;
+  return true;
+}
+
+bool CommandLineFlag::InvokeValidator(const void* value) const
+    EXCLUSIVE_LOCKS_REQUIRED(this->locks->primary_mu) {
+  if (!this->validator) {
+    return true;
+  }
+
+  (void)value;
+
+  ABSL_INTERNAL_LOG(
+      FATAL,
+      absl::StrCat("Flag '", Name(),
+                   "' of encapsulated type should not have a validator"));
+
+  return false;
+}
+
 void CommandLineFlag::SetCallback(
     const flags_internal::FlagCallback mutation_callback) {
-  absl::Mutex* mu = InitFlagIfNecessary(this);
-  absl::MutexLock l(mu);
+  absl::MutexLock l(InitFlagIfNecessary());
 
   callback = mutation_callback;
 
-  InvokeCallback(this, mu);
+  InvokeCallback();
+}
+
+// If the flag has a mutation callback this function invokes it. While the
+// callback is being invoked the primary flag's mutex is unlocked and it is
+// re-locked back after call to callback is completed. Callback invocation is
+// guarded by flag's secondary mutex instead which prevents concurrent callback
+// invocation. Note that it is possible for other thread to grab the primary
+// lock and update flag's value at any time during the callback invocation.
+// This is by design. Callback can get a value of the flag if necessary, but it
+// might be different from the value initiated the callback and it also can be
+// different by the time the callback invocation is completed.
+// Requires that *primary_lock be held in exclusive mode; it may be released
+// and reacquired by the implementation.
+void CommandLineFlag::InvokeCallback()
+    EXCLUSIVE_LOCKS_REQUIRED(this->locks->primary_mu) {
+  if (!this->callback) return;
+
+  // The callback lock is guaranteed initialized, because *locks->primary_mu
+  // exists.
+  absl::Mutex* callback_mu = &this->locks->callback_mu;
+
+  // When executing the callback we need the primary flag's mutex to be unlocked
+  // so that callback can retrieve the flag's value.
+  this->locks->primary_mu.Unlock();
+
+  {
+    absl::MutexLock lock(callback_mu);
+    this->callback();
+  }
+
+  this->locks->primary_mu.Lock();
 }
 
 // Attempts to parse supplied `value` string using parsing routine in the `flag`
@@ -282,8 +246,9 @@ void CommandLineFlag::SetCallback(
 // parsed value in 'dst' assuming it is a pointer to the flag's value type. In
 // case if any error is encountered in either step, the error message is stored
 // in 'err'
-static bool TryParseLocked(CommandLineFlag* flag, void* dst,
-                           absl::string_view value, std::string* err) {
+bool TryParseLocked(CommandLineFlag* flag, void* dst, absl::string_view value,
+                    std::string* err)
+    EXCLUSIVE_LOCKS_REQUIRED(flag->locks->primary_mu) {
   void* tentative_value = Clone(flag->op, flag->def);
   std::string parse_err;
   if (!Parse(flag->marshalling_op, value, tentative_value, &parse_err)) {
@@ -297,7 +262,7 @@ static bool TryParseLocked(CommandLineFlag* flag, void* dst,
     return false;
   }
 
-  if (!Validate(flag, tentative_value)) {
+  if (!flag->InvokeValidator(tentative_value)) {
     *err = absl::StrCat("Failed validation of new value '",
                         Unparse(flag->marshalling_op, tentative_value),
                         "' for flag '", flag->Name(), "'");
@@ -324,17 +289,23 @@ bool CommandLineFlag::SetFromString(absl::string_view value,
                                     ValueSource source, std::string* err) {
   if (IsRetired()) return false;
 
-  UpdateModifiedBit(this);
+  absl::MutexLock l(InitFlagIfNecessary());
 
-  absl::Mutex* mu = InitFlagIfNecessary(this);
-  absl::MutexLock l(mu);
+  // Direct-access flags can be modified without going through the
+  // flag API. Detect such changes and update the flag->modified bit.
+  if (!IsAbseilFlag()) {
+    if (!this->modified && ChangedDirectly(this, this->cur, this->def)) {
+      this->modified = true;
+    }
+  }
 
   switch (set_mode) {
     case SET_FLAGS_VALUE: {
       // set or modify the flag's value
       if (!TryParseLocked(this, this->cur, value, err)) return false;
       this->modified = true;
-      UpdateCopy(this, mu);
+      UpdateCopy(this);
+      InvokeCallback();
 
       if (source == kCommandLine) {
         this->on_command_line = true;
@@ -346,7 +317,8 @@ bool CommandLineFlag::SetFromString(absl::string_view value,
       if (!this->modified) {
         if (!TryParseLocked(this, this->cur, value, err)) return false;
         this->modified = true;
-        UpdateCopy(this, mu);
+        UpdateCopy(this);
+        InvokeCallback();
       } else {
         // TODO(rogeeff): review and fix this semantic. Currently we do not fail
         // in this case if flag is modified. This is misleading since the flag's
@@ -365,7 +337,8 @@ bool CommandLineFlag::SetFromString(absl::string_view value,
       if (!this->modified) {
         // Need to set both defvalue *and* current, in this case
         Copy(this->op, this->def, this->cur);
-        UpdateCopy(this, mu);
+        UpdateCopy(this);
+        InvokeCallback();
       }
       break;
     }
@@ -379,5 +352,143 @@ bool CommandLineFlag::SetFromString(absl::string_view value,
   return true;
 }
 
+void CommandLineFlag::StoreAtomic(size_t size) {
+  int64_t t = 0;
+  assert(size <= sizeof(int64_t));
+  memcpy(&t, this->cur, size);
+  this->atomic.store(t, std::memory_order_release);
+}
+
+void CommandLineFlag::CheckDefaultValueParsingRoundtrip() const {
+  std::string v = DefaultValue();
+
+  absl::MutexLock lock(InitFlagIfNecessary());
+
+  void* dst = Clone(this->op, this->def);
+  std::string error;
+  if (!flags_internal::Parse(this->marshalling_op, v, dst, &error)) {
+    ABSL_INTERNAL_LOG(
+        FATAL,
+        absl::StrCat("Flag ", Name(), " (from ", Filename(),
+                     "): std::string form of default value '", v,
+                     "' could not be parsed; error=", error));
+  }
+
+  // We do not compare dst to def since parsing/unparsing may make
+  // small changes, e.g., precision loss for floating point types.
+  Delete(this->op, dst);
+}
+
+bool CommandLineFlag::ValidateDefaultValue() const {
+  absl::MutexLock lock(InitFlagIfNecessary());
+  return InvokeValidator(this->def);
+}
+
+bool CommandLineFlag::ValidateInputValue(absl::string_view value) const {
+  absl::MutexLock l(InitFlagIfNecessary());  // protect default value access
+
+  void* obj = Clone(this->op, this->def);
+  std::string ignored_error;
+  const bool result =
+      flags_internal::Parse(this->marshalling_op, value, obj, &ignored_error) &&
+      InvokeValidator(obj);
+  Delete(this->op, obj);
+  return result;
+}
+
+const int64_t CommandLineFlag::kAtomicInit;
+
+void CommandLineFlag::Read(void* dst,
+                           const flags_internal::FlagOpFn dst_op) const {
+  absl::ReaderMutexLock l(InitFlagIfNecessary());
+
+  // `dst_op` is the unmarshaling operation corresponding to the declaration
+  // visibile at the call site. `op` is the Flag's defined unmarshalling
+  // operation. They must match for this operation to be well-defined.
+  if (ABSL_PREDICT_FALSE(dst_op != op)) {
+    ABSL_INTERNAL_LOG(
+        ERROR,
+        absl::StrCat("Flag '", name,
+                     "' is defined as one type and declared as another"));
+  }
+  CopyConstruct(op, cur, dst);
+}
+
+void CommandLineFlag::Write(const void* src,
+                            const flags_internal::FlagOpFn src_op) {
+  absl::MutexLock l(InitFlagIfNecessary());
+
+  // `src_op` is the marshalling operation corresponding to the declaration
+  // visible at the call site. `op` is the Flag's defined marshalling operation.
+  // They must match for this operation to be well-defined.
+  if (ABSL_PREDICT_FALSE(src_op != op)) {
+    ABSL_INTERNAL_LOG(
+        ERROR,
+        absl::StrCat("Flag '", name,
+                     "' is defined as one type and declared as another"));
+  }
+
+  if (ShouldValidateFlagValue(*this)) {
+    void* obj = Clone(op, src);
+    std::string ignored_error;
+    std::string src_as_str = Unparse(marshalling_op, src);
+    if (!Parse(marshalling_op, src_as_str, obj, &ignored_error) ||
+        !InvokeValidator(obj)) {
+      ABSL_INTERNAL_LOG(ERROR, absl::StrCat("Attempt to set flag '", name,
+                                            "' to invalid value ", src_as_str));
+    }
+    Delete(op, obj);
+  }
+
+  modified = true;
+  counter++;
+  Copy(op, src, cur);
+
+  UpdateCopy(this);
+  InvokeCallback();
+}
+
+std::string HelpText::GetHelpText() const {
+  if (help_function_) return help_function_();
+  if (help_message_) return help_message_;
+
+  return {};
+}
+
+// Update any copy of the flag value that is stored in an atomic word.
+// In addition if flag has a mutation callback this function invokes it.
+void UpdateCopy(CommandLineFlag* flag) {
+#define STORE_ATOMIC(T)           \
+  else if (flag->IsOfType<T>()) { \
+    flag->StoreAtomic(sizeof(T)); \
+  }
+
+  if (false) {
+  }
+  ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(STORE_ATOMIC)
+#undef STORE_ATOMIC
+}
+
+// Return true iff flag value was changed via direct-access.
+bool ChangedDirectly(CommandLineFlag* flag, const void* a, const void* b) {
+  if (!flag->IsAbseilFlag()) {
+// Need to compare values for direct-access flags.
+#define CHANGED_FOR_TYPE(T)                                                  \
+  if (flag->IsOfType<T>()) {                                                 \
+    return *reinterpret_cast<const T*>(a) != *reinterpret_cast<const T*>(b); \
+  }
+
+    CHANGED_FOR_TYPE(bool);
+    CHANGED_FOR_TYPE(int32_t);
+    CHANGED_FOR_TYPE(int64_t);
+    CHANGED_FOR_TYPE(uint64_t);
+    CHANGED_FOR_TYPE(double);
+    CHANGED_FOR_TYPE(std::string);
+#undef CHANGED_FOR_TYPE
+  }
+
+  return false;
+}
+
 }  // namespace flags_internal
 }  // namespace absl
diff --git a/absl/flags/internal/commandlineflag.h b/absl/flags/internal/commandlineflag.h
index 4815cdc7c9a2..d82443a7760e 100644
--- a/absl/flags/internal/commandlineflag.h
+++ b/absl/flags/internal/commandlineflag.h
@@ -69,11 +69,15 @@ using HelpGenFunc = std::string (*)();
 // based on default value supplied in flag's definition)
 using InitialValGenFunc = void* (*)();
 
+struct CommandLineFlagInfo;
+
 // Signature for the mutation callback used by watched Flags
 // The callback is noexcept.
 // TODO(rogeeff): add noexcept after C++17 support is added.
 using FlagCallback = void (*)();
 
+using FlagValidator = bool (*)();
+
 extern const char kStrippedFlagHelp[];
 
 // The per-type function
@@ -217,6 +221,9 @@ struct CommandLineFlag {
         atomic(kAtomicInit),
         locks(nullptr) {}
 
+  // Revert the init routine.
+  void Destroy() const;
+
   // Not copyable/assignable.
   CommandLineFlag(const CommandLineFlag&) = delete;
   CommandLineFlag& operator=(const CommandLineFlag&) = delete;
@@ -224,7 +231,9 @@ struct CommandLineFlag {
   absl::string_view Name() const { return name; }
   std::string Help() const { return help.GetHelpText(); }
   bool IsRetired() const { return this->retired; }
-  bool IsSpecifiedOnCommandLine() const { return on_command_line; }
+  bool IsModified() const;
+  void SetModified(bool is_modified);
+  bool IsSpecifiedOnCommandLine() const;
   // Returns true iff this is a handle to an Abseil Flag.
   bool IsAbseilFlag() const {
     // Set to null for V1 flags
@@ -236,6 +245,10 @@ struct CommandLineFlag {
   std::string DefaultValue() const;
   std::string CurrentValue() const;
 
+  bool HasValidatorFn() const;
+  bool SetValidatorFn(FlagValidator fn);
+  bool InvokeValidator(const void* value) const;
+
   // Return true iff flag has type T.
   template <typename T>
   inline bool IsOfType() const {
@@ -245,7 +258,7 @@ struct CommandLineFlag {
   // Attempts to retrieve the flag value. Returns value on success,
   // absl::nullopt otherwise.
   template <typename T>
-  absl::optional<T> Get() {
+  absl::optional<T> Get() const {
     if (IsRetired() || flags_internal::FlagOps<T> != this->op)
       return absl::nullopt;
 
@@ -256,6 +269,7 @@ struct CommandLineFlag {
   }
 
   void SetCallback(const flags_internal::FlagCallback mutation_callback);
+  void InvokeCallback();
 
   // Sets the value of the flag based on specified std::string `value`. If the flag
   // was successfully set to new value, it returns true. Otherwise, sets `error`
@@ -269,27 +283,35 @@ struct CommandLineFlag {
                      flags_internal::FlagSettingMode set_mode,
                      flags_internal::ValueSource source, std::string* error);
 
+  void StoreAtomic(size_t size);
+
+  void CheckDefaultValueParsingRoundtrip() const;
+  // Invoke the flag validators for old flags.
+  // TODO(rogeeff): implement proper validators for Abseil Flags
+  bool ValidateDefaultValue() const;
+  bool ValidateInputValue(absl::string_view value) const;
+
   // Constant configuration for a particular flag.
  private:
   const char* const name;
   const HelpText help;
   const char* const filename;
 
- public:
-  const FlagOpFn op;                  // Type-specific handler
+ protected:
+  const FlagOpFn op;                         // Type-specific handler
   const FlagMarshallingOpFn marshalling_op;  // Marshalling ops handler
   const InitialValGenFunc make_init_value;   // Makes initial value for the flag
-  const bool retired;                 // Is the flag retired?
-  std::atomic<bool> inited;           // fields have been lazily initialized
+  const bool retired;                        // Is the flag retired?
+  std::atomic<bool> inited;  // fields have been lazily initialized
 
   // Mutable state (guarded by locks->primary_mu).
-  bool modified;          // Has flag value been modified?
-  bool on_command_line;   // Specified on command line.
-  bool (*validator)();    // Validator function, or nullptr
-  FlagCallback callback;  // Mutation callback, or nullptr
-  void* def;              // Lazily initialized pointer to default value
-  void* cur;              // Lazily initialized pointer to current value
-  int64_t counter;          // Mutation counter
+  bool modified;            // Has flag value been modified?
+  bool on_command_line;     // Specified on command line.
+  FlagValidator validator;  // Validator function, or nullptr
+  FlagCallback callback;    // Mutation callback, or nullptr
+  void* def;                // Lazily initialized pointer to default value
+  void* cur;                // Lazily initialized pointer to current value
+  int64_t counter;            // Mutation counter
 
   // For some types, a copy of the current value is kept in an atomically
   // accessible field.
@@ -302,24 +324,26 @@ struct CommandLineFlag {
   // TODO(rogeeff): fix it once Mutex has constexpr constructor
   struct CommandLineFlagLocks* locks;  // locks, laziliy allocated.
 
+  // Ensure that the lazily initialized fields of *flag have been initialized,
+  // and return the lock which should be locked when flag's state is mutated.
+  absl::Mutex* InitFlagIfNecessary() const;
+
   // copy construct new value of flag's type in a memory referenced by dst
   // based on current flag's value
   void Read(void* dst, const flags_internal::FlagOpFn dst_op) const;
   // updates flag's value to *src (locked)
   void Write(const void* src, const flags_internal::FlagOpFn src_op);
 
-  ABSL_DEPRECATED(
-      "temporary until FlagName call sites are migrated and validator API is "
-      "changed")
-  const char* NameAsCString() const { return name; }
-
- private:
   friend class FlagRegistry;
+  friend class FlagPtrMap;
+  friend class FlagSaverImpl;
+  friend void FillCommandLineFlagInfo(CommandLineFlag* flag,
+                                      CommandLineFlagInfo* result);
+  friend bool TryParseLocked(CommandLineFlag* flag, void* dst,
+                             absl::string_view value, std::string* err);
+  friend absl::Mutex* InitFlag(CommandLineFlag* flag);
 };
 
-// Ensure that the lazily initialized fields of *flag have been initialized,
-// and return &flag->locks->primary_mu.
-absl::Mutex* InitFlagIfNecessary(CommandLineFlag* flag);
 // Update any copy of the flag value that is stored in an atomic word.
 // In addition if flag has a mutation callback this function invokes it. While
 // callback is being invoked the primary flag's mutex is unlocked and it is
@@ -332,15 +356,9 @@ absl::Mutex* InitFlagIfNecessary(CommandLineFlag* flag);
 // different by the time the callback invocation is completed.
 // Requires that *primary_lock be held in exclusive mode; it may be released
 // and reacquired by the implementation.
-void UpdateCopy(CommandLineFlag* flag, absl::Mutex* primary_lock);
+void UpdateCopy(CommandLineFlag* flag);
 // Return true iff flag value was changed via direct-access.
 bool ChangedDirectly(CommandLineFlag* flag, const void* a, const void* b);
-// Direct-access flags can be modified without going through the
-// flag API.  Detect such changes and updated the modified bit.
-void UpdateModifiedBit(CommandLineFlag* flag);
-// Invoke the flag validators for old flags.
-// TODO(rogeeff): implement proper validators for Abseil Flags
-bool Validate(CommandLineFlag* flag, const void* value);
 
 // This macro is the "source of truth" for the list of supported flag types we
 // expect to perform lock free operations on. Specifically it generates code,
diff --git a/absl/flags/internal/commandlineflag_test.cc b/absl/flags/internal/commandlineflag_test.cc
index 705f301ce7a0..f0d57adbd260 100644
--- a/absl/flags/internal/commandlineflag_test.cc
+++ b/absl/flags/internal/commandlineflag_test.cc
@@ -100,39 +100,39 @@ TEST_F(CommandLineFlagTest, TestSetFromStringCurrentValue) {
   std::string err;
 
   auto* flag_01 = flags::FindCommandLineFlag("int_flag");
-  EXPECT_FALSE(flag_01->on_command_line);
+  EXPECT_FALSE(flag_01->IsSpecifiedOnCommandLine());
 
   EXPECT_TRUE(flag_01->SetFromString("11", flags::SET_FLAGS_VALUE,
                                      flags::kProgrammaticChange, &err));
   EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 11);
-  EXPECT_FALSE(flag_01->on_command_line);
+  EXPECT_FALSE(flag_01->IsSpecifiedOnCommandLine());
 
   EXPECT_TRUE(flag_01->SetFromString("-123", flags::SET_FLAGS_VALUE,
                                      flags::kProgrammaticChange, &err));
   EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), -123);
-  EXPECT_FALSE(flag_01->on_command_line);
+  EXPECT_FALSE(flag_01->IsSpecifiedOnCommandLine());
 
   EXPECT_TRUE(!flag_01->SetFromString("xyz", flags::SET_FLAGS_VALUE,
                                       flags::kProgrammaticChange, &err));
   EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), -123);
   EXPECT_EQ(err, "Illegal value 'xyz' specified for flag 'int_flag'");
-  EXPECT_FALSE(flag_01->on_command_line);
+  EXPECT_FALSE(flag_01->IsSpecifiedOnCommandLine());
 
   EXPECT_TRUE(!flag_01->SetFromString("A1", flags::SET_FLAGS_VALUE,
                                       flags::kProgrammaticChange, &err));
   EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), -123);
   EXPECT_EQ(err, "Illegal value 'A1' specified for flag 'int_flag'");
-  EXPECT_FALSE(flag_01->on_command_line);
+  EXPECT_FALSE(flag_01->IsSpecifiedOnCommandLine());
 
   EXPECT_TRUE(flag_01->SetFromString("0x10", flags::SET_FLAGS_VALUE,
                                      flags::kProgrammaticChange, &err));
   EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 16);
-  EXPECT_FALSE(flag_01->on_command_line);
+  EXPECT_FALSE(flag_01->IsSpecifiedOnCommandLine());
 
   EXPECT_TRUE(flag_01->SetFromString("011", flags::SET_FLAGS_VALUE,
                                      flags::kCommandLine, &err));
   EXPECT_EQ(absl::GetFlag(FLAGS_int_flag), 11);
-  EXPECT_TRUE(flag_01->on_command_line);
+  EXPECT_TRUE(flag_01->IsSpecifiedOnCommandLine());
 
   EXPECT_TRUE(!flag_01->SetFromString("", flags::SET_FLAGS_VALUE,
                                       flags::kProgrammaticChange, &err));
diff --git a/absl/flags/internal/flag.h b/absl/flags/internal/flag.h
index 9b32f46735b6..bc50de5486bc 100644
--- a/absl/flags/internal/flag.h
+++ b/absl/flags/internal/flag.h
@@ -24,40 +24,58 @@ namespace flags_internal {
 
 // This is "unspecified" implementation of absl::Flag<T> type.
 template <typename T>
-class Flag {
+class Flag : public flags_internal::CommandLineFlag {
  public:
   constexpr Flag(const char* name, const flags_internal::HelpGenFunc help_gen,
                  const char* filename,
-                 const flags_internal::FlagMarshallingOpFn marshalling_op,
+                 const flags_internal::FlagMarshallingOpFn marshalling_op_arg,
                  const flags_internal::InitialValGenFunc initial_value_gen)
-      : internal(name, flags_internal::HelpText::FromFunctionPointer(help_gen),
-                 filename, &flags_internal::FlagOps<T>, marshalling_op,
-                 initial_value_gen,
-                 /*retired_arg=*/false, /*def_arg=*/nullptr,
-                 /*cur_arg=*/nullptr) {}
-
-  // Not copyable/assignable.
-  Flag(const Flag<T>&) = delete;
-  Flag<T>& operator=(const Flag<T>&) = delete;
-
-  absl::string_view Name() const { return internal.Name(); }
-  std::string Help() const { return internal.Help(); }
-  std::string Filename() const { return internal.Filename(); }
+      : flags_internal::CommandLineFlag(
+            name, flags_internal::HelpText::FromFunctionPointer(help_gen),
+            filename, &flags_internal::FlagOps<T>, marshalling_op_arg,
+            initial_value_gen,
+            /*retired_arg=*/false, /*def_arg=*/nullptr,
+            /*cur_arg=*/nullptr) {}
+
+  T Get() const {
+    // Implementation notes:
+    //
+    // We are wrapping a union around the value of `T` to serve three purposes:
+    //
+    //  1. `U.value` has correct size and alignment for a value of type `T`
+    //  2. The `U.value` constructor is not invoked since U's constructor does
+    //  not
+    //     do it explicitly.
+    //  3. The `U.value` destructor is invoked since U's destructor does it
+    //     explicitly. This makes `U` a kind of RAII wrapper around non default
+    //     constructible value of T, which is destructed when we leave the
+    //     scope. We do need to destroy U.value, which is constructed by
+    //     CommandLineFlag::Read even though we left it in a moved-from state
+    //     after std::move.
+    //
+    // All of this serves to avoid requiring `T` being default constructible.
+    union U {
+      T value;
+      U() {}
+      ~U() { value.~T(); }
+    };
+    U u;
+
+    this->Read(&u.value, &flags_internal::FlagOps<T>);
+    return std::move(u.value);
+  }
 
-  absl::flags_internal::CommandLineFlag internal;
+  bool AtomicGet(T* v) const {
+    const int64_t r = this->atomic.load(std::memory_order_acquire);
+    if (r != flags_internal::CommandLineFlag::kAtomicInit) {
+      memcpy(v, &r, sizeof(T));
+      return true;
+    }
 
-  void SetCallback(const flags_internal::FlagCallback mutation_callback) {
-    internal.SetCallback(mutation_callback);
+    return false;
   }
 
- private:
-  // TODO(rogeeff): add these validations once UnparseFlag invocation is fixed
-  // for built-in types and when we cleanup existing code from operating on
-  // forward declared types.
-  //  auto IsCopyConstructible(const T& v) -> decltype(T(v));
-  //  auto HasAbslParseFlag(absl::string_view in, T* dst, std::string* err)
-  //      -> decltype(AbslParseFlag(in, dst, err));
-  //  auto HasAbslUnparseFlag(const T& v) -> decltype(AbslUnparseFlag(v));
+  void Set(const T& v) { this->Write(&v, &flags_internal::FlagOps<T>); }
 };
 
 // This class facilitates Flag object registration and tail expression-based
@@ -67,7 +85,7 @@ template <typename T, bool do_register>
 class FlagRegistrar {
  public:
   explicit FlagRegistrar(Flag<T>* flag) : flag_(flag) {
-    if (do_register) flags_internal::RegisterCommandLineFlag(&flag_->internal);
+    if (do_register) flags_internal::RegisterCommandLineFlag(flag_);
   }
 
   FlagRegistrar& OnUpdate(flags_internal::FlagCallback cb) && {
diff --git a/absl/flags/internal/registry.cc b/absl/flags/internal/registry.cc
index e52cbb683b9e..f37871c9d40f 100644
--- a/absl/flags/internal/registry.cc
+++ b/absl/flags/internal/registry.cc
@@ -34,13 +34,7 @@ namespace flags_internal {
 namespace {
 
 void DestroyFlag(CommandLineFlag* flag) NO_THREAD_SAFETY_ANALYSIS {
-  // Values are heap allocated for retired and Abseil Flags.
-  if (flag->IsRetired() || flag->IsAbseilFlag()) {
-    if (flag->cur) Delete(flag->op, flag->cur);
-    if (flag->def) Delete(flag->op, flag->def);
-  }
-
-  delete flag->locks;
+  flag->Destroy();
 
   // CommandLineFlag handle object is heap allocated for non Abseil Flags.
   if (!flag->IsAbseilFlag()) {
@@ -48,6 +42,8 @@ void DestroyFlag(CommandLineFlag* flag) NO_THREAD_SAFETY_ANALYSIS {
   }
 }
 
+}  // namespace
+
 // --------------------------------------------------------------------
 // FlagRegistry
 //    A FlagRegistry singleton object holds all flag objects indexed
@@ -105,8 +101,6 @@ class FlagPtrMap {
 };
 constexpr size_t FlagPtrMap::kNumBuckets;
 
-}  // namespace
-
 class FlagRegistry {
  public:
   FlagRegistry() = default;
@@ -292,10 +286,10 @@ class FlagSaverImpl {
       saved.op = flag->op;
       saved.marshalling_op = flag->marshalling_op;
       {
-        absl::MutexLock l(InitFlagIfNecessary(flag));
+        absl::MutexLock l(flag->InitFlagIfNecessary());
         saved.validator = flag->validator;
         saved.modified = flag->modified;
-        saved.on_command_line = flag->IsSpecifiedOnCommandLine();
+        saved.on_command_line = flag->on_command_line;
         saved.current = Clone(saved.op, flag->cur);
         saved.default_value = Clone(saved.op, flag->def);
         saved.counter = flag->counter;
@@ -318,34 +312,34 @@ class FlagSaverImpl {
 
       bool restored = false;
       {
-        absl::Mutex* mu = InitFlagIfNecessary(flag);
-        absl::MutexLock l(mu);
+        absl::MutexLock l(flag->InitFlagIfNecessary());
         flag->validator = src.validator;
         flag->modified = src.modified;
         flag->on_command_line = src.on_command_line;
         if (flag->counter != src.counter ||
             ChangedDirectly(flag, src.default_value, flag->def)) {
-          flag->counter++;
+          restored = true;
           Copy(src.op, src.default_value, flag->def);
         }
         if (flag->counter != src.counter ||
             ChangedDirectly(flag, src.current, flag->cur)) {
           restored = true;
-          flag->counter++;
           Copy(src.op, src.current, flag->cur);
-          UpdateCopy(flag, mu);
-
-          // Revalidate the flag because the validator might store state based
-          // on the flag's value, which just changed due to the restore.
-          // Failing validation is ignored because it's assumed that the flag
-          // was valid previously and there's little that can be done about it
-          // here, anyway.
-          Validate(flag, flag->cur);
+          UpdateCopy(flag);
+          flag->InvokeCallback();
         }
       }
 
-      // Log statements must be done when no flag lock is held.
       if (restored) {
+        flag->counter++;
+
+        // Revalidate the flag because the validator might store state based
+        // on the flag's value, which just changed due to the restore.
+        // Failing validation is ignored because it's assumed that the flag
+        // was valid previously and there's little that can be done about it
+        // here, anyway.
+        flag->ValidateInputValue(flag->CurrentValue());
+
         ABSL_INTERNAL_LOG(
             INFO, absl::StrCat("Restore saved value of ", flag->Name(), ": ",
                                Unparse(src.marshalling_op, src.current)));
@@ -412,13 +406,17 @@ void FillCommandLineFlagInfo(CommandLineFlag* flag,
   result->description = flag->Help();
   result->filename = flag->Filename();
 
-  UpdateModifiedBit(flag);
+  if (!flag->IsAbseilFlag()) {
+    if (!flag->IsModified() && ChangedDirectly(flag, flag->cur, flag->def)) {
+      flag->modified = true;
+    }
+  }
 
-  absl::MutexLock l(InitFlagIfNecessary(flag));
   result->current_value = flag->CurrentValue();
   result->default_value = flag->DefaultValue();
-  result->is_default = !flag->modified;
-  result->has_validator_fn = (flag->validator != nullptr);
+  result->is_default = !flag->IsModified();
+  result->has_validator_fn = flag->HasValidatorFn();
+  absl::MutexLock l(flag->InitFlagIfNecessary());
   result->flag_ptr = flag->IsAbseilFlag() ? nullptr : flag->cur;
 }
 
diff --git a/absl/flags/internal/type_erased.cc b/absl/flags/internal/type_erased.cc
index cc103983672b..2984291c87c7 100644
--- a/absl/flags/internal/type_erased.cc
+++ b/absl/flags/internal/type_erased.cc
@@ -32,7 +32,6 @@ bool GetCommandLineOption(absl::string_view name, std::string* value) {
     return false;
   }
 
-  absl::MutexLock l(InitFlagIfNecessary(flag));
   *value = flag->CurrentValue();
   return true;
 }
@@ -88,22 +87,9 @@ bool SetCommandLineOptionWithMode(absl::string_view name,
 
 bool IsValidFlagValue(absl::string_view name, absl::string_view value) {
   CommandLineFlag* flag = flags_internal::FindCommandLineFlag(name);
-  if (flag == nullptr) {
-    return false;
-  }
-
-  if (flag->IsRetired()) {
-    return true;
-  }
 
-  // No need to lock the flag since we are not mutating it.
-  void* obj = Clone(flag->op, flag->def);
-  std::string ignored_error;
-  const bool result =
-      flags_internal::Parse(flag->marshalling_op, value, obj, &ignored_error) &&
-      Validate(flag, obj);
-  Delete(flag->op, obj);
-  return result;
+  return flag != nullptr &&
+         (flag->IsRetired() || flag->ValidateInputValue(value));
 }
 
 // --------------------------------------------------------------------
@@ -111,7 +97,6 @@ bool IsValidFlagValue(absl::string_view name, absl::string_view value) {
 bool SpecifiedOnCommandLine(absl::string_view name) {
   CommandLineFlag* flag = flags_internal::FindCommandLineFlag(name);
   if (flag != nullptr && !flag->IsRetired()) {
-    absl::MutexLock l(InitFlagIfNecessary(flag));
     return flag->IsSpecifiedOnCommandLine();
   }
   return false;
diff --git a/absl/flags/internal/usage.cc b/absl/flags/internal/usage.cc
index b1ff8b83c223..aac02db6c27a 100644
--- a/absl/flags/internal/usage.cc
+++ b/absl/flags/internal/usage.cc
@@ -21,11 +21,11 @@
 #include "absl/flags/flag.h"
 #include "absl/flags/internal/path_util.h"
 #include "absl/flags/internal/program_name.h"
-#include "absl/flags/usage.h"
 #include "absl/flags/usage_config.h"
 #include "absl/strings/ascii.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_split.h"
+#include "absl/strings/string_view.h"
 #include "absl/synchronization/mutex.h"
 
 ABSL_FLAG(bool, help, false,
@@ -185,7 +185,7 @@ void FlagHelpHumanReadable(const flags_internal::CommandLineFlag& flag,
   }
   printer.Write(absl::StrCat("default: ", dflt_val, ";"));
 
-  if (flag.modified) {
+  if (flag.IsModified()) {
     std::string curr_val = flag.CurrentValue();
     if (flag.IsOfType<std::string>()) {
       curr_val = absl::StrCat("\"", curr_val, "\"");
@@ -202,10 +202,10 @@ void FlagHelpHumanReadable(const flags_internal::CommandLineFlag& flag,
 // STRIP_FLAG_HELP 1' then this flag will not be displayed by '--help'
 // and its variants.
 void FlagsHelpImpl(std::ostream& out, flags_internal::FlagKindFilter filter_cb,
-                   HelpFormat format = HelpFormat::kHumanReadable) {
+                   HelpFormat format, absl::string_view program_usage_message) {
   if (format == HelpFormat::kHumanReadable) {
     out << flags_internal::ShortProgramInvocationName() << ": "
-        << absl::ProgramUsageMessage() << "\n\n";
+        << program_usage_message << "\n\n";
   } else {
     // XML schema is not a part of our public API for now.
     out << "<?xml version=\"1.0\"?>\n"
@@ -214,7 +214,7 @@ void FlagsHelpImpl(std::ostream& out, flags_internal::FlagKindFilter filter_cb,
         // The program name and usage.
         << XMLElement("program", flags_internal::ShortProgramInvocationName())
         << '\n'
-        << XMLElement("usage", absl::ProgramUsageMessage()) << '\n';
+        << XMLElement("usage", program_usage_message) << '\n';
   }
 
   // Map of package name to
@@ -228,8 +228,6 @@ void FlagsHelpImpl(std::ostream& out, flags_internal::FlagKindFilter filter_cb,
       matching_flags;
 
   flags_internal::ForEachFlag([&](flags_internal::CommandLineFlag* flag) {
-    absl::MutexLock l(InitFlagIfNecessary(flag));
-
     std::string flag_filename = flag->Filename();
 
     // Ignore retired flags.
@@ -292,44 +290,51 @@ void FlagHelp(std::ostream& out, const flags_internal::CommandLineFlag& flag,
 // --------------------------------------------------------------------
 // Produces the help messages for all flags matching the filter.
 // If filter is empty produces help messages for all flags.
-void FlagsHelp(std::ostream& out, absl::string_view filter, HelpFormat format) {
+void FlagsHelp(std::ostream& out, absl::string_view filter, HelpFormat format,
+               absl::string_view program_usage_message) {
   flags_internal::FlagKindFilter filter_cb = [&](absl::string_view filename) {
     return filter.empty() || filename.find(filter) != absl::string_view::npos;
   };
-  flags_internal::FlagsHelpImpl(out, filter_cb, format);
+  flags_internal::FlagsHelpImpl(out, filter_cb, format, program_usage_message);
 }
 
 // --------------------------------------------------------------------
 // Checks all the 'usage' command line flags to see if any have been set.
 // If so, handles them appropriately.
-int HandleUsageFlags(std::ostream& out) {
+int HandleUsageFlags(std::ostream& out,
+                     absl::string_view program_usage_message) {
   if (absl::GetFlag(FLAGS_helpshort)) {
     flags_internal::FlagsHelpImpl(
         out, flags_internal::GetUsageConfig().contains_helpshort_flags,
-        HelpFormat::kHumanReadable);
+        HelpFormat::kHumanReadable, program_usage_message);
     return 1;
   }
 
   if (absl::GetFlag(FLAGS_helpfull)) {
     // show all options
-    flags_internal::FlagsHelp(out);
+    flags_internal::FlagsHelp(out, "", HelpFormat::kHumanReadable,
+                              program_usage_message);
     return 1;
   }
 
   if (!absl::GetFlag(FLAGS_helpon).empty()) {
     flags_internal::FlagsHelp(
-        out, absl::StrCat("/", absl::GetFlag(FLAGS_helpon), "."));
+        out, absl::StrCat("/", absl::GetFlag(FLAGS_helpon), "."),
+        HelpFormat::kHumanReadable, program_usage_message);
     return 1;
   }
 
   if (!absl::GetFlag(FLAGS_helpmatch).empty()) {
-    flags_internal::FlagsHelp(out, absl::GetFlag(FLAGS_helpmatch));
+    flags_internal::FlagsHelp(out, absl::GetFlag(FLAGS_helpmatch),
+                              HelpFormat::kHumanReadable,
+                              program_usage_message);
     return 1;
   }
 
   if (absl::GetFlag(FLAGS_help)) {
     flags_internal::FlagsHelpImpl(
-        out, flags_internal::GetUsageConfig().contains_help_flags);
+        out, flags_internal::GetUsageConfig().contains_help_flags,
+        HelpFormat::kHumanReadable, program_usage_message);
 
     out << "\nTry --helpfull to get a list of all flags.\n";
 
@@ -338,7 +343,8 @@ int HandleUsageFlags(std::ostream& out) {
 
   if (absl::GetFlag(FLAGS_helppackage)) {
     flags_internal::FlagsHelpImpl(
-        out, flags_internal::GetUsageConfig().contains_helppackage_flags);
+        out, flags_internal::GetUsageConfig().contains_helppackage_flags,
+        HelpFormat::kHumanReadable, program_usage_message);
 
     out << "\nTry --helpfull to get a list of all flags.\n";
 
diff --git a/absl/flags/internal/usage.h b/absl/flags/internal/usage.h
index 33f3f969bfaf..76075b0829d9 100644
--- a/absl/flags/internal/usage.h
+++ b/absl/flags/internal/usage.h
@@ -47,8 +47,8 @@ void FlagHelp(std::ostream& out, const flags_internal::CommandLineFlag& flag,
 //  .../path/to/file.<ext>
 // for any extension 'ext'. If the filter is empty this function produces help
 // messages for all flags.
-void FlagsHelp(std::ostream& out, absl::string_view filter = {},
-               HelpFormat format = HelpFormat::kHumanReadable);
+void FlagsHelp(std::ostream& out, absl::string_view filter,
+               HelpFormat format, absl::string_view program_usage_message);
 
 // --------------------------------------------------------------------
 
@@ -60,7 +60,8 @@ void FlagsHelp(std::ostream& out, absl::string_view filter = {},
 // -1 - if no usage flags were set on a commmand line.
 // Non negative return values are expected to be used as an exit code for a
 // binary.
-int HandleUsageFlags(std::ostream& out);
+int HandleUsageFlags(std::ostream& out,
+                     absl::string_view program_usage_message);
 
 }  // namespace flags_internal
 }  // namespace absl
diff --git a/absl/flags/internal/usage_test.cc b/absl/flags/internal/usage_test.cc
index fa121fc933fc..5e5f7a84cc43 100644
--- a/absl/flags/internal/usage_test.cc
+++ b/absl/flags/internal/usage_test.cc
@@ -36,6 +36,8 @@ ABSL_FLAG(double, usage_reporting_test_flag_03, 1.03,
 ABSL_FLAG(int64_t, usage_reporting_test_flag_04, 1000000000000004L,
           "usage_reporting_test_flag_04 help message");
 
+static const char kTestUsageMessage[] = "Custom usage message";
+
 struct UDT {
   UDT() = default;
   UDT(const UDT&) = default;
@@ -83,7 +85,7 @@ class UsageReportingTest : public testing::Test {
 using UsageReportingDeathTest = UsageReportingTest;
 
 TEST_F(UsageReportingDeathTest, TestSetProgramUsageMessage) {
-  EXPECT_EQ(absl::ProgramUsageMessage(), "Custom usage message");
+  EXPECT_EQ(absl::ProgramUsageMessage(), kTestUsageMessage);
 
 #ifndef _WIN32
   // TODO(rogeeff): figure out why this does not work on Windows.
@@ -175,22 +177,22 @@ TEST_F(UsageReportingTest, TestFlagsHelpHRF) {
 
   std::stringstream test_buf_01;
   flags::FlagsHelp(test_buf_01, "usage_test.cc",
-                   flags::HelpFormat::kHumanReadable);
+                   flags::HelpFormat::kHumanReadable, kTestUsageMessage);
   EXPECT_EQ(test_buf_01.str(), usage_test_flags_out);
 
   std::stringstream test_buf_02;
   flags::FlagsHelp(test_buf_02, "flags/internal/usage_test.cc",
-                   flags::HelpFormat::kHumanReadable);
+                   flags::HelpFormat::kHumanReadable, kTestUsageMessage);
   EXPECT_EQ(test_buf_02.str(), usage_test_flags_out);
 
   std::stringstream test_buf_03;
-  flags::FlagsHelp(test_buf_03, "usage_test",
-                   flags::HelpFormat::kHumanReadable);
+  flags::FlagsHelp(test_buf_03, "usage_test", flags::HelpFormat::kHumanReadable,
+                   kTestUsageMessage);
   EXPECT_EQ(test_buf_03.str(), usage_test_flags_out);
 
   std::stringstream test_buf_04;
   flags::FlagsHelp(test_buf_04, "flags/invalid_file_name.cc",
-                   flags::HelpFormat::kHumanReadable);
+                   flags::HelpFormat::kHumanReadable, kTestUsageMessage);
   EXPECT_EQ(test_buf_04.str(),
             R"(usage_test: Custom usage message
 
@@ -198,7 +200,8 @@ TEST_F(UsageReportingTest, TestFlagsHelpHRF) {
 )");
 
   std::stringstream test_buf_05;
-  flags::FlagsHelp(test_buf_05, "", flags::HelpFormat::kHumanReadable);
+  flags::FlagsHelp(test_buf_05, "", flags::HelpFormat::kHumanReadable,
+                   kTestUsageMessage);
   std::string test_out = test_buf_05.str();
   absl::string_view test_out_str(test_out);
   EXPECT_TRUE(
@@ -217,7 +220,7 @@ TEST_F(UsageReportingTest, TestFlagsHelpHRF) {
 
 TEST_F(UsageReportingTest, TestNoUsageFlags) {
   std::stringstream test_buf;
-  EXPECT_EQ(flags::HandleUsageFlags(test_buf), -1);
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage), -1);
 }
 
 // --------------------------------------------------------------------
@@ -226,7 +229,7 @@ TEST_F(UsageReportingTest, TestUsageFlag_helpshort) {
   absl::SetFlag(&FLAGS_helpshort, true);
 
   std::stringstream test_buf;
-  EXPECT_EQ(flags::HandleUsageFlags(test_buf), 1);
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage), 1);
   EXPECT_EQ(test_buf.str(),
             R"(usage_test: Custom usage message
 
@@ -250,7 +253,7 @@ TEST_F(UsageReportingTest, TestUsageFlag_help) {
   absl::SetFlag(&FLAGS_help, true);
 
   std::stringstream test_buf;
-  EXPECT_EQ(flags::HandleUsageFlags(test_buf), 1);
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage), 1);
   EXPECT_EQ(test_buf.str(),
             R"(usage_test: Custom usage message
 
@@ -276,7 +279,7 @@ TEST_F(UsageReportingTest, TestUsageFlag_helppackage) {
   absl::SetFlag(&FLAGS_helppackage, true);
 
   std::stringstream test_buf;
-  EXPECT_EQ(flags::HandleUsageFlags(test_buf), 1);
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage), 1);
   EXPECT_EQ(test_buf.str(),
             R"(usage_test: Custom usage message
 
@@ -302,10 +305,9 @@ TEST_F(UsageReportingTest, TestUsageFlag_version) {
   absl::SetFlag(&FLAGS_version, true);
 
   std::stringstream test_buf;
-  EXPECT_EQ(flags::HandleUsageFlags(test_buf), 0);
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage), 0);
 #ifndef NDEBUG
-  EXPECT_EQ(test_buf.str(),
-            "usage_test\nDebug build (NDEBUG not #defined)\n");
+  EXPECT_EQ(test_buf.str(), "usage_test\nDebug build (NDEBUG not #defined)\n");
 #else
   EXPECT_EQ(test_buf.str(), "usage_test\n");
 #endif
@@ -317,7 +319,7 @@ TEST_F(UsageReportingTest, TestUsageFlag_only_check_args) {
   absl::SetFlag(&FLAGS_only_check_args, true);
 
   std::stringstream test_buf;
-  EXPECT_EQ(flags::HandleUsageFlags(test_buf), 0);
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf, kTestUsageMessage), 0);
   EXPECT_EQ(test_buf.str(), "");
 }
 
@@ -327,7 +329,7 @@ TEST_F(UsageReportingTest, TestUsageFlag_helpon) {
   absl::SetFlag(&FLAGS_helpon, "bla-bla");
 
   std::stringstream test_buf_01;
-  EXPECT_EQ(flags::HandleUsageFlags(test_buf_01), 1);
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf_01, kTestUsageMessage), 1);
   EXPECT_EQ(test_buf_01.str(),
             R"(usage_test: Custom usage message
 
@@ -337,7 +339,7 @@ TEST_F(UsageReportingTest, TestUsageFlag_helpon) {
   absl::SetFlag(&FLAGS_helpon, "usage_test");
 
   std::stringstream test_buf_02;
-  EXPECT_EQ(flags::HandleUsageFlags(test_buf_02), 1);
+  EXPECT_EQ(flags::HandleUsageFlags(test_buf_02, kTestUsageMessage), 1);
   EXPECT_EQ(test_buf_02.str(),
             R"(usage_test: Custom usage message
 
@@ -362,7 +364,7 @@ TEST_F(UsageReportingTest, TestUsageFlag_helpon) {
 int main(int argc, char* argv[]) {
   absl::GetFlag(FLAGS_undefok);  // Force linking of parse.cc
   flags::SetProgramInvocationName("usage_test");
-  absl::SetProgramUsageMessage("Custom usage message");
+  absl::SetProgramUsageMessage(kTestUsageMessage);
   ::testing::InitGoogleTest(&argc, argv);
 
   return RUN_ALL_TESTS();
diff --git a/absl/flags/parse.cc b/absl/flags/parse.cc
index 3caaa1c256cb..e9dd4204e8a8 100644
--- a/absl/flags/parse.cc
+++ b/absl/flags/parse.cc
@@ -16,6 +16,7 @@
 #include "absl/flags/parse.h"
 
 #include <stdlib.h>
+
 #include <fstream>
 #include <iostream>
 #include <tuple>
@@ -28,6 +29,7 @@
 #include "absl/flags/internal/program_name.h"
 #include "absl/flags/internal/registry.h"
 #include "absl/flags/internal/usage.h"
+#include "absl/flags/usage.h"
 #include "absl/flags/usage_config.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/strip.h"
@@ -280,22 +282,7 @@ void CheckDefaultValuesParsingRoundtrip() {
     IGNORE_TYPE(std::vector<std::string>)
 #undef IGNORE_TYPE
 
-    absl::MutexLock lock(InitFlagIfNecessary(flag));
-
-    std::string v = flag->DefaultValue();
-    void* dst = Clone(flag->op, flag->def);
-    std::string error;
-    if (!flags_internal::Parse(flag->marshalling_op, v, dst, &error)) {
-      ABSL_INTERNAL_LOG(
-          FATAL,
-          absl::StrCat("Flag ", flag->Name(), " (from ", flag->Filename(),
-                       "): std::string form of default value '", v,
-                       "' could not be parsed; error=", error));
-    }
-
-    // We do not compare dst to def since parsing/unparsing may make
-    // small changes, e.g., precision loss for floating point types.
-    Delete(flag->op, dst);
+    flag->CheckDefaultValueParsingRoundtrip();
   });
 #endif
 }
@@ -717,12 +704,14 @@ std::vector<char*> ParseCommandLineImpl(int argc, char* argv[],
 #endif
 
   if (!success) {
-    flags_internal::HandleUsageFlags(std::cout);
+    flags_internal::HandleUsageFlags(std::cout,
+                                     ProgramUsageMessage());
     std::exit(1);
   }
 
   if (usage_flag_act == UsageFlagsAction::kHandleUsage) {
-    int exit_code = flags_internal::HandleUsageFlags(std::cout);
+    int exit_code = flags_internal::HandleUsageFlags(
+        std::cout, ProgramUsageMessage());
 
     if (exit_code != -1) {
       std::exit(exit_code);
diff --git a/absl/hash/hash_test.cc b/absl/hash/hash_test.cc
index bab560bd4e64..9a667ba703b4 100644
--- a/absl/hash/hash_test.cc
+++ b/absl/hash/hash_test.cc
@@ -288,7 +288,6 @@ TEST(HashValueTest, Strings) {
   // Also check that nested types maintain the same hash.
   const WrapInTuple t{};
   EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
-      //
       t(std::string()), t(absl::string_view()),
       t(std::string("")), t(absl::string_view("")),
       t(std::string(small)), t(absl::string_view(small)),
diff --git a/absl/hash/internal/hash.h b/absl/hash/internal/hash.h
index 18665173350e..4ff4a126f0cd 100644
--- a/absl/hash/internal/hash.h
+++ b/absl/hash/internal/hash.h
@@ -640,7 +640,8 @@ class CityHashState : public HashStateBase<CityHashState> {
 #endif  // ABSL_HAVE_INTRINSIC_INT128
 
   static constexpr uint64_t kMul =
-      sizeof(size_t) == 4 ? uint64_t{0xcc9e2d51} : uint64_t{0x9ddfea08eb382d69};
+      sizeof(size_t) == 4 ? uint64_t{0xcc9e2d51}
+                          : uint64_t{0x9ddfea08eb382d69};
 
   template <typename T>
   using IntegralFastPath =
diff --git a/absl/numeric/int128.h b/absl/numeric/int128.h
index 2f5b8ad7a874..10be8eca42e7 100644
--- a/absl/numeric/int128.h
+++ b/absl/numeric/int128.h
@@ -19,8 +19,8 @@
 //
 // 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.
+// Currently, this file defines `uint128`, an unsigned 128-bit integer;
+// a signed 128-bit integer is forthcoming.
 
 #ifndef ABSL_NUMERIC_INT128_H_
 #define ABSL_NUMERIC_INT128_H_
diff --git a/absl/random/internal/fast_uniform_bits.h b/absl/random/internal/fast_uniform_bits.h
index 184a2708b2af..e8df92f3d99f 100644
--- a/absl/random/internal/fast_uniform_bits.h
+++ b/absl/random/internal/fast_uniform_bits.h
@@ -22,11 +22,18 @@
 
 namespace absl {
 namespace random_internal {
+// Returns true if the input value is zero or a power of two. Useful for
+// determining if the range of output values in a URBG
+template <typename UIntType>
+constexpr bool IsPowerOfTwoOrZero(UIntType n) {
+  return (n == 0) || ((n & (n - 1)) == 0);
+}
+
 // Computes the length of the range of values producible by the URBG, or returns
 // zero if that would encompass the entire range of representable values in
 // URBG::result_type.
 template <typename URBG>
-constexpr typename URBG::result_type constexpr_range() {
+constexpr typename URBG::result_type RangeSize() {
   using result_type = typename URBG::result_type;
   return ((URBG::max)() == (std::numeric_limits<result_type>::max)() &&
           (URBG::min)() == std::numeric_limits<result_type>::lowest())
@@ -34,6 +41,42 @@ constexpr typename URBG::result_type constexpr_range() {
              : (URBG::max)() - (URBG::min)() + result_type{1};
 }
 
+template <typename UIntType>
+constexpr UIntType LargestPowerOfTwoLessThanOrEqualTo(UIntType n) {
+  return n < 2 ? n : 2 * LargestPowerOfTwoLessThanOrEqualTo(n / 2);
+}
+
+// Given a URBG generating values in the closed interval [Lo, Hi], returns the
+// largest power of two less than or equal to `Hi - Lo + 1`.
+template <typename URBG>
+constexpr typename URBG::result_type PowerOfTwoSubRangeSize() {
+  return LargestPowerOfTwoLessThanOrEqualTo(RangeSize<URBG>());
+}
+
+// Computes the floor of the log. (i.e., std::floor(std::log2(N));
+template <typename UIntType>
+constexpr UIntType IntegerLog2(UIntType n) {
+  return (n <= 1) ? 0 : 1 + IntegerLog2(n / 2);
+}
+
+// Returns the number of bits of randomness returned through
+// `PowerOfTwoVariate(urbg)`.
+template <typename URBG>
+constexpr size_t NumBits() {
+  return RangeSize<URBG>() == 0
+             ? std::numeric_limits<typename URBG::result_type>::digits
+             : IntegerLog2(PowerOfTwoSubRangeSize<URBG>());
+}
+
+// Given a shift value `n`, constructs a mask with exactly the low `n` bits set.
+// If `n == 0`, all bits are set.
+template <typename UIntType>
+constexpr UIntType MaskFromShift(UIntType n) {
+  return ((n % std::numeric_limits<UIntType>::digits) == 0)
+             ? ~UIntType{0}
+             : (UIntType{1} << n) - UIntType{1};
+}
+
 // FastUniformBits implements a fast path to acquire uniform independent bits
 // from a type which conforms to the [rand.req.urbg] concept.
 // Parameterized by:
@@ -45,14 +88,6 @@ constexpr typename URBG::result_type constexpr_range() {
 // generator that will outlive the std::independent_bits_engine instance.
 template <typename UIntType = uint64_t>
 class FastUniformBits {
-  static_assert(std::is_unsigned<UIntType>::value,
-                "Class-template FastUniformBits<> must be parameterized using "
-                "an unsigned type.");
-
-  // `kWidth` is the width, in binary digits, of the output. By default it is
-  // the number of binary digits in the `result_type`.
-  static constexpr size_t kWidth = std::numeric_limits<UIntType>::digits;
-
  public:
   using result_type = UIntType;
 
@@ -65,14 +100,47 @@ class FastUniformBits {
   result_type operator()(URBG& g);  // NOLINT(runtime/references)
 
  private:
-  // Variate() generates a single random variate, always returning a value
-  // in the closed interval [0 ... FastUniformBitsURBGConstants::kRangeMask]
-  // (kRangeMask+1 is a power of 2).
+  static_assert(std::is_unsigned<UIntType>::value,
+                "Class-template FastUniformBits<> must be parameterized using "
+                "an unsigned type.");
+
+  // PowerOfTwoVariate() generates a single random variate, always returning a
+  // value in the half-open interval `[0, PowerOfTwoSubRangeSize<URBG>())`. If
+  // the URBG already generates values in a power-of-two range, the generator
+  // itself is used. Otherwise, we use rejection sampling on the largest
+  // possible power-of-two-sized subrange.
+  struct PowerOfTwoTag {};
+  struct RejectionSamplingTag {};
   template <typename URBG>
-  typename URBG::result_type Variate(URBG& g);  // NOLINT(runtime/references)
+  static typename URBG::result_type PowerOfTwoVariate(
+      URBG& g) {  // NOLINT(runtime/references)
+    using tag =
+        typename std::conditional<IsPowerOfTwoOrZero(RangeSize<URBG>()),
+                                  PowerOfTwoTag, RejectionSamplingTag>::type;
+    return PowerOfTwoVariate(g, tag{});
+  }
+
+  template <typename URBG>
+  static typename URBG::result_type PowerOfTwoVariate(
+      URBG& g,  // NOLINT(runtime/references)
+      PowerOfTwoTag) {
+    return g() - (URBG::min)();
+  }
 
-  // generate() generates a random value, dispatched on whether
-  // the underlying URNG must loop over multiple calls or not.
+  template <typename URBG>
+  static typename URBG::result_type PowerOfTwoVariate(
+      URBG& g,  // NOLINT(runtime/references)
+      RejectionSamplingTag) {
+    // Use rejection sampling to ensure uniformity across the range.
+    typename URBG::result_type u;
+    do {
+      u = g() - (URBG::min)();
+    } while (u >= PowerOfTwoSubRangeSize<URBG>());
+    return u;
+  }
+
+  // Generate() generates a random value, dispatched on whether
+  // the underlying URBG must loop over multiple calls or not.
   template <typename URBG>
   result_type Generate(URBG& g,  // NOLINT(runtime/references)
                        std::true_type /* avoid_looping */);
@@ -82,196 +150,107 @@ class FastUniformBits {
                        std::false_type /* avoid_looping */);
 };
 
-// FastUniformBitsURBGConstants computes the URBG-derived constants used
-// by FastUniformBits::Generate and FastUniformBits::Variate.
-// Parameterized by the FastUniformBits parameter:
-//   `URBG`: The underlying UniformRandomNumberGenerator.
-//
-// The values here indicate the URBG range as well as providing an indicator
-// whether the URBG output is a power of 2, and kRangeMask, which allows masking
-// the generated output to kRangeBits.
+template <typename UIntType>
 template <typename URBG>
-class FastUniformBitsURBGConstants {
-  // Computes the floor of the log. (i.e., std::floor(std::log2(N));
-  static constexpr size_t constexpr_log2(size_t n) {
-    return (n <= 1) ? 0 : 1 + constexpr_log2(n / 2);
-  }
-
-  // Computes a mask of n bits for the URBG::result_type.
-  static constexpr typename URBG::result_type constexpr_mask(size_t n) {
-    return (typename URBG::result_type(1) << n) - 1;
-  }
-
- public:
-  using result_type = typename URBG::result_type;
-
-  // The range of the URNG, max - min + 1, or zero if that result would cause
-  // overflow.
-  static constexpr result_type kRange = constexpr_range<URBG>();
-
-  static constexpr bool kPowerOfTwo =
-      (kRange == 0) || ((kRange & (kRange - 1)) == 0);
-
-  // kRangeBits describes the number number of bits suitable to mask off of URNG
-  // variate, which is:
-  // kRangeBits = floor(log2(kRange))
-  static constexpr size_t kRangeBits =
-      kRange == 0 ? std::numeric_limits<result_type>::digits
-                  : constexpr_log2(kRange);
-
-  // kRangeMask is the mask used when sampling variates from the URNG when the
-  // width of the URNG range is not a power of 2.
+typename FastUniformBits<UIntType>::result_type
+FastUniformBits<UIntType>::operator()(URBG& g) {  // NOLINT(runtime/references)
+  // kRangeMask is the mask used when sampling variates from the URBG when the
+  // width of the URBG range is not a power of 2.
   // Y = (2 ^ kRange) - 1
-  static constexpr result_type kRangeMask =
-      kRange == 0 ? (std::numeric_limits<result_type>::max)()
-                  : constexpr_mask(kRangeBits);
-
-  static_assert((URBG::max)() != (URBG::min)(),
-                "Class-template FastUniformBitsURBGConstants<> "
+  static_assert((URBG::max)() > (URBG::min)(),
                 "URBG::max and URBG::min may not be equal.");
-
-  static_assert(std::is_unsigned<result_type>::value,
-                "Class-template FastUniformBitsURBGConstants<> "
-                "URBG::result_type must be unsigned.");
-
-  static_assert(kRangeMask > 0,
-                "Class-template FastUniformBitsURBGConstants<> "
-                "URBG does not generate sufficient random bits.");
-
-  static_assert(kRange == 0 ||
-                    kRangeBits < std::numeric_limits<result_type>::digits,
-                "Class-template FastUniformBitsURBGConstants<> "
-                "URBG range computation error.");
-};
-
-// FastUniformBitsLoopingConstants computes the looping constants used
-// by FastUniformBits::Generate. These constants indicate how multiple
-// URBG::result_type values are combined into an output_value.
-// Parameterized by the FastUniformBits parameters:
-//  `UIntType`: output type.
-//  `URNG`: The underlying UniformRandomNumberGenerator.
-//
-// The looping constants describe the sets of loop counters and mask values
-// which control how individual variates are combined the final output.  The
-// algorithm ensures that the number of bits used by any individual call differs
-// by at-most one bit from any other call. This is simplified into constants
-// which describe two loops, with the second loop parameters providing one extra
-// bit per variate.
-//
-// See [rand.adapt.ibits] for more details on the use of these constants.
-template <typename UIntType, typename URBG>
-class FastUniformBitsLoopingConstants {
- private:
-  static constexpr size_t kWidth = std::numeric_limits<UIntType>::digits;
   using urbg_result_type = typename URBG::result_type;
-  using uint_result_type = UIntType;
-
- public:
-  using result_type =
-      typename std::conditional<(sizeof(urbg_result_type) <=
-                                 sizeof(uint_result_type)),
-                                uint_result_type, urbg_result_type>::type;
-
- private:
-  // Estimate N as ceil(width / urng width), and W0 as (width / N).
-  static constexpr size_t kRangeBits =
-      FastUniformBitsURBGConstants<URBG>::kRangeBits;
-
-  // The range of the URNG, max - min + 1, or zero if that result would cause
-  // overflow.
-  static constexpr result_type kRange = constexpr_range<URBG>();
-  static constexpr size_t kEstimateN =
-      kWidth / kRangeBits + (kWidth % kRangeBits != 0);
-  static constexpr size_t kEstimateW0 = kWidth / kEstimateN;
-  static constexpr result_type kEstimateY0 = (kRange >> kEstimateW0)
-                                             << kEstimateW0;
-
- public:
-  // Parameters for the two loops:
-  // kN0, kN1 are the number of underlying calls required for each loop.
-  // KW0, kW1 are shift widths for each loop.
-  //
-  static constexpr size_t kN1 = (kRange - kEstimateY0) >
-                                        (kEstimateY0 / kEstimateN)
-                                    ? kEstimateN + 1
-                                    : kEstimateN;
-  static constexpr size_t kN0 = kN1 - (kWidth % kN1);
-  static constexpr size_t kW0 = kWidth / kN1;
-  static constexpr size_t kW1 = kW0 + 1;
-
-  static constexpr result_type kM0 = (result_type(1) << kW0) - 1;
-  static constexpr result_type kM1 = (result_type(1) << kW1) - 1;
-
-  static_assert(
-      kW0 <= kRangeBits,
-      "Class-template FastUniformBitsLoopingConstants::kW0 too large.");
-
-  static_assert(
-      kW0 > 0,
-      "Class-template FastUniformBitsLoopingConstants::kW0 too small.");
-};
-
-template <typename UIntType>
-template <typename URBG>
-typename FastUniformBits<UIntType>::result_type
-FastUniformBits<UIntType>::operator()(
-    URBG& g) {  // NOLINT(runtime/references)
-  using constants = FastUniformBitsURBGConstants<URBG>;
-  return Generate(
-      g, std::integral_constant<bool, constants::kRangeMask >= (max)()>{});
-}
-
-template <typename UIntType>
-template <typename URBG>
-typename URBG::result_type FastUniformBits<UIntType>::Variate(
-    URBG& g) {  // NOLINT(runtime/references)
-  using constants = FastUniformBitsURBGConstants<URBG>;
-  if (constants::kPowerOfTwo) {
-    return g() - (URBG::min)();
-  }
-
-  // Use rejection sampling to ensure uniformity across the range.
-  typename URBG::result_type u;
-  do {
-    u = g() - (URBG::min)();
-  } while (u > constants::kRangeMask);
-  return u;
+  constexpr urbg_result_type kRangeMask =
+      RangeSize<URBG>() == 0
+          ? (std::numeric_limits<urbg_result_type>::max)()
+          : static_cast<urbg_result_type>(PowerOfTwoSubRangeSize<URBG>() - 1);
+  return Generate(g, std::integral_constant<bool, (kRangeMask >= (max)())>{});
 }
 
 template <typename UIntType>
 template <typename URBG>
 typename FastUniformBits<UIntType>::result_type
-FastUniformBits<UIntType>::Generate(
-    URBG& g,  // NOLINT(runtime/references)
-    std::true_type /* avoid_looping */) {
+FastUniformBits<UIntType>::Generate(URBG& g,  // NOLINT(runtime/references)
+                                    std::true_type /* avoid_looping */) {
   // The width of the result_type is less than than the width of the random bits
-  // provided by URNG.  Thus, generate a single value and then simply mask off
+  // provided by URBG.  Thus, generate a single value and then simply mask off
   // the required bits.
-  return Variate(g) & (max)();
+
+  return PowerOfTwoVariate(g) & (max)();
 }
 
 template <typename UIntType>
 template <typename URBG>
 typename FastUniformBits<UIntType>::result_type
-FastUniformBits<UIntType>::Generate(
-    URBG& g,  // NOLINT(runtime/references)
-    std::false_type /* avoid_looping */) {
-  // The width of the result_type is wider than the number of random bits
-  // provided by URNG. Thus we merge several variates of URNG into the result
-  // using a shift and mask.  The constants type generates the parameters used
-  // ensure that the bits are distributed across all the invocations of the
-  // underlying URNG.
-  using constants = FastUniformBitsLoopingConstants<UIntType, URBG>;
+FastUniformBits<UIntType>::Generate(URBG& g,  // NOLINT(runtime/references)
+                                    std::false_type /* avoid_looping */) {
+  // See [rand.adapt.ibits] for more details on the constants calculated below.
+  //
+  // It is preferable to use roughly the same number of bits from each generator
+  // call, however this is only possible when the number of bits provided by the
+  // URBG is a divisor of the number of bits in `result_type`. In all other
+  // cases, the number of bits used cannot always be the same, but it can be
+  // guaranteed to be off by at most 1. Thus we run two loops, one with a
+  // smaller bit-width size (`kSmallWidth`) and one with a larger width size
+  // (satisfying `kLargeWidth == kSmallWidth + 1`). The loops are run
+  // `kSmallIters` and `kLargeIters` times respectively such
+  // that
+  //
+  //    `kTotalWidth == kSmallIters * kSmallWidth
+  //                    + kLargeIters * kLargeWidth`
+  //
+  // where `kTotalWidth` is the total number of bits in `result_type`.
+  //
+  constexpr size_t kTotalWidth = std::numeric_limits<result_type>::digits;
+  constexpr size_t kUrbgWidth = NumBits<URBG>();
+  constexpr size_t kTotalIters =
+      kTotalWidth / kUrbgWidth + (kTotalWidth % kUrbgWidth != 0);
+  constexpr size_t kSmallWidth = kTotalWidth / kTotalIters;
+  constexpr size_t kLargeWidth = kSmallWidth + 1;
+  //
+  // Because `kLargeWidth == kSmallWidth + 1`, it follows that
+  //
+  //     `kTotalWidth == kTotalIters * kSmallWidth + kLargeIters`
+  //
+  // and therefore
+  //
+  //     `kLargeIters == kTotalWidth % kSmallWidth`
+  //
+  // Intuitively, each iteration with the large width accounts for one unit
+  // of the remainder when `kTotalWidth` is divided by `kSmallWidth`. As
+  // mentioned above, if the URBG width is a divisor of `kTotalWidth`, then
+  // there would be no need for any large iterations (i.e., one loop would
+  // suffice), and indeed, in this case, `kLargeIters` would be zero.
+  constexpr size_t kLargeIters = kTotalWidth % kSmallWidth;
+  constexpr size_t kSmallIters =
+      (kTotalWidth - (kLargeWidth * kLargeIters)) / kSmallWidth;
+
+  static_assert(
+      kTotalWidth == kSmallIters * kSmallWidth + kLargeIters * kLargeWidth,
+      "Error in looping constant calculations.");
 
   result_type s = 0;
-  for (size_t n = 0; n < constants::kN0; ++n) {
-    auto u = Variate(g);
-    s = (s << constants::kW0) + (u & constants::kM0);
+
+  constexpr size_t kSmallShift = kSmallWidth % kTotalWidth;
+  constexpr result_type kSmallMask = MaskFromShift(result_type{kSmallShift});
+  for (size_t n = 0; n < kSmallIters; ++n) {
+    s = (s << kSmallShift) +
+        (static_cast<result_type>(PowerOfTwoVariate(g)) & kSmallMask);
   }
-  for (size_t n = constants::kN0; n < constants::kN1; ++n) {
-    auto u = Variate(g);
-    s = (s << constants::kW1) + (u & constants::kM1);
+
+  constexpr size_t kLargeShift = kLargeWidth % kTotalWidth;
+  constexpr result_type kLargeMask = MaskFromShift(result_type{kLargeShift});
+  for (size_t n = 0; n < kLargeIters; ++n) {
+    s = (s << kLargeShift) +
+        (static_cast<result_type>(PowerOfTwoVariate(g)) & kLargeMask);
   }
+
+  static_assert(
+      kLargeShift == kSmallShift + 1 ||
+          (kLargeShift == 0 &&
+           kSmallShift == std::numeric_limits<result_type>::digits - 1),
+      "Error in looping constant calculations");
+
   return s;
 }
 
diff --git a/absl/random/internal/fast_uniform_bits_test.cc b/absl/random/internal/fast_uniform_bits_test.cc
index 183779445a52..9f2e82687dd6 100644
--- a/absl/random/internal/fast_uniform_bits_test.cc
+++ b/absl/random/internal/fast_uniform_bits_test.cc
@@ -18,6 +18,8 @@
 
 #include "gtest/gtest.h"
 
+namespace absl {
+namespace random_internal {
 namespace {
 
 template <typename IntType>
@@ -29,7 +31,7 @@ TYPED_TEST_SUITE(FastUniformBitsTypedTest, IntTypes);
 
 TYPED_TEST(FastUniformBitsTypedTest, BasicTest) {
   using Limits = std::numeric_limits<TypeParam>;
-  using FastBits = absl::random_internal::FastUniformBits<TypeParam>;
+  using FastBits = FastUniformBits<TypeParam>;
 
   EXPECT_EQ(0, FastBits::min());
   EXPECT_EQ(Limits::max(), FastBits::max());
@@ -45,91 +47,226 @@ TYPED_TEST(FastUniformBitsTypedTest, BasicTest) {
   }
 }
 
-class UrngOddbits {
- public:
-  using result_type = uint8_t;
-  static constexpr result_type min() { return 1; }
-  static constexpr result_type max() { return 0xfe; }
-  result_type operator()() { return 2; }
-};
+template <typename UIntType, UIntType Lo, UIntType Hi, UIntType Val = Lo>
+struct FakeUrbg {
+  using result_type = UIntType;
 
-class Urng4bits {
- public:
-  using result_type = uint8_t;
-  static constexpr result_type min() { return 1; }
-  static constexpr result_type max() { return 0xf + 1; }
-  result_type operator()() { return 2; }
+  static constexpr result_type(max)() { return Hi; }
+  static constexpr result_type(min)() { return Lo; }
+  result_type operator()() { return Val; }
 };
 
-class Urng32bits {
- public:
-  using result_type = uint32_t;
-  static constexpr result_type min() { return 0; }
-  static constexpr result_type max() { return 0xffffffff; }
-  result_type operator()() { return 1; }
-};
+using UrngOddbits = FakeUrbg<uint8_t, 1, 0xfe, 0x73>;
+using Urng4bits = FakeUrbg<uint8_t, 1, 0x10, 2>;
+using Urng31bits = FakeUrbg<uint32_t, 1, 0xfffffffe, 0x60070f03>;
+using Urng32bits = FakeUrbg<uint32_t, 0, 0xffffffff, 0x74010f01>;
 
-// Compile-time test to validate the helper classes used by FastUniformBits
-TEST(FastUniformBitsTest, FastUniformBitsDetails) {
-  using absl::random_internal::FastUniformBitsLoopingConstants;
-  using absl::random_internal::FastUniformBitsURBGConstants;
+TEST(FastUniformBitsTest, IsPowerOfTwoOrZero) {
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint8_t{0}));
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint8_t{1}));
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint8_t{2}));
+  EXPECT_FALSE(IsPowerOfTwoOrZero(uint8_t{3}));
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint8_t{16}));
+  EXPECT_FALSE(IsPowerOfTwoOrZero(uint8_t{17}));
+  EXPECT_FALSE(IsPowerOfTwoOrZero((std::numeric_limits<uint8_t>::max)()));
 
-  // 4-bit URBG
-  {
-    using constants = FastUniformBitsURBGConstants<Urng4bits>;
-    static_assert(constants::kPowerOfTwo == true,
-                  "constants::kPowerOfTwo == false");
-    static_assert(constants::kRange == 16, "constants::kRange == false");
-    static_assert(constants::kRangeBits == 4, "constants::kRangeBits == false");
-    static_assert(constants::kRangeMask == 0x0f,
-                  "constants::kRangeMask == false");
-  }
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint16_t{0}));
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint16_t{1}));
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint16_t{2}));
+  EXPECT_FALSE(IsPowerOfTwoOrZero(uint16_t{3}));
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint16_t{16}));
+  EXPECT_FALSE(IsPowerOfTwoOrZero(uint16_t{17}));
+  EXPECT_FALSE(IsPowerOfTwoOrZero((std::numeric_limits<uint16_t>::max)()));
 
-  // ~7-bit URBG
-  {
-    using constants = FastUniformBitsURBGConstants<UrngOddbits>;
-    static_assert(constants::kPowerOfTwo == false,
-                  "constants::kPowerOfTwo == false");
-    static_assert(constants::kRange == 0xfe, "constants::kRange == 0xfe");
-    static_assert(constants::kRangeBits == 7, "constants::kRangeBits == 7");
-    static_assert(constants::kRangeMask == 0x7f,
-                  "constants::kRangeMask == 0x7f");
-  }
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint32_t{0}));
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint32_t{1}));
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint32_t{2}));
+  EXPECT_FALSE(IsPowerOfTwoOrZero(uint32_t{3}));
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint32_t{32}));
+  EXPECT_FALSE(IsPowerOfTwoOrZero(uint32_t{17}));
+  EXPECT_FALSE(IsPowerOfTwoOrZero((std::numeric_limits<uint32_t>::max)()));
+
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint64_t{0}));
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint64_t{1}));
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint64_t{2}));
+  EXPECT_FALSE(IsPowerOfTwoOrZero(uint64_t{3}));
+  EXPECT_TRUE(IsPowerOfTwoOrZero(uint64_t{64}));
+  EXPECT_FALSE(IsPowerOfTwoOrZero(uint64_t{17}));
+  EXPECT_FALSE(IsPowerOfTwoOrZero((std::numeric_limits<uint64_t>::max)()));
+}
+
+TEST(FastUniformBitsTest, IntegerLog2) {
+  EXPECT_EQ(IntegerLog2(uint16_t{0}), 0);
+  EXPECT_EQ(IntegerLog2(uint16_t{1}), 0);
+  EXPECT_EQ(IntegerLog2(uint16_t{2}), 1);
+  EXPECT_EQ(IntegerLog2(uint16_t{3}), 1);
+  EXPECT_EQ(IntegerLog2(uint16_t{4}), 2);
+  EXPECT_EQ(IntegerLog2(uint16_t{5}), 2);
+  EXPECT_EQ(IntegerLog2(std::numeric_limits<uint64_t>::max()), 63);
+}
+
+TEST(FastUniformBitsTest, RangeSize) {
+  EXPECT_EQ((RangeSize<FakeUrbg<uint8_t, 0, 3>>()), 4);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint8_t, 2, 2>>()), 1);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint8_t, 2, 5>>()), 4);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint8_t, 2, 6>>()), 5);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint8_t, 2, 10>>()), 9);
+  EXPECT_EQ(
+      (RangeSize<FakeUrbg<uint8_t, 0, std::numeric_limits<uint8_t>::max()>>()),
+      0);
+
+  EXPECT_EQ((RangeSize<FakeUrbg<uint16_t, 0, 3>>()), 4);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint16_t, 2, 2>>()), 1);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint16_t, 2, 5>>()), 4);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint16_t, 2, 6>>()), 5);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint16_t, 1000, 1017>>()), 18);
+  EXPECT_EQ((RangeSize<
+                FakeUrbg<uint16_t, 0, std::numeric_limits<uint16_t>::max()>>()),
+            0);
+
+  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 0, 3>>()), 4);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 2, 2>>()), 1);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 2, 5>>()), 4);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 2, 6>>()), 5);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 1000, 1017>>()), 18);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 0, 0xffffffff>>()), 0);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 1, 0xffffffff>>()), 0xffffffff);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 1, 0xfffffffe>>()), 0xfffffffe);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint32_t, 2, 0xfffffffe>>()), 0xfffffffd);
+  EXPECT_EQ((RangeSize<
+                FakeUrbg<uint32_t, 0, std::numeric_limits<uint32_t>::max()>>()),
+            0);
+
+  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 0, 3>>()), 4);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 2, 2>>()), 1);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 2, 5>>()), 4);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 2, 6>>()), 5);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 1000, 1017>>()), 18);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 0, 0xffffffff>>()), 0x100000000ull);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 1, 0xffffffff>>()), 0xffffffffull);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 1, 0xfffffffe>>()), 0xfffffffeull);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 2, 0xfffffffe>>()), 0xfffffffdull);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 0, 0xffffffffffffffffull>>()), 0ull);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 1, 0xffffffffffffffffull>>()),
+            0xffffffffffffffffull);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 1, 0xfffffffffffffffeull>>()),
+            0xfffffffffffffffeull);
+  EXPECT_EQ((RangeSize<FakeUrbg<uint64_t, 2, 0xfffffffffffffffeull>>()),
+            0xfffffffffffffffdull);
+  EXPECT_EQ((RangeSize<
+                FakeUrbg<uint64_t, 0, std::numeric_limits<uint64_t>::max()>>()),
+            0);
+}
+
+TEST(FastUniformBitsTest, PowerOfTwoSubRangeSize) {
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint8_t, 0, 3>>()), 4);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint8_t, 2, 2>>()), 1);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint8_t, 2, 5>>()), 4);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint8_t, 2, 6>>()), 4);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint8_t, 2, 10>>()), 8);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<
+                FakeUrbg<uint8_t, 0, std::numeric_limits<uint8_t>::max()>>()),
+            0);
+
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint16_t, 0, 3>>()), 4);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint16_t, 2, 2>>()), 1);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint16_t, 2, 5>>()), 4);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint16_t, 2, 6>>()), 4);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint16_t, 1000, 1017>>()), 16);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<
+                FakeUrbg<uint16_t, 0, std::numeric_limits<uint16_t>::max()>>()),
+            0);
+
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint32_t, 0, 3>>()), 4);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint32_t, 2, 2>>()), 1);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint32_t, 2, 5>>()), 4);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint32_t, 2, 6>>()), 4);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint32_t, 1000, 1017>>()), 16);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint32_t, 0, 0xffffffff>>()), 0);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint32_t, 1, 0xffffffff>>()),
+            0x80000000);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint32_t, 1, 0xfffffffe>>()),
+            0x80000000);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<
+                FakeUrbg<uint32_t, 0, std::numeric_limits<uint32_t>::max()>>()),
+            0);
+
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 0, 3>>()), 4);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 2, 2>>()), 1);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 2, 5>>()), 4);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 2, 6>>()), 4);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 1000, 1017>>()), 16);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 0, 0xffffffff>>()),
+            0x100000000ull);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 1, 0xffffffff>>()),
+            0x80000000ull);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 1, 0xfffffffe>>()),
+            0x80000000ull);
+  EXPECT_EQ(
+      (PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 0, 0xffffffffffffffffull>>()),
+      0);
+  EXPECT_EQ(
+      (PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 1, 0xffffffffffffffffull>>()),
+      0x8000000000000000ull);
+  EXPECT_EQ(
+      (PowerOfTwoSubRangeSize<FakeUrbg<uint64_t, 1, 0xfffffffffffffffeull>>()),
+      0x8000000000000000ull);
+  EXPECT_EQ((PowerOfTwoSubRangeSize<
+                FakeUrbg<uint64_t, 0, std::numeric_limits<uint64_t>::max()>>()),
+            0);
 }
 
 TEST(FastUniformBitsTest, Urng4_VariousOutputs) {
   // Tests that how values are composed; the single-bit deltas should be spread
   // across each invocation.
   Urng4bits urng4;
+  Urng31bits urng31;
   Urng32bits urng32;
 
   // 8-bit types
   {
-    absl::random_internal::FastUniformBits<uint8_t> fast8;
+    FastUniformBits<uint8_t> fast8;
     EXPECT_EQ(0x11, fast8(urng4));
+    EXPECT_EQ(0x2, fast8(urng31));
     EXPECT_EQ(0x1, fast8(urng32));
   }
 
   // 16-bit types
   {
-    absl::random_internal::FastUniformBits<uint16_t> fast16;
+    FastUniformBits<uint16_t> fast16;
     EXPECT_EQ(0x1111, fast16(urng4));
-    EXPECT_EQ(0x1, fast16(urng32));
+    EXPECT_EQ(0xf02, fast16(urng31));
+    EXPECT_EQ(0xf01, fast16(urng32));
   }
 
   // 32-bit types
   {
-    absl::random_internal::FastUniformBits<uint32_t> fast32;
+    FastUniformBits<uint32_t> fast32;
     EXPECT_EQ(0x11111111, fast32(urng4));
-    EXPECT_EQ(0x1, fast32(urng32));
+    EXPECT_EQ(0x0f020f02, fast32(urng31));
+    EXPECT_EQ(0x74010f01, fast32(urng32));
   }
 
   // 64-bit types
   {
-    absl::random_internal::FastUniformBits<uint64_t> fast64;
+    FastUniformBits<uint64_t> fast64;
     EXPECT_EQ(0x1111111111111111, fast64(urng4));
-    EXPECT_EQ(0x0000000100000001, fast64(urng32));
+    EXPECT_EQ(0x387811c3c0870f02, fast64(urng31));
+    EXPECT_EQ(0x74010f0174010f01, fast64(urng32));
   }
 }
 
+TEST(FastUniformBitsTest, URBG32bitRegression) {
+  // Validate with deterministic 32-bit std::minstd_rand
+  // to ensure that operator() performs as expected.
+  std::minstd_rand gen(1);
+  FastUniformBits<uint64_t> fast64;
+
+  EXPECT_EQ(0x05e47095f847c122ull, fast64(gen));
+  EXPECT_EQ(0x8f82c1ba30b64d22ull, fast64(gen));
+  EXPECT_EQ(0x3b971a3558155039ull, fast64(gen));
+}
+
 }  // namespace
+}  // namespace random_internal
+}  // namespace absl
diff --git a/absl/strings/str_join.h b/absl/strings/str_join.h
index 7345b9627bdd..4772f5d17c89 100644
--- a/absl/strings/str_join.h
+++ b/absl/strings/str_join.h
@@ -254,7 +254,7 @@ std::string StrJoin(const Range& range, absl::string_view separator,
 
 template <typename T, typename Formatter>
 std::string StrJoin(std::initializer_list<T> il, absl::string_view separator,
-               Formatter&& fmt) {
+                    Formatter&& fmt) {
   return strings_internal::JoinRange(il, separator, fmt);
 }
 
@@ -275,7 +275,8 @@ std::string StrJoin(const Range& range, absl::string_view separator) {
 }
 
 template <typename T>
-std::string StrJoin(std::initializer_list<T> il, absl::string_view separator) {
+std::string StrJoin(std::initializer_list<T> il,
+                    absl::string_view separator) {
   return strings_internal::JoinRange(il, separator);
 }
 
diff --git a/absl/synchronization/internal/create_thread_identity.cc b/absl/synchronization/internal/create_thread_identity.cc
index 6e93605de061..ce3676a88482 100644
--- a/absl/synchronization/internal/create_thread_identity.cc
+++ b/absl/synchronization/internal/create_thread_identity.cc
@@ -31,7 +31,8 @@ namespace synchronization_internal {
 
 // ThreadIdentity storage is persistent, we maintain a free-list of previously
 // released ThreadIdentity objects.
-static base_internal::SpinLock freelist_lock(base_internal::kLinkerInitialized);
+static base_internal::SpinLock freelist_lock(
+    base_internal::kLinkerInitialized);
 static base_internal::ThreadIdentity* thread_identity_freelist;
 
 // A per-thread destructor for reclaiming associated ThreadIdentity objects.
diff --git a/absl/time/time.h b/absl/time/time.h
index 0534780516d0..e236870bd829 100644
--- a/absl/time/time.h
+++ b/absl/time/time.h
@@ -179,6 +179,7 @@ class Duration {
   Duration& operator%=(Duration rhs);
 
   // Overloads that forward to either the int64_t or double overloads above.
+  // Integer operands must be representable as int64_t.
   template <typename T>
   Duration& operator*=(T r) {
     int64_t x = r;
@@ -221,6 +222,7 @@ inline Duration operator+(Duration lhs, Duration rhs) { return lhs += rhs; }
 inline Duration operator-(Duration lhs, Duration rhs) { return lhs -= rhs; }
 
 // Multiplicative Operators
+// Integer operands must be representable as int64_t.
 template <typename T>
 Duration operator*(Duration lhs, T rhs) {
   return lhs *= rhs;
@@ -375,7 +377,8 @@ constexpr Duration InfiniteDuration();
 // Hours()
 //
 // Factory functions for constructing `Duration` values from an integral number
-// of the unit indicated by the factory function's name.
+// of the unit indicated by the factory function's name. The number must be
+// representable as int64_t.
 //
 // Note: no "Days()" factory function exists because "a day" is ambiguous.
 // Civil days are not always 24 hours long, and a 24-hour duration often does
diff --git a/ci/linux_clang-latest_libcxx_asan_bazel.sh b/ci/linux_clang-latest_libcxx_asan_bazel.sh
index 4d1d28a9c571..07af64d0fee4 100755
--- a/ci/linux_clang-latest_libcxx_asan_bazel.sh
+++ b/ci/linux_clang-latest_libcxx_asan_bazel.sh
@@ -67,13 +67,21 @@ for std in ${STD}; do
         --compilation_mode=${compilation_mode} \
         --copt="-DDYNAMIC_ANNOTATIONS_ENABLED=1" \
         --copt="-DADDRESS_SANITIZER" \
+        --copt="-DUNDEFINED_BEHAVIOR_SANITIZER" \
         --copt="-fsanitize=address" \
+        --copt="-fsanitize=float-divide-by-zero" \
+        --copt="-fsanitize=nullability" \
+        --copt="-fsanitize=undefined" \
+        --copt="-fno-sanitize=vptr" \
         --copt=-Werror \
         --keep_going \
         --linkopt="-fsanitize=address" \
+        --linkopt="-fsanitize-link-c++-runtime" \
         --show_timestamps \
         --test_env="ASAN_SYMBOLIZER_PATH=/opt/llvm/clang/bin/llvm-symbolizer" \
         --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
+        --test_env="UBSAN_OPTIONS=print_stacktrace=1" \
+        --test_env="UBSAN_SYMBOLIZER_PATH=/opt/llvm/clang/bin/llvm-symbolizer" \
         --test_output=errors \
         --test_tag_filters="-benchmark,-noasan" \
         ${BAZEL_EXTRA_ARGS:-}