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

14 files changed, 516 insertions, 387 deletions

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);