Export of internal Abseil changes

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3d018c03a34bf273a4b24b3584ed77f0a6d21686 by Abseil Team <absl-team@google.com>:

Fix a spelling typo (s/boundries/boundaries).

PiperOrigin-RevId: 442041877
Change-Id: I608020697d37b85316bb9a0838e4b457659c926c

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518b8119e51db24ce7fb0fd2fe537ec43825c3e6 by Dino Radakovic <dinor@google.com>:

absl/types/internal/variant: Make include guard uppercase

https://google.github.io/styleguide/cppguide.html#The__define_Guard

PiperOrigin-RevId: 441911692
Change-Id: I9837dd07f20204d8253f20627b0917a34dc21825

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b91696c38310a7cae8c1ea9e2d479495f5dc3f69 by Greg Falcon <gfalcon@google.com>:

Add an internal-only API to wrap __builtin_prefetch() if available.

This private API is intended for future use by the Abseil implementation.  Like any internal-namespaced function, it may be changed or removed at any time.

PiperOrigin-RevId: 441894616
Change-Id: Iaa48bd4680b373f4a0d5afab0cb35e2a1908595f

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0f01e8b0551a662e02dff60840c54320f987315f by Derek Mauro <dmauro@google.com>:

C++20: Use the standard `constinit` keyword for `ABSL_CONST_INIT` when available

PiperOrigin-RevId: 441778874
Change-Id: I70c616469752ff23b326b1c615437599f42cc6aa
GitOrigin-RevId: 3d018c03a34bf273a4b24b3584ed77f0a6d21686

1 files changed, 20 insertions, 6 deletions

diff --git a/absl/base/attributes.h b/absl/base/attributes.h
index c71038c7..e4e7a3d8 100644
--- a/absl/base/attributes.h
+++ b/absl/base/attributes.h
@@ -682,9 +682,18 @@
 // not compile (on supported platforms) unless the variable has a constant
 // initializer. This is useful for variables with static and thread storage
 // duration, because it guarantees that they will not suffer from the so-called
-// "static init order fiasco".  Prefer to put this attribute on the most visible
-// declaration of the variable, if there's more than one, because code that
-// accesses the variable can then use the attribute for optimization.
+// "static init order fiasco".
+//
+// This attribute must be placed on the initializing declaration of the
+// variable. Some compilers will give a -Wmissing-constinit warning when this
+// attribute is placed on some other declaration but missing from the
+// initializing declaration.
+//
+// In some cases (notably with thread_local variables), `ABSL_CONST_INIT` can
+// also be used in a non-initializing declaration to tell the compiler that a
+// variable is already initialized, reducing overhead that would otherwise be
+// incurred by a hidden guard variable. Thus annotating all declarations with
+// this attribute is recommended to potentially enhance optimization.
 //
 // Example:
 //
@@ -693,14 +702,19 @@
 //     ABSL_CONST_INIT static MyType my_var;
 //   };
 //
-//   MyType MyClass::my_var = MakeMyType(...);
+//   ABSL_CONST_INIT MyType MyClass::my_var = MakeMyType(...);
+//
+// For code or headers that are assured to only build with C++20 and up, prefer
+// just using the standard `constinit` keyword directly over this macro.
 //
 // Note that this attribute is redundant if the variable is declared constexpr.
-#if ABSL_HAVE_CPP_ATTRIBUTE(clang::require_constant_initialization)
+#if defined(__cpp_constinit) && __cpp_constinit >= 201907L
+#define ABSL_CONST_INIT constinit
+#elif ABSL_HAVE_CPP_ATTRIBUTE(clang::require_constant_initialization)
 #define ABSL_CONST_INIT [[clang::require_constant_initialization]]
 #else
 #define ABSL_CONST_INIT
-#endif  // ABSL_HAVE_CPP_ATTRIBUTE(clang::require_constant_initialization)
+#endif
 
 // ABSL_ATTRIBUTE_PURE_FUNCTION
 //