Export of internal Abseil changes.

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3f04cd3c25a99df91ff913977b8c5b343532db5d by Abseil Team <absl-team@google.com>:

Stricter memory order constraints for CycleClock callback.

PiperOrigin-RevId: 242670115

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216db48375306490f1722a11aaf33080939d9f2f by Abseil Team <absl-team@google.com>:

internal/optional.h: move macro from types/optional.h

ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS is only used within this file.
additionally check the macro with #ifdef rather than #if, fixes -Wundef
warning:
'ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS' is not defined, evaluates to 0
PiperOrigin-RevId: 242548205

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fbe22e7d8dc5c0b3d43ac26297e97ddbaeab3d39 by Samuel Benzaquen <sbenza@google.com>:

Implement %f natively for any input.
It evaluates the input at runtime and allocates stack space accordingly.

This removes a potential fallback into snprintf, improves performance, and removes all memory allocations in this formatting path.

PiperOrigin-RevId: 242531736

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1458f9ba2a79ef0534e46527cd34770dee54164d by Greg Falcon <gfalcon@google.com>:

Add explicit check for NVCC in compressed_tuple.h.

NVCC claims to be MSVC, but does not implement this MSVC attribute.

PiperOrigin-RevId: 242513453
GitOrigin-RevId: 3f04cd3c25a99df91ff913977b8c5b343532db5d
Change-Id: I0742e8619c5248c7607961113e406486bc0e279b

1 files changed, 16 insertions, 2 deletions

diff --git a/absl/base/internal/cycleclock.cc b/absl/base/internal/cycleclock.cc
index 4b553c2..e9844b7 100644
--- a/absl/base/internal/cycleclock.cc
+++ b/absl/base/internal/cycleclock.cc
@@ -55,10 +55,23 @@ static constexpr int32_t kShift = 2;
 static constexpr double kFrequencyScale = 1.0 / (1 << kShift);
 static std::atomic<CycleClockSourceFunc> cycle_clock_source;
 
+CycleClockSourceFunc LoadCycleClockSource() {
+  // Optimize for the common case (no callback) by first doing a relaxed load;
+  // this is significantly faster on non-x86 platforms.
+  if (cycle_clock_source.load(std::memory_order_relaxed) == nullptr) {
+    return nullptr;
+  }
+  // This corresponds to the store(std::memory_order_release) in
+  // CycleClockSource::Register, and makes sure that any updates made prior to
+  // registering the callback are visible to this thread before the callback is
+  // invoked.
+  return cycle_clock_source.load(std::memory_order_acquire);
+}
+
 }  // namespace
 
 int64_t CycleClock::Now() {
-  auto fn = cycle_clock_source.load(std::memory_order_relaxed);
+  auto fn = LoadCycleClockSource();
   if (fn == nullptr) {
     return base_internal::UnscaledCycleClock::Now() >> kShift;
   }
@@ -70,7 +83,8 @@ double CycleClock::Frequency() {
 }
 
 void CycleClockSource::Register(CycleClockSourceFunc source) {
-  cycle_clock_source.store(source, std::memory_order_relaxed);
+  // Corresponds to the load(std::memory_order_acquire) in LoadCycleClockSource.
+  cycle_clock_source.store(source, std::memory_order_release);
 }
 
 #else