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Optimize atomic_thread_fence #740

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merged 8 commits into from
Apr 30, 2020
Merged

Optimize atomic_thread_fence #740

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ad3adbd
Fix #739 (without saving stack variable)
AlexGuteniev Apr 23, 2020
764c824
Suppress warning, explain it
AlexGuteniev Apr 23, 2020
d38cf4b
Quotes
AlexGuteniev Apr 23, 2020
1ec772f
Avoid destination register by using xor
AlexGuteniev Apr 24, 2020
a09db16
`lock inc` has smaller encoding
AlexGuteniev Apr 24, 2020
2eb0922
Update stl/inc/atomic
AlexGuteniev Apr 25, 2020
61cd41b
Shorten comment to avoid enforced bad newline
AlexGuteniev Apr 25, 2020
44a7f31
Merge branch 'optimize_atomic_thread_fence
AlexGuteniev Apr 25, 2020
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11 changes: 9 additions & 2 deletions stl/inc/atomic
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Original file line number Diff line number Diff line change
Expand Up @@ -1942,8 +1942,15 @@ extern "C" inline void atomic_thread_fence(const memory_order _Order) noexcept {
#else // ^^^ ARM32/ARM64 hardware / x86/x64 hardware vvv
_Compiler_barrier();
if (_Order == memory_order_seq_cst) {
static long _Guard;
(void) _InterlockedCompareExchange(&_Guard, 0, 0);
volatile long _Guard; // Not initialized to avoid an unnecessary operation; the value does not matter

// _mm_mfence could have been used, but it is not supported on older x86 CPUs and is slower on some recent CPUs.
// The memory fence provided by interlocked operations has some exceptions, but this is fine:
// std::atomic_thread_fence works with respect to other atomics only; it may not be a full fence for all ops.
#pragma warning(suppress : 6001) // "Using uninitialized memory '_Guard'"
#pragma warning(suppress : 28113) // "Accessing a local variable _Guard via an Interlocked function: This is an unusual
// usage which could be reconsidered."
(void) _InterlockedIncrement(&_Guard);
_Compiler_barrier();
}
#endif // hardware
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