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Deprecate constant reference API to DoNotOptimize. (#1493)

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The compiler assume that a constant reference, even though escaped via asm
volatile, is unchanged.  The const-ref interface is deprecated to discourage
new uses of it, as subtle compiler optimizations (invariant hoisting, etc.) can
occur.

Within microbenchmarks for Abseil's hashtables, BM_FindMiss_Hot
(c0eaa90671/fleetbench/swissmap/hot_swissmap_benchmark.cc (L48))
has a `const uint32_t key` is passed to to the lookup of a hashtable.
With the `key` marked `const`, LLVM hoists part of the lookup
calculation outside of the loop.

With the `const` removed, this hoisting does not occur.

Co-authored-by: Dominic Hamon <dominichamon@users.noreply.github.com>
Co-authored-by: dominic <510002+dmah42@users.noreply.github.com>
This commit is contained in:
Chris Kennelly 2023-02-06 10:50:37 -05:00 committed by GitHub
parent 94083ca441
commit 53df805dc8
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GPG Key ID: 4AEE18F83AFDEB23
1 changed files with 18 additions and 0 deletions
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18
include/benchmark/benchmark.h
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@ -435,6 +435,9 @@ inline BENCHMARK_ALWAYS_INLINE void ClobberMemory() {
#ifndef BENCHMARK_HAS_NO_INLINE_ASSEMBLY
#if !defined(__GNUC__) || defined(__llvm__) || defined(__INTEL_COMPILER)
template <class Tp>
BENCHMARK_DEPRECATED_MSG(
"The const-ref version of this method can permit "
"undesired compiler optimizations in benchmarks")
inline BENCHMARK_ALWAYS_INLINE void DoNotOptimize(Tp const& value) {
asm volatile("" : : "r,m"(value) : "memory");
}
@ -451,6 +454,9 @@ inline BENCHMARK_ALWAYS_INLINE void DoNotOptimize(Tp& value) {
// Workaround for a bug with full argument copy overhead with GCC.
// See: #1340 and https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105519
template <class Tp>
BENCHMARK_DEPRECATED_MSG(
"The const-ref version of this method can permit "
"undesired compiler optimizations in benchmarks")
inline BENCHMARK_ALWAYS_INLINE
typename std::enable_if<std::is_trivially_copyable<Tp>::value &&
(sizeof(Tp) <= sizeof(Tp*))>::type
@ -459,6 +465,9 @@ inline BENCHMARK_ALWAYS_INLINE
}
template <class Tp>
BENCHMARK_DEPRECATED_MSG(
"The const-ref version of this method can permit "
"undesired compiler optimizations in benchmarks")
inline BENCHMARK_ALWAYS_INLINE
typename std::enable_if<!std::is_trivially_copyable<Tp>::value ||
(sizeof(Tp) > sizeof(Tp*))>::type
@ -487,6 +496,9 @@ inline BENCHMARK_ALWAYS_INLINE
// to use memory operations instead of operations with registers.
// TODO: Remove if GCC < 5 will be unsupported.
template <class Tp>
BENCHMARK_DEPRECATED_MSG(
"The const-ref version of this method can permit "
"undesired compiler optimizations in benchmarks")
inline BENCHMARK_ALWAYS_INLINE void DoNotOptimize(Tp const& value) {
asm volatile("" : : "m"(value) : "memory");
}
@ -504,6 +516,9 @@ inline BENCHMARK_ALWAYS_INLINE void ClobberMemory() {
#endif
#elif defined(_MSC_VER)
template <class Tp>
BENCHMARK_DEPRECATED_MSG(
"The const-ref version of this method can permit "
"undesired compiler optimizations in benchmarks")
inline BENCHMARK_ALWAYS_INLINE void DoNotOptimize(Tp const& value) {
internal::UseCharPointer(&reinterpret_cast<char const volatile&>(value));
_ReadWriteBarrier();
@ -514,6 +529,9 @@ inline BENCHMARK_ALWAYS_INLINE void ClobberMemory() { _ReadWriteBarrier(); }
#endif
#else
template <class Tp>
BENCHMARK_DEPRECATED_MSG(
"The const-ref version of this method can permit "
"undesired compiler optimizations in benchmarks")
inline BENCHMARK_ALWAYS_INLINE void DoNotOptimize(Tp const& value) {
internal::UseCharPointer(&reinterpret_cast<char const volatile&>(value));
}