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Force DoNotOptimize operand to memory for both gcc and clang

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Albert Pretorius 2016-05-25 07:31:20 +01:00 committed by Albert Pretorius
parent a38f022b5a
commit 74e82e822f
5 changed files with 45 additions and 6 deletions
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1
AUTHORS
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@ -8,6 +8,7 @@
#
# Please keep the list sorted.
Albert Pretorius <pretoalb@gmail.com>
Arne Beer <arne@twobeer.de>
Christopher Seymour <chris.j.seymour@hotmail.com>
David Coeurjolly <david.coeurjolly@liris.cnrs.fr>

1
CONTRIBUTORS
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@ -22,6 +22,7 @@
#
# Please keep the list sorted.
Albert Pretorius <pretoalb@gmail.com>
Arne Beer <arne@twobeer.de>
Billy Robert O'Neal III <billy.oneal@gmail.com> <bion@microsoft.com>
Chris Kennelly <ckennelly@google.com> <ckennelly@ckennelly.com>

10
include/benchmark/benchmark_api.h
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@ -210,20 +210,18 @@ Benchmark* RegisterBenchmarkInternal(Benchmark*);
// expression from being optimized away by the compiler. This function is
// intented to add little to no overhead.
// See: http://stackoverflow.com/questions/28287064
#if defined(__clang__) && defined(__GNUC__)
#if defined(__GNUC__)
// TODO(ericwf): Clang has a bug where it tries to always use a register
// even if value must be stored in memory. This causes codegen to fail.
// To work around this we remove the "r" modifier so the operand is always
// loaded into memory.
// GCC also has a bug where it complains about inconsistent operand constraints
// when "+rm" is used for a type larger than can fit in a register or two.
// For now force the operand to memory for both GCC and Clang.
template <class Tp>
inline BENCHMARK_ALWAYS_INLINE void DoNotOptimize(Tp const& value) {
asm volatile("" : "+m" (const_cast<Tp&>(value)));
}
#elif defined(__GNUC__)
template <class Tp>
inline BENCHMARK_ALWAYS_INLINE void DoNotOptimize(Tp const& value) {
asm volatile("" : "+rm" (const_cast<Tp&>(value)));
}
#else
template <class Tp>
inline BENCHMARK_ALWAYS_INLINE void DoNotOptimize(Tp const& value) {

3
test/CMakeLists.txt
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@ -39,6 +39,9 @@ add_test(diagnostics_test diagnostics_test --benchmark_min_time=0.01)
compile_benchmark_test(skip_with_error_test)
add_test(skip_with_error_test skip_with_error_test --benchmark_min_time=0.01)
compile_benchmark_test(donotoptimize_test)
add_test(donotoptimize_test donotoptimize_test --benchmark_min_time=0.01)
compile_benchmark_test(fixture_test)
add_test(fixture_test fixture_test --benchmark_min_time=0.01)

36
test/donotoptimize_test.cc Normal file
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@ -0,0 +1,36 @@
#include "benchmark/benchmark.h"
#include <cstdint>
namespace {
#if defined(__GNUC__)
std::uint64_t double_up(const std::uint64_t x) __attribute__ ((const));
#endif
std::uint64_t double_up(const std::uint64_t x) {
return x * 2;
}
}
int main(int, char*[]) {
// this test verifies compilation of DoNotOptimize() for some types
char buffer8[8];
benchmark::DoNotOptimize(buffer8);
char buffer20[20];
benchmark::DoNotOptimize(buffer20);
char buffer1024[1024];
benchmark::DoNotOptimize(buffer1024);
benchmark::DoNotOptimize(&buffer1024[0]);
int x = 123;
benchmark::DoNotOptimize(x);
benchmark::DoNotOptimize(&x);
benchmark::DoNotOptimize(x += 42);
benchmark::DoNotOptimize(double_up(x));
return 0;
}