[BUG] DoNotOptimize build break with GCC

[marcalff]

See downstream bug:

• Build error: read-only reference ‘value’ used as ‘asm’ output open-telemetry/opentelemetry-cpp#2343

When building the following code:

void BM_ParentBasedSamplerConstruction(benchmark::State &state)
{
  while (state.KeepRunning())
  {
    benchmark::DoNotOptimize(ParentBasedSampler(std::make_shared<AlwaysOnSampler>())); // line 49
  }
}

and

void BM_TraceIdRatioBasedSamplerConstruction(benchmark::State &state)
{
  while (state.KeepRunning())
  {
    benchmark::DoNotOptimize(TraceIdRatioBasedSampler(0.01)); // line 58
  }
}

the build fails with GCC 7.4.1, GCC 7.5.0, GCC 12.3.0.

The error reported is:

[ 56%] Building CXX object sdk/test/trace/CMakeFiles/sampler_benchmark.dir/sampler_benchmark.cc.o
In file included from /home/malff/CODE/MARC_GITHUB/opentelemetry-cpp/sdk/test/trace/sampler_benchmark.cc:17:
/opt/benchmark-1.8.2/include/benchmark/benchmark.h: In instantiation of ‘typename std::enable_if<((! std::is_trivially_copyable<_Tp>::value) || (sizeof (Tp) > sizeof (Tp*)))>::type benchmark::DoNotOptimize(Tp&&) [with Tp = opentelemetry::v1::sdk::trace::ParentBasedSampler; typename std::enable_if<((! std::is_trivially_copyable<_Tp>::value) || (sizeof (Tp) > sizeof (Tp*)))>::type = void]’:
/home/malff/CODE/MARC_GITHUB/opentelemetry-cpp/sdk/test/trace/sampler_benchmark.cc:49:29:   required from here
/opt/benchmark-1.8.2/include/benchmark/benchmark.h:540:3: error: read-only reference ‘value’ used as ‘asm’ output
  540 |   asm volatile("" : "+m"(value) : : "memory");
      |   ^~~
/opt/benchmark-1.8.2/include/benchmark/benchmark.h: In instantiation of ‘typename std::enable_if<((! std::is_trivially_copyable<_Tp>::value) || (sizeof (Tp) > sizeof (Tp*)))>::type benchmark::DoNotOptimize(Tp&&) [with Tp = opentelemetry::v1::sdk::trace::TraceIdRatioBasedSampler; typename std::enable_if<((! std::is_trivially_copyable<_Tp>::value) || (sizeof (Tp) > sizeof (Tp*)))>::type = void]’:
/home/malff/CODE/MARC_GITHUB/opentelemetry-cpp/sdk/test/trace/sampler_benchmark.cc:58:29:   required from here
/opt/benchmark-1.8.2/include/benchmark/benchmark.h:540:3: error: read-only reference ‘value’ used as ‘asm’ output
make[2]: *** [sdk/test/trace/CMakeFiles/sampler_benchmark.dir/build.make:76: sdk/test/trace/CMakeFiles/sampler_benchmark.dir/sampler_benchmark.cc.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:4217: sdk/test/trace/CMakeFiles/sampler_benchmark.dir/all] Error 2
make: *** [Makefile:146: all] Error 2

Verified with benchmark 1.8.2 and 1.8.3.

Note that the same code builds fine with CLang.

If this matters, ParentBasedSampler and TraceIdRatioBasedSampler are classes: what is benchmarked here is a call to the constructor.

Note how the benchmark header file is different for GCC, with:

• https://github.com/google/benchmark/blob/main/include/benchmark/benchmark.h#L486-L488

The build failure occurs in:

template <class Tp>
inline BENCHMARK_ALWAYS_INLINE
    typename std::enable_if<!std::is_trivially_copyable<Tp>::value ||
                            (sizeof(Tp) > sizeof(Tp*))>::type
    DoNotOptimize(Tp&& value) {
  asm volatile("" : "+m"(value) : : "memory"); // line 540
}

Expected result is a successful build with both GCC and Clang.

[marcalff]

Reproducible test case:

// Build with:
// g++-12 -I/opt/benchmark-1.8.3/include/ foo.cc
//
// Fails:
//
// In file included from foo.cc:5:
// /opt/benchmark-1.8.3/include/benchmark/benchmark.h: In instantiation of ‘typename std::enable_if<((! std::is_trivially_copyable<_Tp>::value) || (sizeof (Tp) > sizeof (Tp*)))>::type benchmark::DoNotOptimize(Tp&&) [with Tp = TraceIdRatioBasedSampler; typename std::enable_if<((! std::is_trivially_copyable<_Tp>::value) || (sizeof (Tp) > sizeof (Tp*)))>::type = void]’:
// foo.cc:30:29:   required from here
// /opt/benchmark-1.8.3/include/benchmark/benchmark.h:540:3: error: read-only reference ‘value’ used as ‘asm’ output
//   540 |   asm volatile("" : "+m"(value) : : "memory");
//       |   ^~~


#include "benchmark/benchmark.h"

class Sampler
{
public:
  virtual ~Sampler() = default;

  virtual int GetDescription() const noexcept = 0;
};

class TraceIdRatioBasedSampler : public Sampler
{
public:
  explicit TraceIdRatioBasedSampler(double ratio);

  int GetDescription() const noexcept override;

private:
  // uint64_t threshold_;
  const uint64_t threshold_;
};

void BM_TraceIdRatioBasedSamplerConstruction(benchmark::State &state)
{
  while (state.KeepRunning())
  {
    benchmark::DoNotOptimize(TraceIdRatioBasedSampler(0.01));
  }
}

Note that changing

const uint64_t threshold_;

to

uint64_t threshold_;

resolves the build break.

[LebedevRI]

I'm 99% sure this is working as intended as per the deprecation of non-non-const DoNotOptimize,
but the error message is rather sub-par here.