[C++] Dynamic hook simd level seems not involve sse4.2

[chasingegg]

Describe the usage question you have. Please include as many useful details as possible.

I have briefly go through the building related code, it seems that we can not use SSE4_2 simd level at runtime?

template <typename KernelType>
const KernelType* DispatchExactImpl(const std::vector<KernelType*>& kernels,
                                    const std::vector<TypeHolder>& values) {
  const KernelType* kernel_matches[SimdLevel::MAX] = {nullptr};

  // Validate arity
  for (const auto& kernel : kernels) {
    if (kernel->signature->MatchesInputs(values)) {
      kernel_matches[kernel->simd_level] = kernel;
    }
  }

  // Dispatch as the CPU feature
#if defined(ARROW_HAVE_RUNTIME_AVX512) || defined(ARROW_HAVE_RUNTIME_AVX2)
  auto cpu_info = arrow::internal::CpuInfo::GetInstance();
#endif
#if defined(ARROW_HAVE_RUNTIME_AVX512)
  if (cpu_info->IsSupported(arrow::internal::CpuInfo::AVX512)) {
    if (kernel_matches[SimdLevel::AVX512]) {
      return kernel_matches[SimdLevel::AVX512];
    }
  }
#endif
#if defined(ARROW_HAVE_RUNTIME_AVX2)
  if (cpu_info->IsSupported(arrow::internal::CpuInfo::AVX2)) {
    if (kernel_matches[SimdLevel::AVX2]) {
      return kernel_matches[SimdLevel::AVX2];
    }
  }
#endif
  if (kernel_matches[SimdLevel::NONE]) {
    return kernel_matches[SimdLevel::NONE];
  }

  return nullptr;
}

ARROW_HAVE_RUNTIME_SSE4_2 never used at runtime? Do I miss something, if this is a bug, I'd like to fix it.

Component(s)

C++

[kou]

It seems that we don't have any kernel for SSE4.2. So I think that this is not a bug.

Anyway, do you want to implement a kernel for SSE4.2? Then we can support SSE4.2 in the dispatch code.

[js8544]

Adding to kou's comment, the code you pasted are specific to the arrow compute functions. Currently no compute functions have a SSE kernel implementation so there's no need yet to check SSE in runtime.

There are other places where arrow uses SSE such as in handling csv and json files. ARROW_HAVE_SSE4_2 is used instead of ARROW_HAVE_RUNTIME_SSE4_2 because they don't require runtime dispatching, unlike compute functions.

[chasingegg]

Thanks for your clarification! When I compile arrow by default options, is it supposed to run any x86_64 platforms even without sse4_2 since it will dynamically choose max simd level we can use?

[js8544]

is it supposed to run any x86_64 platforms even without sse4_2

It depends. Do you plan to compile it on a machine with SSE4.2 support and run it on machines without? Then you should propably pass -DARROW_C_FLAGS_${BUILD_TYPE}=-march=<your target architecture> (i.e. cross compiling) to CMake to disable auto-generated SSE4.2 instructions (see this), as well as -DARROW_SIMD_LEVEL=NONE for arrow's explicit SSE4.2 intrinsics.

Also if you plan to use it on Windows, POPCNT may be a required instruction (see #21840).

it will dynamically choose max simd level we can use

Nope. Only the compute functions do this. Other places use the compile time constant ARROW_HAVE_SSE4_2.

[chasingegg]

is it supposed to run any x86_64 platforms even without sse4_2

It depends. Do you plan to compile it on a machine with SSE4.2 support and run it on machines without? Then you should propably pass -DARROW_C_FLAGS_${BUILD_TYPE}=-march=<your target architecture> (i.e. cross compiling) to CMake to disable auto-generated SSE4.2 instructions (see this), as well as -DARROW_HAVE_SSE4_2=OFF for arrow's explicit SSE4.2 intrinsics.

Also if you plan to use it on Windows, POPCNT may be a required instruction (see #21840).

it will dynamically choose max simd level we can use

Nope. Only the compute functions do this. Other places use the compile time constant ARROW_HAVE_SSE4_2.

Thank you for your reply, sorry I don't make my case clear, I want to compile it on a machine with avx and run it on SSE4.2 machine, it should work right? Also as you mentioned, some compile time constant is ARROW_HAVE_SSE4_2.

[js8544]

I see. The default ARROW_SIMD_LEVEL is SSE4_2 so it should be fine.

[chasingegg]

Thanks for your help!