ternary operator for clang's extended vectors

问题

I've tried playing with clang's extended vectors. The ternary operator is supposed to work, but it is not working for me. Example:

int main()
{
  using int4 = int __attribute__((ext_vector_type(4)));

  int4 a{0, 1, 3, 4};
  int4 b{2, 1, 4, 5};

  auto const r(a - b ? a : b);

  return 0;
}

Please provide examples on how I might make it work, like it works under OpenCL. I am using clang-3.4.2.

Error:

t.cpp:8:16: error: value of type 'int __attribute__((ext_vector_type(4)))' is not contextually convertible to 'bool'
  auto const r(a - b ? a : b);
               ^~~~~
1 error generated.

回答1:

You can loop over the elements directly in Clang. Here is a solution for GCC and Clang.

#include <inttypes.h>
#include <x86intrin.h>

#if defined(__clang__)
typedef float float4 __attribute__ ((ext_vector_type(4)));
typedef   int   int4 __attribute__ ((ext_vector_type(4)));
#else
typedef float float4 __attribute__ ((vector_size (sizeof(float)*4)));
typedef   int   int4 __attribute__ ((vector_size (sizeof(int)*4)));
#endif

float4 select(int4 s, float4 a, float4 b) {
  float4 c;
  #if defined(__GNUC__) && !defined(__INTEL_COMPILER) && !defined(__clang__)
  c = s ? a : b;
  #else
  for(int i=0; i<4; i++) c[i] = s[i] ? a[i] : b[i];
  #endif
  return c;
}

The both generate

select(int __vector(4), float __vector(4), float __vector(4)):
  pxor xmm3, xmm3
  pcmpeqd xmm0, xmm3
  blendvps xmm1, xmm2, xmm0
  movaps xmm0, xmm1
  ret

Nehalem: https://godbolt.org/g/cVWYym
Skylake: https://godbolt.org/g/LhEpnN
KNL: https://godbolt.org/g/NFrFKg

But with AVX512 it's better to use masks (e.g. __mmask16).

回答2:

This works in a pinch:

auto const diff = a-b;
auto const ra( - (diff!=zero) * a - (diff==zero) *b);

I guess this is a bug in the compiler, or in the documentation you linked.

回答3:

In the end I went with this:

#if defined(__clang__)
template <typename U, typename V>
constexpr inline std::enable_if_t<
  !std::is_arithmetic<V>{},
  V
>
select(V const a, V const b, U const c) noexcept
{
  return V((c & U(a)) | (~c & U(b)));
}
#else
template <typename U, typename V>
constexpr inline std::enable_if_t<
  !std::is_arithmetic<V>{},
  V
>
select(V const a, V const b, U const c) noexcept
{
  return c ? a : b;
}
#endif

The same could have been accomplished in other ways, using the indices trick, for example, but it might not optimize very well (I didn't want any conditionals in there).

来源：https://stackoverflow.com/questions/25345585/ternary-operator-for-clangs-extended-vectors

标签

c++

clang

ternary-operator

simd