Optimal branchless conditional selection of two SSE2 packed doubles - c++

Question

I'm trying to write a branchless bit select function for packed SSE2 doubles:
#include <iostream>
#include <emmintrin.h>
inline __m128d select(bool expression, const __m128d& x, const __m128d& y)
{
const int conditional_mask = expression ? -1 : 0;
const auto mask = _mm_castsi128_pd(_mm_set_epi64x(conditional_mask, conditional_mask));
return _mm_or_pd(_mm_and_pd(mask, x), _mm_andnot_pd(mask, y));
}
int main()
{
auto r1 = _mm_setr_pd(1, 2);
auto r2 = _mm_setr_pd(5, 6);
auto result = select(true, r1, r2);
auto packed = reinterpret_cast<double*>(&result);
std::cout << "result = " << packed[0] << ", " << packed[1] << std::endl;
std::getchar();
return EXIT_SUCCESS;
}
Is there a simpler approach for SSE2 and SSE4 that would be more optimal on x64?

Answer 1

You've specified that SSE4 is allowed, SSE4.1 has blendvpd so you can blend with a built-in blend: (not tested, but compiled)
inline __m128d select(bool expression, const __m128d& x, const __m128d& y)
{
const int c_mask = expression ? -1 : 0;
const auto mask = _mm_castsi128_pd(_mm_set_epi64x(c_mask, c_mask));
return _mm_blendv_pd(y, x, mask);
}
I would also not take SSE vectors as argument by reference, copying them is trivial so not something to be avoided and taking them by reference encourages the compiler to bounce them through memory (for non-inlined calls).