Get index of first element that is not zero in a __m256 variable

Question

__m256  dst = _mm256_cmp_ps(value1, value2, _CMP_LE_OQ);

If dst is [0,0,0,-nan, 0,0,0,-nan]; I want to be able to know the first -nan index, in this case 3 without doing a for loop with 8 iterations. Is this possible?

Answer 1

I would movmskps the result of the comparison and then do a bitscan forward.

Using intrinsics (this works with gcc/clang, see here for alternatives):

int pos = __builtin_ctz(_mm256_movemask_ps(dst));

Note that the result of bsf is unspecified if no bit is set. To work around this you can, e.g., write this to get 8, if no other bit is set:

int pos = __builtin_ctz(_mm256_movemask_ps(dst) | 0x100);