How to examine a 256i (16-bit) vector to know if it contains any element greater than zero?

Question

I am converting a vectorized code from SSE2 intrinsics to AVX2 intrinsics, and would like to know how to check if a 256i (16-bit) vector contains any element greater than zero or not. Below is the code used in the SSE2:

int check2(__m128i vector1, __m128i vector2)
{
  __m128i vcmp =  _mm_cmplt_epi16(vector2, vector1);
  int cmp = _mm_movemask_epi8(vcmp);
  return ((cmp>0) ? 1 : 0) ;
}

I thought that the following code will work, bit it didn't.

int check2(__m256i vector1, __m256i vector2)
{
  __m256i vcmp = _mm256_cmpgt_epi16(vector1, vector2);
  int cmp = _mm256_movemask_epi8(vcmp);
  return ((cmp>0) ? 1 : 0) ;
}

I would be thankful if somebody can advise

Answer 1

I think you just have a trivial bug - your function should be:

int check2(__m256i vector1, __m256i vector2)
{
    __m256i vcmp = _mm256_cmpgt_epi16(vector1, vector2);
    int cmp = _mm256_movemask_epi8(vcmp);
    return cmp != 0;
}

The problem is that _mm256_movemask_epi8 returns 32 bit flags as a signed int, and you were testing this for > 0. Obviously if the MS bit is 1 then this test will fail (since the result will be < 0). You did not see this problem with the SSE version because it only returns 16 bits.