Find min/max value from a __m128i

Question

I want to find the minimum/maximum value into an array of byte using SIMD operations. So far I was able to go through the array and store the minimum/maximum value into a __m128

Answer 1

Alternatively, convert to words and use phminposuw (not tested)

int hminu8(__m128i x)
{
  __m128i l = _mm_unpacklo_epi8(x, _mm_setzero_si128());
  __m128i h = _mm_unpackhi_epi8(x, _mm_setzero_si128());
  l = _mm_minpos_epu16(l);
  h = _mm_minpos_epu16(h);
  return _mm_extract_epi16(_mm_min_epu16(l, h), 0);
}

By my quick count, the latency is a bit worse than a min/shuffle cascade, but the throughput a bit better. The linked answer with phminposuw is probably better though. Adapted for unsigned bytes (but not tested)

uint8_t hminu8(__m128i x)
{
  x = _mm_min_epu8(x, _mm_srli_epi16(x, 8));
  x = _mm_minpos_epu16(x);
  return _mm_cvtsi128_si32(x);
}

You could use it for max too, but with a bit of overhead: complement the input and result.