Question about the SLP vectorizer failed to perform automatic vectorization in one case

[Hanke Zhang via Gcc]

Hi,
I'm trying to studing the automatic vectorization optimization in GCC,
but I found one case that SLP vectorizer failed to do such things.

Here is the sample code: (also a simplification version of a function
from the 625/525.x264 source code in SPEC CPU 2017)

void pixel_sub_wxh(int16_t *diff, uint8_t *pix1, uint8_t *pix2) {
  for (int y = 0; y < 4; y++) {
    for (int x = 0; x < 4; x++)
      diff[x + y * 4] = pix1[x] - pix2[x];
    pix1 += 16;
    pix2 += 32;
  }
}

When I compiled with `-O3 -mavx2/-msse4.2`, SLP vectorizer failed to
vectorize it, and I got the following message when adding
`-fopt-info-vec-all`. (The inner loop will be unrolled)

<source>:6:21: optimized: loop vectorized using 8 byte vectors
<source>:6:21: optimized:  loop versioned for vectorization because of
possible aliasing
<source>:5:6: note: vectorized 1 loops in function.
<source>:5:6: note: ***** Analysis failed with vector mode V8SI
<source>:5:6: note: ***** The result for vector mode V32QI would be the same
<source>:5:6: note: ***** Re-trying analysis with vector mode V16QI
<source>:5:6: note: ***** Analysis failed with vector mode V16QI
<source>:5:6: note: ***** Re-trying analysis with vector mode V8QI
<source>:5:6: note: ***** Analysis failed with vector mode V8QI
<source>:5:6: note: ***** Re-trying analysis with vector mode V4QI
<source>:5:6: note: ***** Analysis failed with vector mode V4QI

If I manually use the type declaration provided by `immintrin.h` to
rewrite the code, the code is as follows (which I hope the SLP
vectorizer to be able to do)

void pixel_sub_wxh_vec(int16_t *diff, uint8_t *pix1, uint8_t *pix2) {
  for (int y = 0; y < 4; y++) {
    __v4hi pix1_v = {pix1[0], pix1[1], pix1[2], pix1[3]};
    __v4hi pix2_v = {pix2[0], pix2[1], pix2[2], pix2[3]};
    __v4hi diff_v = pix1_v - pix2_v;
    *(long long *)(diff + y * 4) = (long long)diff_v;
    pix1 += 16;
    pix2 += 32;
  }
}

What I want to know is why SLP vectorizer can't vectorize the code
here, and what changes do I need to make to SLP vectorizer or the
source code if I want it to do so?

Thanks
Hanke Zhang