Re: [PATCH] A new reload-rewrite pattern recognizer for GCC vectorizer.

[Jakub Jelinek <jakub at redhat dot com>]

On Thu, Apr 24, 2014 at 05:32:54PM -0700, Cong Hou wrote:
> In this patch a new reload-rewrite pattern detector is composed to
> handle the following pattern in the loop being vectorized:
> 
>    x = *p;
>    ...
>    y = *p;
> 
>    or
> 
>    *p = x;
>    ...
>    y = *p;
> 
> In both cases, *p is reloaded because there may exist other defs to
> another memref that may alias with p.  However, aliasing is eliminated
> with alias checks.  Then we can safely replace the last statement in
> above cases by y = x.

Not safely, at least not for #pragma omp simd/#pragma simd/#pragma ivdep
loops if alias analysis hasn't proven there is no aliasing.

So, IMNSHO you need to guard this with LOOP_VINFO_NO_DATA_DEPENDENCIES,
assuming it has been computed at that point already (otherwise you need to
do it elsewhere).

Consider:

void
foo (int *p, int *q)
{
  int i;
  #pragma omp simd safelen(16)
  for (i = 0; i < 128; i++)
    {
      int x = *p;
      *q += 8;
      *p = *p + x;
      p++;
      q++;
    }
}

It is valid to call the above with completely unrelated p and q, but
also e.g. p == q, or q == p + 16 or p == q + 16.
Your patch would certainly break it e.g. for p == q.

	Jakub