[Bug tree-optimization/84485] New: [6/7 Regression] Vectorising zero-stride rmw operation

["rsandifo at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84485

            Bug ID: 84485
           Summary: [6/7 Regression] Vectorising zero-stride rmw operation
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rsandifo at gcc dot gnu.org
  Target Milestone: ---

r223486 meant that we could vectorise:

void
f (unsigned long incx, unsigned long incy,
   float *restrict dx, float *restrict dy)
{
  unsigned long ix = 0, iy = 0;
  for (unsigned long i = 0; i < 512; ++i)
    {
      dy[iy] += dx[ix];
      ix += incx;
      iy += incy;
    }
}

without first proving that incy is nonzero.  At the time this
particular testcase needed -fno-vect-cost-model on both x86_64 and
aarch64, but by the time of the GCC 6 and 7 releases, the aarch64
vectorisation costs allowed it to be vectorised at -O3.

This was fixed for GCC 8 with r256644, but that's obviously
much too invasive to backport.