[Bug tree-optimization/96783] [11 Regression] std::rotate miscompiled

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96783

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
Simplified testcase - it's the negative step grouped SLP loads that start at
the wrong address when the VF is > 1.

long a[1024];
long b[1024];

void __attribute__((noipa)) foo ()
{
  for (int i = 0; i < 256; ++i)
    {
      a[3*i] = b[1023 - 3*i - 2];
      a[3*i + 1] = b[1023 - 3*i - 1];
      a[3*i + 2] = b[1023 - 3*i];
    }
}

int main()
{
  for (int i = 0; i < 1024; ++i)
    b[i] = i;
  foo ();
  for (int i = 0; i < 256; ++i)
    if (a[3*i] != 1023 - 3*i - 2
        || a[3*i+1] != 1023 - 3*i - 1
        || a[3*i+2] != 1023 - 3*i)
      __builtin_abort ();
  return 0;
}