[Bug tree-optimization/127877] [14/15/16/17 Regression] Wrong code with -O2 -march=x86-64-v3: nonlinear shift induction gives 0 instead of -1 since r13-2503-gc13223b790bbc5e4

[rguenth at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127877

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2026-10-13
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot 
gnu.org
             Status|UNCONFIRMED                 |ASSIGNED

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed, mine.  A bit reduced:

__attribute__((noipa)) void
f (unsigned char *bb, int64_t init)
{
  int64_t x = init;
  unsigned s = 0;
  for (int i = 0; i < 256; i++)
    {
      s += bb[i];
      x >>= 3;
    }
  res_x = x;
  res_s = s;
}

int main ()
{
#pragma GCC novector
  for (int i = 0; i < 256; i++)
    b[i] = (unsigned char) (i + 1);
  f (b, -1);
  if (res_x != -1 || res_s != 32640)
    __builtin_abort ();
  return 0;
}