[Bug middle-end/121509] [16 Regression] runtime differences with -march=znver3

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121509

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|needs-reduction             |missed-optimization

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
This is also a missed optimization in final value replacement which fails to
replace the PHI.  Of course it requires a high level of obfuscation to get this
past unrolling, VRP and other opts that would do this.

Testcase:

long g_73[2] = {6L,6L};
int __GIMPLE (ssa,startwith("loop")) __attribute__((noipa))
foo ()
{
  signed char g;
  int l;
  int _1;
  unsigned char _3;
  unsigned char _4;

  __BB(2):
  goto __BB3;

  __BB(3,loop_header(1)):
  l_5 = __PHI (__BB2: _Literal (int) -511973466, __BB3: 1);
  g_6 = __PHI (__BB2: _Literal (signed char) 0, __BB3: g_12);
  _1 = (int) g_6;
  g_73[_1] = 0l;
  _3 = (unsigned char) g_6;
  _4 = _3 + _Literal (unsigned char) 1;
  g_12 = (signed char) _4;
  if (g_12 > _Literal (signed char) 1)
    goto __BB4;
  else
    goto __BB3;

  __BB(4):
  l_14 = __PHI (__BB3: l_5);
  return l_14;
}

int main()
{
  if (foo () != 1)
    __builtin_abort ();
  return 0;
}