[Bug tree-optimization/106523] New: forwprop miscompile

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106523

            Bug ID: 106523
           Summary: forwprop miscompile
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kristerw at gcc dot gnu.org
  Target Milestone: ---

The function f7 from testsuite/c-c++-common/rotate-2.c is miscompiled by
forwprop. This can be seen by running the function as

__attribute__((noinline)) unsigned char
f7 (unsigned char x, unsigned int y)
{
  unsigned int t = x;
  return (t << y) | (t >> ((-y) & 7));
}

int
main (void)
{
  volatile unsigned char x = 152;
  volatile unsigned int y = 19;
  if (f7(x, y) != 4)
    __builtin_abort ();

  return 0;
}

This fails at -O1 and higher optimization levels.

What is happening here is that forwprop1 has optimized the function
to
  _10 = x_7(D) r<< y_9(D);
  return _10;