[Bug tree-optimization/127292] New: `(a > 1) != a` should be optimized to `a != 0`

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127292

            Bug ID: 127292
           Summary: `(a > 1) != a` should be optimized to `a != 0`
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
            Blocks: 126043
  Target Milestone: ---

Reduced from testsuite/gcc.dg/tree-ssa/pr108357.c  after a phiopt change but
can be shown without it.

Take:
```
int f23(int a)
{
  int t = a > 1;
  return t != a;
}
int f23_1(int a)
{
  int t;
  if (a > 1)
    t = 1;
  else
    t = 0;
  return t != a;
}
int f23_2(int a)
{
  int t;
  if (a > 1)
    t = 1;
  else
    t = 0;
  if (t != a)
    t = 1;
  else
    t = 0;
  return t;
}
```

These all three should produce the same code.
Currently only f23_2 produces the best at `a!=0`.

Note LLVM is able to handle f23_1 and f23_2 but not f23.

This is related to PR 110293 but slightly different.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126043
[Bug 126043] early phiopt should allow CMP, (cast)CMP, -(cast)CMP