[Bug tree-optimization/114093] New: Canonicalization of `a == -1 || a == 0`

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114093

            Bug ID: 114093
           Summary: Canonicalization of `a == -1 || a == 0`
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
_Bool f1(int a)
{
        return a == -1 || a == 0;
}

_Bool f0(signed a)
{
        a = -a;
        return a == 1 || a == 0;
}


_Bool f(unsigned a)
{
        return a == -1u || a == 0;
}

_Bool f3(unsigned a)
{
        a = -a;
        return a == 1 || a == 0;
}


_Bool f2(unsigned a)
{
        return (-a) <= 1;
}
```

These all should produce the exact same code as they are all equivalent (if we
ignore the (undefined) overflow possibility for f0).

This is more about canonicalizations rather than anything else.

Though I will note on the riscv and mips targets, f is worse than the others.

LLVM canonical form seems to be `((unsigned)a) + 1 <= 1`.