[Bug tree-optimization/110982] New: (unsigned)(signed_char) != (unsigned)-1 is never changed back into signed_char != -1

[Bug tree-optimization/110982] New: (unsigned)(signed_char) != (unsigned)-1 is never changed back into signed_char != -1

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110982

            Bug ID: 110982
           Summary: (unsigned)(signed_char) != (unsigned)-1 is never
                    changed back into signed_char != -1
           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: ---

While looking into a failure after improving VRP, I see we never translate
this:
```
bool f(signed char t)
{
  unsigned int t1 = t;
  return t1 != (unsigned int)-1;
}
```
into:
```
bool f0(signed char t)
{
  return t != -1;
}
```

[Bug tree-optimization/110982] (unsigned)(signed_char) != (unsigned)-1 is never changed back into signed_char != -1

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110982

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The basic issue is int_fits_type_p does not work the way folks think it works
for casting from signed to unsigned and then casting back ...

[Bug tree-optimization/110982] (unsigned)(signed_char) != (unsigned)-1 is never changed back into signed_char != -1

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110982

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-09-25

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Another case where int_fits_type_p use causes an missed optimization is:
```
unsigned f(int a)
{
        unsigned t = a;
        if (a == -1)
        return t;
        return 0;
}
```

This should be caught in phiopt2 but currently is not due to the
int_fits_type_p usage.

I noticed this in PR 110131 .