[Bug tree-optimization/119471] New: (a * b) != 0 then we know that both a!=0 & b != 0

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119471

            Bug ID: 119471
           Summary: (a * b) != 0 then we know that both a!=0 & b != 0
           Product: gcc
           Version: 14.1.0
            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
            Blocks: 85316, 110992
  Target Milestone: ---

Take:
```
int fa(int a, int b)
{
  int c = a * b;
  if (c != 0)
    return (a != 0);
  return 0;
}
int fb(int a, int b)
{
  int c = a * b;
  if (c != 0)
    return (b != 0);
  return 0;
}
```

This should optimize to just `return (a*b) != 0` (well rather `a & b != 0` but
that is for a different issue). as we know if we multiply together 2 integers
then both of them has to be non-zero.

This is the missing optimization that is causing PR 110992 not to be optimized
any more after the `&-` -> `*` simplification.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85316
[Bug 85316] [meta-bug] VRP range propagation missed cases
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110992
[Bug 110992] [13/14/15 Regression] missed VRP optimization due to
transformation of `a & -zero_one_valued_p` into `a * zero_one_valued_p`