[Bug tree-optimization/128474] New: transform `a/b >= CST` into `a >= b*CST` if `b*CST` does not overflow (unsigned only?)

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128474

            Bug ID: 128474
           Summary: transform `a/b >= CST` into `a >= b*CST` if `b*CST`
                    does not overflow (unsigned only?)
           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
  Target Milestone: ---

Take:
```
#define CST 2
unsigned src(unsigned long long a, unsigned y)
{
    unsigned long long b = y;
    unsigned long long d = a/b;
    return d >= CST; // `d > (CST-1)`
}

unsigned tgt(unsigned long long a, unsigned y)
{
    unsigned long long b = y;
    unsigned long long t = b*CST; // won't overflow
    return a >= t;
}
```

src can be translated to tgt as b*CST does not overflow.
We already do this for a const y via the following pattern:
```
/* X / C1 op C2 into a simple range test.  */
(for cmp (simple_comparison)
 (simplify
  (cmp (trunc_div:s @0 INTEGER_CST@1) INTEGER_CST@2)
```
But this could be generalized too.


Note this is a generalized form from
https://github.com/llvm/llvm-project/issues/238952 .