[Bug tree-optimization/120283] New: `Transform comparisons of the form X +- C1 CMP C2 to X CMP C2 -+ C1` gimple depends on single use

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120283

            Bug ID: 120283
           Summary: `Transform comparisons of the form X +- C1 CMP C2 to X
                    CMP C2 -+ C1` gimple depends on single use
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: internal-improvement
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: pinskia at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
            Blocks: 120206
  Target Milestone: ---

Forwprop for comparisons proping uses generic rather than gimple so it will
cause to ignore single use in many cases.

So in this case of this pattern:
```
/* Transform comparisons of the form X +- C1 CMP C2 to X CMP C2 -+ C1.  */
(for cmp (eq ne)
 (for op (plus minus)
      rop (minus plus)
  (simplify
   (cmp (op@3 @0 INTEGER_CST@1) INTEGER_CST@2)
   (if (!TREE_OVERFLOW (@1) && !TREE_OVERFLOW (@2)
        && !TYPE_OVERFLOW_SANITIZED (TREE_TYPE (@0))
        && !TYPE_OVERFLOW_TRAPS (TREE_TYPE (@0))
        && !TYPE_SATURATING (TREE_TYPE (@0)))
    (with { tree res = int_const_binop (rop, @2, @1); }
     (if (TREE_OVERFLOW (res)
          && TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (@0)))
      { constant_boolean_node (cmp == NE_EXPR, type); }
      (if (single_use (@3))
       (cmp @0 { TREE_OVERFLOW (res)
                 ? drop_tree_overflow (res) : res; }))))))))
```

forwprop ignores the single use there as it does combining into generic and
then uses generic to do the simplification.

I am thinking single use is not important here after all and has been ignored
anyways.

The pattern with single use was added with r7-502-ga8b85ce9e2bffc.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120206
[Bug 120206] Removal of
forward_propagate_into_gimple_cond/forward_propagate_into_comparison