[Bug tree-optimization/92734] Missing match.pd simplification done by fold_binary_loc on generic

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92734

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
We have a set of special-case patterns for simple associations in match.pd, one
for this case seems missing, like the group

  /* Match patterns that allow contracting a plus-minus pair
     irrespective of overflow issues.  */
  /* (A +- B) - A       ->  +- B */
  /* (A +- B) -+ B      ->  A */
  /* A - (A +- B)       -> -+ B */
  /* A +- (B -+ A)      ->  +- B */

closest match is

  /* CST1 - (CST2 - A) -> CST3 + A  */
  (simplify
   (minus CONSTANT_CLASS_P@1 (minus CONSTANT_CLASS_P@2 @0))
   (with { tree cst = const_binop (MINUS_EXPR, type, @1, @2); }
    (if (cst && !TREE_OVERFLOW (cst))
     (plus { cst; } @0))))

which "just" seems to lack a (convert? ...) around the inner minus.

And yes, reassoc would be the pass to handle this, maybe in the very late
if/when
we decide to switch the IL to -fwapv semantics there.