[Bug target/122998] RISC-V optimization: builtin_sub_overflow(unsigned) can make sub-optimal code

[law at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122998

Jeffrey A. Law <law at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2025-12-07
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #1 from Jeffrey A. Law <law at gcc dot gnu.org> ---
I suspect fixing this likely requires work in gimple.

;;   basic block 2, loop depth 0
;;    pred:       ENTRY
  _6 = .SUB_OVERFLOW (x_4(D), y_5(D));
  _2 = IMAGPART_EXPR <_6>;
  if (_2 != 0) 
    goto <bb 4>; [21.72%]
  else  
    goto <bb 3>; [78.28%]
;;    succ:       4
;;                3 

;;   basic block 3, loop depth 0
;;    pred:       2 
  _1 = REALPART_EXPR <_6>;
;;    succ:       4

;;   basic block 4, loop depth 0
;;    pred:       2 
;;                3
  # _3 = PHI <291(2), _1(3)>
  return _3;

In that form there's really nothing to do.  The subtraction is speculative and
it's going to be fairly hard to sink the subtraction later in the RTL pipeline.