[Bug rtl-optimization/97603] Failure to optimize out compare into reuse of subtraction result

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97603

--- Comment #5 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Gabriel Ravier from comment #0)
> int g();
> 
> int f(int a, int b)
> {
>     if (a != b)
>         return a - b;
>     return g();
> }
> 
> This can be optimized to using the result of `a - b` to check for `a != b`.

The above testcase is expanded to:

(insn 7 4 8 2 (set (reg:CCGC 17 flags)
        (compare:CCGC (reg/v:SI 99 [ a ])
            (reg/v:SI 100 [ b ]))) "pr97603.c":5:8 15 {*cmpsi_1}
     (nil))
(jump_insn 8 7 9 2 (set (pc)
        (if_then_else (ge (reg:CCGC 17 flags)
                (const_int 0 [0]))
            (label_ref 13)
            (pc))) "pr97603.c":5:8 1490 {*jcc}
     (int_list:REG_BR_PROB 524845001 (nil))
 -> 13)

(note 9 8 10 4 [bb 4] NOTE_INSN_BASIC_BLOCK)
(insn 10 9 11 4 (parallel [
            (set (reg:SI 98 [ <retval> ])
                (minus:SI (reg/v:SI 99 [ a ])
                    (reg/v:SI 100 [ b ])))
            (clobber (reg:CC 17 flags))
        ]) "pr97603.c":6:18 393 {*subsi_1}
     (nil))

Please note that (insn 7) and (insn 10) could be merged to:

(define_insn "*sub<mode>_3"
  [(set (reg FLAGS_REG)
        (compare (match_operand:SWI 1 "nonimmediate_operand" "0,0,r,r,m")
                 (match_operand:SWI 2 "<general_operand>"
"<r><i>,<m>,r<i>,<m>,r<i>")))
   (set (match_operand:SWI 0 "nonimmediate_operand" "=<r>m,<r>,r,r,r")
        (minus:SWI (match_dup 1) (match_dup 2)))]
  "ix86_match_ccmode (insn, CCmode)
   && ix86_binary_operator_ok (MINUS, <MODE>mode, operands, TARGET_APX_NDD)"

As mentioned in PR3507 c#67, this is the job of pre-reload compare elimination
pass.