https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94798
--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
As for the subtraction followed by comparison, I think this is something
normally optimized through peephole2, but that one doesn't trigger in this case
due to RA decisions, where it sees different operands:
(insn 33 4 26 2 (set (reg:SI 0 ax [86])
(reg/v:SI 4 si [orig:84 b ] [84])) "pr94798.c":4:29 67
{*movsi_internal}
(nil))
(insn 26 33 27 2 (parallel [
(set (reg:SI 0 ax [86])
(minus:SI (reg:SI 0 ax [86])
(reg/v:SI 5 di [orig:83 a ] [83])))
(clobber (reg:CC 17 flags))
]) "pr94798.c":4:29 254 {*subsi_1}
(expr_list:REG_UNUSED (reg:CC 17 flags)
(nil)))
(insn 27 26 31 2 (set (reg:CCGC 17 flags)
(compare:CCGC (reg/v:SI 4 si [orig:84 b ] [84])
(reg/v:SI 5 di [orig:83 a ] [83]))) "pr94798.c":4:29 11 {*cmpsi_1}
(expr_list:REG_DEAD (reg/v:SI 5 di [orig:83 a ] [83])
(expr_list:REG_DEAD (reg/v:SI 4 si [orig:84 b ] [84])
(nil))))
