https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98957
--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
The first change is during cse1:
(insn 10 9 11 2 (parallel [
(set (reg:HI 90)
(and:HI (subreg:HI (reg:QI 89) 0)
(const_int 1 [0x1])))
(clobber (reg:CC 17 flags))
]) "pr98957.c":3:14 489 {*andhi_1}
(expr_list:REG_DEAD (reg:QI 89)
(expr_list:REG_UNUSED (reg:CC 17 flags)
(nil))))
(insn 11 10 12 2 (set (reg:QI 86)
(subreg:QI (reg:HI 90) 0)) "pr98957.c":3:14 77 {*movqi_internal}
(expr_list:REG_DEAD (reg:HI 90)
(nil)))
(insn 12 11 16 2 (set (reg:QI 83 [ <retval> ])
(subreg:QI (reg:HI 90) 0)) "pr98957.c":3:14 77 {*movqi_internal}
(expr_list:REG_DEAD (reg:QI 86)
(nil)))
(insn 16 12 17 2 (set (reg/i:QI 0 ax)
- (subreg:QI (reg:HI 90) 0)) "pr98957.c":4:1 77 {*movqi_internal}
+ (reg:QI 86)) "pr98957.c":4:1 77 {*movqi_internal}
(expr_list:REG_DEAD (reg:QI 83 [ <retval> ])
(nil)))
and that then changes how combine handles it.
