https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99927
--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
It indeed goes wrong in the 105, 107 -> 108 try_combine, but at the start of
that we have:
(insn 105 23 106 6 (set (reg:QI 135)
(const_int 1 [0x1])) "pr99927.c":13:24 77 {*movqi_internal}
(nil))
(note 106 105 107 6 NOTE_INSN_DELETED)
(insn 107 106 108 6 (set (reg:CCZ 17 flags)
(compare:CCZ (reg:SI 107)
(const_int 0 [0]))) "pr99927.c":13:24 7 {*cmpsi_ccno_1}
(nil))
(insn 108 107 111 6 (set (reg:QI 96 [ var_lsm_flag.12 ])
(if_then_else:QI (eq (reg:CCZ 17 flags)
(const_int 0 [0]))
(subreg:QI (reg:SI 107) 0)
(reg:QI 135))) "pr99927.c":13:24 1104 {*movqicc_noc}
(expr_list:REG_DEAD (reg:SI 107)
(expr_list:REG_DEAD (reg:CC 17 flags)
(nil))))
(insn 111 108 85 6 (set (reg:QI 96 [ var_lsm_flag.12 ])
(if_then_else:QI (eq (reg:CCZ 17 flags)
(const_int 0 [0]))
(reg:QI 96 [ var_lsm_flag.12 ])
(reg:QI 135))) "pr99927.c":13:24 1104 {*movqicc_noc}
(expr_list:REG_DEAD (reg:QI 135)
(nil)))
(jump_insn 85 111 35 6 (set (pc)
(if_then_else (ne (reg:CCZ 17 flags)
(const_int 0 [0]))
(label_ref 45)
(pc))) 806 {*jcc}
(expr_list:REG_DEAD (reg:CCZ 17 flags)
(int_list:REG_BR_PROB 536870916 (nil)))
-> 45)
The substitutions of 105 and 107 into 108 properly simplify 108 into
(set (reg:QI 96 [ var_lsm_flag.12 ])
(subreg:QI (reg:SI 107) 0))
because it is:
(set (reg:QI 96 [ var_lsm_flag.12 ])
(if_then_else:QI (ne (reg:SI 107)
(const_int 0 [0]))
(const_int 1 [0x1])
(const_int 0 [0])))
But what is wrong is that try_combine has been called at all, because
(reg:CCZ 17 flags) is used in 3 instructions rather than just one.
