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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/99927] [11 Regression] Maybe wrong code since
 r11-39-gf9e1ea10e657af9f
Date: Tue, 06 Apr 2021 14:44:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99927

--- 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.=