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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/99863] [10/11 Regression] wrong code with -O
 -fno-tree-forwprop -mno-sse2 since r10-7268-g529ea7d9596b26ba
Date: Thu, 01 Apr 2021 12:26:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99863

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #10 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I think the REG_DEAD note isn't the problem here.
At least, when I dump bb 2 in rest_of_handle_dse right before df_analyze ca=
ll,
there is
(insn 6 3 7 2 (set (reg:CCZ 17 flags)
        (compare:CCZ (reg/v:DI 203 [ y ])
            (const_int 0 [0]))) "pr99863.c":14:12 8 {*cmpdi_ccno_1}
     (nil))
(insn 7 6 8 2 (set (reg:QI 205)
        (eq:QI (reg:CCZ 17 flags)
            (const_int 0 [0]))) "pr99863.c":14:12 802 {*setcc_qi}
     (expr_list:REG_DEAD (reg:CCZ 17 flags)
        (nil)))
...
(insn 40 38 41 2 (set (reg:QI 220)
        (ne:QI (reg:CCZ 17 flags)
            (const_int 0 [0]))) "pr99863.c":15:11 802 {*setcc_qi}
     (expr_list:REG_DEAD (reg:CCZ 17 flags)
        (nil)))
but if I dump it right after df_analyze call, I see:
(insn 6 3 7 2 (set (reg:CCZ 17 flags)
        (compare:CCZ (reg/v:DI 203 [ y ])
            (const_int 0 [0]))) "pr99863.c":14:12 8 {*cmpdi_ccno_1}
     (expr_list:REG_DEAD (reg/v:DI 203 [ y ])
        (nil)))
(insn 7 6 8 2 (set (reg:QI 205)
        (eq:QI (reg:CCZ 17 flags)
            (const_int 0 [0]))) "pr99863.c":14:12 802 {*setcc_qi}
     (nil))
...
(insn 40 38 41 2 (set (reg:QI 220)
        (ne:QI (reg:CCZ 17 flags)
            (const_int 0 [0]))) "pr99863.c":15:11 802 {*setcc_qi}
     (expr_list:REG_DEAD (reg:CCZ 17 flags)
        (nil)))

i.e. the problematic REG_DEAD note is gone and I think at this point DF kno=
ws
that CC is live there.
But replace_read has code to check for live hard regs, see
note_stores (this_insn, look_for_hardregs, regs_set);
in there etc.=