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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 15:37:05 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99927

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|jakub at gcc dot gnu.org           |unassigned at gcc d=
ot gnu.org
             Status|ASSIGNED                    |NEW

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Ah, create_log_links wants to work like that.
So, the bug seems to be that insn 108 has REG_DEAD (reg:CC 17 flags) note.
It doesn't initially, but it is added during 106 -> 108 combination
(gdb) p debug_rtx (i3)
(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]))
            (reg:QI 134)
            (reg:QI 135))) "pr99927.c":13:24 1104 {*movqicc_noc}
     (expr_list:REG_DEAD (reg:QI 134)
        (nil)))
$151 =3D void
(gdb) p debug_rtx (i2)
(insn 106 105 107 6 (parallel [
            (set (reg:QI 134)
                (and:QI (subreg:QI (reg:SI 107) 0)
                    (const_int 1 [0x1])))
            (clobber (reg:CC 17 flags))
        ]) "pr99927.c":13:24 491 {*andqi_1}
     (expr_list:REG_UNUSED (reg:CC 17 flags)
        (nil)))

The combination of those 2 insns is successful - into:
(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))))
but the distribute_notes that turned REG_UNUSED (reg:CC 17 flags) note from
insn 106 into REG_DEAD (reg:CC 17 flags) note on insn 108 is what looks bro=
ken
to me,
the flags register is set by insn 107 in between those two and is used by s=
ome
insns after insn 108 (111 and 85) and the new combined pattern certainly
doesn't
kill the register in any way.

Segher, could you please have a look?
Thanks.=