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From: "ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/107172] [13 Regression] wrong code with "-O1 -ftree-vrp"
 on x86_64-linux-gnu since r13-1268-g8c99e307b20c502e
Date: Thu, 13 Oct 2022 08:14:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107172

--- Comment #24 from Uro=C5=A1 Bizjak <ubizjak at gmail dot com> ---
(In reply to Hongtao.liu from comment #23)
> looking at i386.c put_condition_code used by *setcc_qi, it looks like (EQ
> (reg:CCCmode FLAG_REG) (const_int 0)) means get carry flag.
> Not (LTU: (REG:CCCmode FLAGS_REG) (const_int 0)).
> Now I got more confused.

CCCmode means that single flag is tested, it uses EQ and NE, so "c" and "nc"
suffix is emitted. When CCmode is used, LTU/GEU operation on CCmode flags r=
eg
produces "b" and "nb" suffix, which decodes to exactly the same assembly as=
 "c"
and "nc" suffixes.

However, it looks that somewhere LTU/GEU is also generated with CCCmode fla=
gs
reg, and some fixup was introduced to put_condition_code to "fix" this
inconsistency. If LTU/GEU is valid only for CCmode, then the producers of
invalid RTX should be fixed.=