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From: "segher at gcc dot gnu.org" <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 22:07:51 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107172

--- Comment #28 from Segher Boessenkool <segher at gcc dot gnu.org> ---
> So the issue is with the consumer:
>=20
> (insn 50 49 51 2 (parallel [
>             (set (reg:SI 93)
>                 (neg:SI (ltu:SI (reg:CCC 17 flags)
>                         (const_int 0 [0]))))
>             (clobber (reg:CC 17 flags))
>         ]) "107172.c":4:10 1258 {*x86_movsicc_0_m1_neg}
>      (expr_list:REG_DEAD (reg:CCC 17 flags)
>         (expr_list:REG_UNUSED (reg:CC 17 flags)
>             (nil))))
>=20
> There are many similar patterns in different backends.  They work as long=
 as
> the flags register isn't a known constant since simplify-rtx.cc leaves th=
em
> alone.  They become a problem only when the flags register is a known
> constant.

Such patterns are fine.  The problem is that this consumer of MODE_CC does =
not
fit together with the producer of that reg 17: it only has meaning together,
that
is how this stuff works; and it has no meaning at all like this.=