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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 22:20:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107172

--- Comment #30 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to H.J. Lu from comment #26)
> LTU/GEU are only used to check FLAGS_REG against constant 0.

That is not what
  (ltu (reg 17) (const_int 0))
means though?

Together with a previous
  (set (reg 17) (compare A B))

it means the result of A <=3D B.

There is no actual comparison with 0, that is just notation.

> simplify_const_relational_operation has
>=20
>  /* We can't simplify MODE_CC values since we don't know what the
>      actual comparison is.  */
>   if (GET_MODE_CLASS (GET_MODE (op0)) =3D=3D MODE_CC)
>     return 0;

And combine *does* know how to find the actual comparison, in many cases.  =
Some
other passes can as well, there is no magic involved, you just need to look=
 at
other insns as well (just one really, the CC setter).=