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From: "liuhongt at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/114429] [x86] (neg a) ashifrt>> 31 can be optimized to a
 > 0.
Date: Fri, 22 Mar 2024 05:44:30 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114429

Hongtao Liu <liuhongt at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |x86_64-*-* i?86-*-*

--- Comment #1 from Hongtao Liu <liuhongt at gcc dot gnu.org> ---
when x is INT_MIN, I assume -x is UD, so compiler can do anything.
otherwise, (-x) >> 31 is just x > 0.
>From rtl view. neg of INT_MIN is assumed to 0 after it's truncated.
(neg:m x)
(ss_neg:m x)
(us_neg:m x)
These two expressions represent the negation (subtraction from zero) of the
value represented by x, carried out in mode m. They differ in the behavior =
on
overflow of integer modes. In the case of neg, the negation of the operand =
may
be a number not representable in mode m, in which case it is truncated to m.
ss_neg and us_neg ensure that an out-of-bounds result saturates to the maxi=
mum
or minimum signed or unsigned value.

so we can optimize (neg a)>>31 to a>0.=