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From: "tnfchris at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/116229] [15 Regression] wrong code at -Ofast aarch64 due to missing fneg to generate 0x8000000000000000
Date: Tue, 06 Aug 2024 06:23:35 +0000	[thread overview]
Message-ID: <bug-116229-4-HkQiaaeAFR@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-116229-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116229

Tamar Christina <tnfchris at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |tnfchris at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

--- Comment #3 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #2)
>   /* For Advanced SIMD we can create an integer with only the top bit set
>      using fneg (0.0f).  */
> 
> is wrong in aarch64_maybe_generate_simd_constant.
> 
> it should use either an unspec here or an XOR instead of fneg here I think
> especially for -ffast-math reasons.

XOR would defeat the point of the optimization. The original expression is fine
but relied on nothing in the late pipeline being able to fold the zero constant
back in.

It was for this reason that we explicitly forced it to a separate register.
Late combine is just doing something not possible before. I'll fix it.

  parent reply	other threads:[~2024-08-06  6:23 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-08-04 18:09 [Bug target/116229] New: " zsojka at seznam dot cz
2024-08-04 22:27 ` [Bug target/116229] [15 Regression] " pinskia at gcc dot gnu.org
2024-08-04 22:38 ` pinskia at gcc dot gnu.org
2024-08-06  6:23 ` tnfchris at gcc dot gnu.org [this message]
2024-08-08 17:52 ` cvs-commit at gcc dot gnu.org
2024-08-08 17:55 ` tnfchris at gcc dot gnu.org

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