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From: "roger at nextmovesoftware dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/98713] Failure to generate branch version of abs if user requested it
Date: Mon, 18 Jan 2021 09:19:52 +0000 [thread overview]
Message-ID: <bug-98713-4-RKl2QpEXm3@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-98713-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98713
Roger Sayle <roger at nextmovesoftware dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |roger at nextmovesoftware dot com
--- Comment #2 from Roger Sayle <roger at nextmovesoftware dot com> ---
My first impression is that this isn't a bug, it's a feature. In an optimizing
compiler, the user specifies the computation to be performed and the compiler
selects the implementation. Hence "x+0" isn't a user request to perform an
addition.
Perhaps David could provide more information on why a branch implementation is
required/preferred (for example on which target)? On generic x86_64, I believe
the code currently generated is both smaller and faster. Assuming "neg eax"
takes about the same time as "test edi,edi", and that "cmovs" takes about the
same time as (either branch) of "js".
As a workaround a branch version can be implemented in inline assembly using
__asm, but I'm still hazy as to why this would be desirable.
next prev parent reply other threads:[~2021-01-18 9:19 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-17 20:08 [Bug c/98713] New: " david.bolvansky at gmail dot com
2021-01-18 8:44 ` [Bug middle-end/98713] " marxin at gcc dot gnu.org
2021-01-18 9:19 ` roger at nextmovesoftware dot com [this message]
2021-01-18 9:35 ` rguenth at gcc dot gnu.org
2021-01-18 9:45 ` ubizjak at gmail dot com
2021-01-18 10:31 ` david.bolvansky at gmail dot com
2021-01-18 10:34 ` jakub at gcc dot gnu.org
2021-01-18 11:46 ` roger at nextmovesoftware dot com
2021-01-18 14:03 ` jakub at gcc dot gnu.org
2021-09-26 9:41 ` pinskia at gcc dot gnu.org
2021-09-26 9:42 ` pinskia at gcc dot gnu.org
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