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From: "trippels at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/67435] Large performance drop on apparently unrelated changes (potential cause : hot loop alignment)
Date: Fri, 04 Sep 2015 10:23:00 -0000	[thread overview]
Message-ID: <bug-67435-4-wN4c1JooCR@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-67435-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67435

--- Comment #9 from Markus Trippelsdorf <trippels at gcc dot gnu.org> ---
(In reply to Yann Collet from comment #8)
> However, for some reason, in the table provided, both Sandy Bridge and
> Haswell get a default loop alignment value of 16. not 32.
> 
> Is there a reason for that choice ?

These values are normally strait out of the Vendors manuals.
And there are also drawbacks to high alignment values.

> Less precise but still great, having the ability to set this optimization
> parameter for a function or a section code would be great. But my experiment
> seem to show that using #pragma or __attribute__ with align-loops does not
> work, as if the optimization setting was simply ignored.

Well, there already is an aligned attribute for functions, variables and
fields,
see: 
http://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes


  parent reply	other threads:[~2015-09-04 10:23 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-09-02 13:24 [Bug c/67435] New: Large performance drop on apparently unrelated changes (probable cause : strange inlining side-effect) yann.collet.73 at gmail dot com
2015-09-02 14:01 ` [Bug c/67435] " trippels at gcc dot gnu.org
2015-09-02 14:03 ` pinskia at gcc dot gnu.org
2015-09-02 14:51 ` yann.collet.73 at gmail dot com
2015-09-03  2:44 ` yann.collet.73 at gmail dot com
2015-09-03  7:19 ` [Bug c/67435] Large performance drop on apparently unrelated changes (potential cause : critical loop instruction alignment) trippels at gcc dot gnu.org
2015-09-03 18:47 ` yann.collet.73 at gmail dot com
2015-09-04 10:23 ` trippels at gcc dot gnu.org [this message]
2015-09-04 10:34 ` [Bug c/67435] Feature request: Implement align-loops attribute trippels at gcc dot gnu.org
2015-10-20 13:09 ` yann.collet.73 at gmail dot com

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