This is the mail archive of the
gcc-bugs@gcc.gnu.org
mailing list for the GCC project.
[Bug c/67435] Large performance drop on apparently unrelated changes (potential cause : hot loop alignment)
- From: "trippels at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Fri, 04 Sep 2015 10:23:42 +0000
- Subject: [Bug c/67435] Large performance drop on apparently unrelated changes (potential cause : hot loop alignment)
- Auto-submitted: auto-generated
- References: <bug-67435-4 at http dot gcc dot gnu dot 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