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Re: Compilation flags in libgfortran
- From: Kyrill Tkachov <kyrylo dot tkachov at arm dot com>
- To: "pinskia at gmail dot com" <pinskia at gmail dot com>
- Cc: Igor Zamyatin <izamyatin at gmail dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>, Yuri Rumyantsev <ysrumyan at gmail dot com>
- Date: Wed, 16 Oct 2013 11:22:45 +0100
- Subject: Re: Compilation flags in libgfortran
- Authentication-results: sourceware.org; auth=none
- References: <CAKdSQZk4gBwLOXPNL+Z+Nc8476S0acuFMKYS3QLG=hkzzLCqbQ at mail dot gmail dot com> <D1AD949E-E6A9-44F3-BE2F-E850B53139BF at gmail dot com>
On 16/10/13 10:37, pinskia@gmail.com wrote:
On Oct 15, 2013, at 6:58 AM, Igor Zamyatin <izamyatin@gmail.com> wrote:
Hi All!
Is there any particular reason that matmul* modules from libgfortran
are compiled with -O2 -ftree-vectorize?
I see some regressions on Atom processor after r202980
(http://gcc.gnu.org/ml/gcc-cvs/2013-09/msg00846.html)
Why not just use O3 for those modules?
-O3 and -O2 -ftree-vectorize won't give much performance difference. What you are seeing is the cost model needs improvement; at least for atom.
Hi all,
I think http://gcc.gnu.org/ml/gcc-patches/2013-09/msg01908.html introduced the
new "cheap" vectoriser cost model that favors compilation time over runtime
performance and is set as default for -O2. -O3 uses the "dynamic" model which
potentially gives better runtime performance in exchange for longer compile
times (if I understand the new rules correctly).
Therefore, I'd expect -O3 to give a better vector performance than -O2...
Kyrill