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From: "burnus at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/46900] [4.6 Regression] 50% slowdown when linking with LTO in a single step
Date: Sun, 12 Dec 2010 10:50:00 -0000 [thread overview]
Message-ID: <bug-46900-4-Ss3iVAu1jE@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-46900-4@http.gcc.gnu.org/bugzilla/>
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46900
--- Comment #4 from Tobias Burnus <burnus at gcc dot gnu.org> 2010-12-12 10:50:09 UTC ---
(In reply to comment #3)
> (I don't understand why the MATMUL part differs that much - it should call the
> same BLAS function [via the same GCC 4.6 libgfortran.so wrapper] and LTO should
> not affect it.)
Seemingly, LTO is crucial for 4.5 - without LTO dgemm gets slower but the
libgfortran version gets faster:
$ gfortran-4.5 -fexternal-blas -O3 -ffast-math -march=native test.f90 dgemm.f
lsame.f xerbla.f && ./a.out
Time, MATMUL: 1.3200819 53.480084765505403
dgemm: 1.3120821 56.452265589399069
$ gfortran-4.5 -c -flto -fexternal-blas -O3 -ffast-math -march=native test.f90
dgemm.f lsame.f xerbla.f
$ gfortran-4.5 -flto -O3 -ffast-math -march=native test.o dgemm.o lsame.o
xerbla.o
$ ./a.out
Time, MATMUL: 1.3080810 53.480084765505403
dgemm: 1.0800680 56.452265589399069
Here, for GCC 4.5, one sees that for the direct call of dgemm, LTO improves the
performance - and doing a single step compilation+linkage or in two steps does
not matter.
However, also for GCC 4.5 the single-step pessimizes the performance of the
libgfortran MATMUL (which is a wrapper for dgemm).
next prev parent reply other threads:[~2010-12-12 10:50 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-12-12 10:31 [Bug fortran/46900] New: " burnus at gcc dot gnu.org
2010-12-12 10:33 ` [Bug middle-end/46900] " burnus at gcc dot gnu.org
2010-12-12 10:34 ` burnus at gcc dot gnu.org
2010-12-12 10:39 ` [Bug middle-end/46900] [4.6 Regression] " burnus at gcc dot gnu.org
2010-12-12 10:50 ` burnus at gcc dot gnu.org [this message]
2010-12-16 14:40 ` rguenth at gcc dot gnu.org
2010-12-16 15:24 ` burnus at gcc dot gnu.org
2010-12-17 12:14 ` d.g.gorbachev at gmail dot com
2011-01-19 17:10 ` rguenth at gcc dot gnu.org
2011-01-19 22:11 ` burnus at gcc dot gnu.org
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