[Bug middle-end/46900] [4.6 Regression] 50% slowdown when linking with LTO in a single step
burnus at gcc dot gnu.org
gcc-bugzilla@gcc.gnu.org
Sun Dec 12 10:39:00 GMT 2010
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46900
Tobias Burnus <burnus at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target Milestone|--- |4.6.0
Summary|50% slowdown when linking |[4.6 Regression] 50%
|with LTO in a single step |slowdown when linking with
| |LTO in a single step
--- Comment #3 from Tobias Burnus <burnus at gcc dot gnu.org> 2010-12-12 10:39:17 UTC ---
The linkage seems to be a regression. If I compile with GCC 4.5, I get for the
direct dgemm call the same performance if I link in a single step:
$ gfortran-4.5 -fexternal-blas -flto -O3 -ffast-math -march=native \
test.f90 dgemm.f lsame.f xerbla.f
$ ./a.out
Time, MATMUL: 1.4160880 53.480084765505403
dgemm: 1.0840679 56.452265589399069
(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.)
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