[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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