[Bug tree-optimization/27855] New: reassociation pass produces ~30% slower matrix multiplication code

["uros at kss-loka dot si" <gcc-bugzilla at gcc dot gnu dot org>]

The testcase from PR target/27827 shows another problem, this time with
-ffast-math. The runtime performance of -ffast-math code drops for ~30%.

The problem could be traced down to reassociation tree pass, because the
performance jumps back when "flag_unsafe_math_optimizations" switch is disabled
by changing every occurence in tree-ssa-reassoc.c with
(flag_unsafe_math_optimizations && 0).

To see the problem, -funsafe-math-optimizations should be added to MMFLAGS in
target/27827 example Makefile:

MM4FLAGS = $(GMMFLAGS) -funsafe-math-optimizations


Current mainline gcc produces code with following results:

-O -mfpmath=387

ALGORITHM     NB   REPS        TIME      MFLOPS
=========  =====  =====  ==========  ==========

atlasmm       60   1000       0.260     1663.04

-O -msse2 -mfpmath=sse

ALGORITHM     NB   REPS        TIME      MFLOPS
=========  =====  =====  ==========  ==========

atlasmm       60   1000       0.229     1890.47


gcc with disabled reassoc pass for floating point values:

-O -mfpmath=387

ALGORITHM     NB   REPS        TIME      MFLOPS
=========  =====  =====  ==========  ==========

atlasmm       60   1000       0.162     2664.87

-O -msse2 -mfpmath=sse

ALGORITHM     NB   REPS        TIME      MFLOPS
=========  =====  =====  ==========  ==========

atlasmm       60   1000       0.164     2633.15


-- 
           Summary: reassociation pass produces ~30% slower matrix
                    multiplication code
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: uros at kss-loka dot si
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu
OtherBugsDependingO 27827
             nThis:


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27855