[Bug fortran/29549] matmul slow for complex matrices

["jb at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #4 from jb at gcc dot gnu dot org  2006-11-04 22:16 -------
For the C version with 1d arrays, the benchmark results, with gfortran results
for comparison, are

Complex version:
-O3 funroll-loops -mfpmath=sse -msse2
    1.32
above + fast-math
    0.38
gfortran -O2:
0.32

Real version:
0.07 s
fast-math, same thing.

gfortran -O2 -g
0.07

So it seems the culprit is some optimization that -ffast-math enables that
makes a huge difference for C99 complex arithmetic. However, compiling matmul
in libgfortran with -ffast-math almost certainly won't fly.. 

So ideally we should find exactly what flag enables this performance
improvement, and see if we can enable only that without bringing in all the
-ffast-math baggage. Or then we should bugger the optimizer guys, if this is an
optimization that could be enabled also without -ffast-math.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29549