The problem came up with octopus, http://www.tddft.org/programs/octopus/
http://www.tddft.org/pipermail/octopus-devel/2007-March/003398.html
(Somehow, all my messages are not archived?)
The problem is that "sum(w_re(1:nn,1)*fi(i(1:nn, ii)))" can be much slower. For
the original program, one finds the following timings:
Core 2 Duo + gfortan + gcc (v. 4.1.x), total cpu time:
SSE2 120 s
PLAIN C 160 s
FORTRAN 331 s
(Fortran = assumed shape arrays)
* * *
I tried to reproduce this with a smaller test case (see attachment) - and with
explicit shape arrays. Here, SSE and non-SSE version made little difference.
Result for gcc/gfortran 4.3.0 20070311 on an Athlon 64 X2 4800+.
-O3 -march=opteron -funroll-loops -msse3 -ftree-vectorize -m64:
Fortran: 0.8240519, real 0m7.661s, user 0m7.232s
Fortran: 0.8240528, real 0m7.654s, user 0m7.232s
c_nosse: 0.2320137, real 0m7.071s, user 0m6.652s
c_nosse: 0.2320151, real 0m7.062s, user 0m6.672s
-O3 -march=opteron -msse3 -ftree-vectorize -m32:
Fortran: 0.3840241, real 0m7.714s, user 0m7.280s
Fortran: 0.3840246, real 0m7.701s, user 0m7.328s
c_nosse: 0.3480220, real 0m7.687s, user 0m7.256s
c_nosse: 0.3400207, real 0m7.670s, user 0m7.236s
And with ifort/x86-64:
gcc -std=c99 -O3 -funroll-loops -ftree-vectorize -march=opteron -msse3 -m64
ifort -xW -O3
Fortran: 0.3280210, real 0m0.855s, user 0m0.624s
Fortran: 0.3280210, real 0m0.856s, user 0m0.624s
c_nosse: 0.2320140, real 0m0.753s, user 0m0.492s
c_nosse: 0.2280150, real 0m0.756s, user 0m0.464s
and with ifort/ia32:
Fortran: 0.3000200, real 0m0.818s, user 0m0.516s
Fortran: 0.2960190, real 0m0.826s, user 0m0.528s
c_nosse: 0.3760230, real 0m0.904s, user 0m0.652s
c_nosse: 0.3800240, real 0m0.902s, user 0m0.624s
I did no yet check which of the problems are Fortran, Backend and Target
problems.
Summary:
- GCC -m32 is much slower than -m64
- gfortran is slower (-m32) / much slower (-m64) than the C version
- ifort is faster than gfortran and similarly fast on both -m32 and -m64.
--
Summary: sum(w_re(1:nn,1)*fi(i(1:nn, ii))) up to 3.5x slower
than C version
Product: gcc
Version: 4.3.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: enhancement
Priority: P3
Component: fortran
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: burnus at gcc dot gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31139
