Re: benchmarking (or almabench)

[Daniel Berlin <dberlin at dberlin dot org>]

On Tuesday, April 22, 2003, at 11:08  AM, S. Bosscher wrote:

> -march=pentium4 is known to pessimise code compared to -march=i686 for 
> some
> benchmarks, see PR 8474.  Maybe you're seeing the same problem?

Actually, if i had to guess, i'd put my money on the vectorization.
Notice ICC vectorized two loops in his example, and obviously, we 
vectorized 0.
:)

If those were compute intensive loops, ....

> Greetz
> Steven
>
>
> -----Original Message-----
> From: Jeremy Sanders
> To: gcc at gcc dot gnu dot org
> Sent: 22-4-03 16:43
> Subject: benchmarking (or almabench)
>
> I've been looking at compiling the almabench benchmark again with gcc.
> See:
>
> http://gcc.gnu.org/ml/gcc/2003-01/msg00037.html
>
> With a pentium4 processor I'm getting drastically different times for
> the
> running the code output from icc and gcc. icc produces code which is up
> to
> 2.7 times faster than gcc code for this program.
>
> (with gcc mainline)
>
> /data/jss/gcc-3.3/bin/g++ -o almabench.o -O2 -mfpmath=sse -msse -msse2
> -march=pentium4 -finline-limit=10000 -c almabench.cpp
> /data/jss/gcc-3.3/bin/g++ -o almabench -O2 -mfpmath=sse -msse -msse2
> -march=pentium4 -finline-limit=10000 almabench.o
> xpc5:/<3>almabench-1.0.1/cpp> time ./almabench
> 31.121u 0.060s 0:33.31 93.6%	0+0k 0+0io 212pf+0w
> xpc5:/<3>almabench-1.0.1/cpp> time ./almabench
> 31.148u 0.052s 0:33.61 92.7%	0+0k 0+0io 212pf+0w
>
> (I've also tried without sse and march, and there's little difference.
> I've also tried fprofile-arcs, which doesn't do anything. inline-limit
> has no real effect).
>
> With icc 7.1.
>
> xpc5:/<3>almabench-1.0.1/cpp> make
> icc -o almabench.o -O2 -c almabench.cpp
> icc -o almabench -O2 almabench.o
> xpc5:/<3>almabench-1.0.1/cpp> time ./almabench
> 16.494u 0.013s 0:17.71 93.1%	0+0k 0+0io 116pf+0w
> xpc5:/<3>almabench-1.0.1/cpp> time ./almabench
> 16.445u 0.029s 0:17.53 93.8%	0+0k 0+0io 116pf+0w
>
> That's 88% faster than gcc.
>
>
> Enabling P4 optimisation (okay gcc can't do vectorization):
>
> xpc5:/<3>almabench-1.0.1/cpp> make
> icc -o almabench.o -O2 -tpp7 -xW -march=pentium4 -c almabench.cpp
> almabench.cpp(219) : (col. 5) remark: LOOP WAS VECTORIZED.
> almabench.cpp(230) : (col. 5) remark: LOOP WAS VECTORIZED.
> icc -o almabench -O2 -tpp7 -xW -march=pentium4 almabench.o
> xpc5:/<3>almabench-1.0.1/cpp> time ./almabench
> 11.318u 0.005s 0:12.09 93.5%	0+0k 0+0io 116pf+0w
> xpc5:/<3>almabench-1.0.1/cpp> time ./almabench
> 11.277u 0.007s 0:12.08 93.2%	0+0k 0+0io 116pf+0w
>
> That's 2.75 times faster than gcc's code.
>
>
> Obviously this benchmark is synthetic, but it suggests gcc isn't
> optimising something in this code very well. We've also seen similar
> effects with other floating-point intensive code. Any suggestions? I 
> can
> supply assembler output for both if anyone would like a look!
>
> Jeremy
>
> --
> Jeremy Sanders <jss at ast dot cam dot ac dot uk>   
> http://www-xray.ast.cam.ac.uk/~jss/
> X-Ray Group, Institute of Astronomy, University of Cambridge, UK.
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