I think it is a glibc issue.
H.J.
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On Tue, Mar 14, 2006 at 01:18:34PM -0800, Torsten Rohlfing wrote:
> Greetings.
>
> I am experiencing a major performance problem with the log() function on
> the x86_64 platform. It can be illustrated with the following little
> test program:
>
> ====testlog.cxx=======
> #include <math.h>
>
> main()
> {
> float f = 0;
> for ( int i = 0; i < 1e8; ++i )
> f += log( i );
> }
> ==================
>
> I compile this twice, on the same machine, once as a 64bit binary and
> once as 32bits:
>
> g++ -mtune=nocona -msse -msse2 -msse3 -O3 -o testlog64 testlog.cxx
> g++ -m32 -mtune=nocona -msse -msse2 -msse3 -O3 -o testlog32 testlog.cx
>
> Compiler config is:
>
> Using built-in specs.
> Target: x86_64-redhat-linux
> Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
> --infodir=/usr/share/info --enable-shared --enable-threads=posix
> --enable-checking=release --with-system-zlib --enable-__cxa_atexit
> --disable-libunwind-exceptions --enable-libgcj-multifile
> --enable-languages=c,c++,objc,java,f95,ada --enable-java-awt=gtk
> --with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre
> --host=x86_64-redhat-linux
> Thread model: posix
> gcc version 4.0.2 20051125 (Red Hat 4.0.2-8)
>
> When I run the two binaries on the exact same box and time them, I get
> the following outputs:
>
> time ./testlog64
> 13.264u 0.000s 0:13.26 100.0% 0+0k 0+0io 0pf+0w
>
> time ./testlog32
> 6.960u 0.004s 0:06.96 100.0% 0+0k 0+0io 0pf+0w
>
> In other words, the log function is approximately twice as fast in the
> 32bit binary as it is in the 64bit binary. Does anyone have any idea
> what this is caused by or how I could further diagnose the problem? It
> seems that using logf() I get approximately identical performance for
> both targets, but unfortunately then the results I get are slightly
> different for the two, and that's also not acceptable.
>
> Thanks to everyone in advance for any insights you may be able to provide!
> Torsten
>
> --
> Torsten Rohlfing, PhD SRI International, Neuroscience Program
> Research Scientist 333 Ravenswood Ave, Menlo Park, CA 94025
> Phone: ++1 (650) 859-3379 Fax: ++1 (650) 859-2743
> [EMAIL PROTECTED] http://www.stanford.edu/~rohlfing/
>
> "Though this be madness, yet there is a method in't"
