gcc: poor log() performance on Intel x86_64

[Torsten Rohlfing <torsten at synapse dot sri dot com>]

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
  torsten@synapse.sri.com        http://www.stanford.edu/~rohlfing/

    "Though this be madness, yet there is a method in't"