Re: gcc: poor log() performance on Intel x86_64

[Andrew Haley <aph at redhat dot com>]

Torsten Rohlfing writes:
 > 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

You have weird hardware.

[aph@zorro ~]$ time ./testlog64

real    0m7.330s
user    0m7.328s
sys     0m0.004s
[aph@zorro ~]$ time ./testlog32

real    0m7.538s
user    0m7.528s
sys     0m0.000s

Andrew.