Paul Moore writes: > Dear List, > > If this isn't the right place to ask, please advise. > > I've come across different program execution times based on the gcc > version used to compile C code. I understand that this is expected > between compiler generations, but I expected to see better results > with newer compiler generations. In fact, the results were worse. > > The program/shared libraries involved are > - coded in C > - compiled as version 1: gcc 3.3.3 on SLES 9 > as version 2: gcc 4.1.2 on SLES 10 SP1 > - gcc flags used for both compilation were identical: > gcc -c -DLINUX -D_GNU_SOURCE -fPIC -funsigned-char -m32 -g > > The application is single threaded, CPU bound. The utilization of one > CPU core is in both cases near or on 100%. > > The averaged test run times were > - 4m07s for the program compiled with gcc 3.3.3 > - 5m24s for the program compiled with gcc 4.1.2 > > The test runs were performed on the same system, a 8 CPU AMD Opteron > with 2.6 GHz each, SLES 10 SP1. The program does pattern matching and > format generation, nothing fancy. No fp calculations or similar math > stuff is involved. > > Using optimization flags is not possible due to the fact that large > parts of the application failed in -O test runs and would need to be > fixed/tested extensively first. Profiling with -pg/gprof has not been > produced usable results as shared libraries are not being tracked. > > Is there any explanation for this or am I missing something? gcc 4.x generates code in a very different way from 3.x. It's quite likely that at -O0 there will me more temporaries and more intermediate steps. Using optimization is the only way to increase performance. Andrew.
