Re: program executin times differ between gcc versions

[Andrew Haley <aph-gcc at littlepinkcloud dot COM>]

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.