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FW: very very very slow when optimized


 
Dear List:
We have some software , mostly written by me, in C ,  that seems to
become very very slow when optimized. The user (who has also written
some of the routines) complained that it was taking longer than it
should -- but when compiled for debugging without any -O options and
with -g  suddenly it was much faster! (34 seconds instead of 500
seconds!)
 
 
So far we have repeated a test case
using gcc 3.3.3   gcc (GCC) 3.3.3 (SuSE Linux)
with:
 
-O3 -march=nocona (slow)
-O3 (slow)
-O2 (slow)
[no -O flag] (fast)
-g (fast)
 
and then with self-built gcc4.0.2 that I just compiled, with similar
results 
gccv4 (GCC) 4.0.2   Copyright (C) 2005 Free Software Foundation, Inc.
 
-O3 (slow)
-g   (fast)
 
 
This is on a dual Intel em64t Xeon running Suse gnu/linux  9.1 + "yast
online updates" , Linux kernel  2.6.5-7.151-smp from Suse, The machine
has 8Gb memory and the job uses about 2Gb as seen in the Top output:
 
 PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
14454 someuser  39  19 2153m 1.2g 1296 R 99.9 15.6 132:57.79 my_program

I suggested also trying -m32 and also building and running on another
machine that has ia32 (Pentium 4 Xeon) processors also using gcc-3.3.3
(not Suse though, I compiled it on the Red hat machine) but .. he's gone
home for the day... 
 
Before I spend more time attempting to locate the problem, perhaps
someone can recommend how I can best do so? Or, are there some really
obvious reasons why -O3 would make it slow down dramatically? Other
information about the machine /compiler that would help?
 
Thanks
Robert
 
 
 
 
 


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