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
