Hi all, Thanks for your helpful hints. The first thing I did was compare the compiled program's output with -O2 and -O2 -fno-strict-aliasing. The differences did not go away using -fno-strict-aliasing, but it gave me a clue that I should be increasing the precision on the output to check for differences. Below is some output which does not match when viewed at high precision. The output should match, because the number is calculated from the string sequence. (The output is two individuals' fitness in a genetic algorithm if that matters to you.) -O2 0.879923389326927374298747963621281087398529052734375, "dadabda" -O2 -fno-strict-aliasing 0.87992338932692748532105042613693512976169586181640625, "dadabda" This makes me think that there is something going on with calculation of the number from the string of letters. The calculation includes use of exp(), but I think that is the only special thing other than + - / *. I played around with "-msse -mfpunit=sse", but this doesn't seem to make a difference. Are there any other math type stuff to try? Does the output of a program compiled with "-O2 -fno-strict-aliasing" not being the same give you all any clues? Also, I tried -Wstrict-aliasing and -Wstrict-aliasing=1, but they didn't give any warnings/errors. Here are the output tests I've tried so far, and whether the output matches or not: (-O1 == -O2 == -O2 -msse -mfpunit=sse == -O3) != (-O2 -fno-strict-aliasing) != (-O0 == -O0 -msse -mfpunit=sse) Does this suggest any further tests? Thanks again, C. P.S. Thanks for the hint on how to view all the optimization flags activated by Ox. I tried some tests adding the flags activated by O1 to the command line, but that didn't cause the output to be the same. It seems the difference in output is from one of the non-flag optimizations activated by O1-3.
