CSights wrote: > I'm trying to debug some mismatching results from a program compiled with > O1,2,3) and without (-O0 or nothing) optimization flags. > My thought was to individually turn on optimization flags and see which one > changes the program's output. -O is not just a combination of a bunch of -f flags. It doesn't work that way. There are optimizations that are controlled directly by -O, with no corresponding -f. The manual says this at the top of <http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html>: > Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed. If you have a program that behaves differently with and without optimization, then it's probably relying on undefined behavior. A common mistake is to violate the C aliasing rules. Compile your code with -O2 -fno-strict-aliasing and see if that makes the problem go away. If it does that's a good indication that it's an aliasing issue. Then compile with -O2 -Wstrict-aliasing (i.e. remove -fno-strict-aliasing) and see if any of the warnings give you a clue as to the problem. You can try -Wstrict-aliasing=1 if the default did not give any warnings, at the cost of potentially more false positives. Brian
