The best I’ve found so far for generic use, is this:
-O3 -march=opteron -mtune=amdfam10 -m64 -mfpmath=sse -mcrc32 -ffast-math -fomit-frame-pointer
If you are using a latest-generation AMD64 CPU, made by AMD, the following is recommended:
-O3 -march=amdfam10 -m64 -mfpmath=sse -mcrc32 -ffast-math -fomit-frame-pointer
For many applications, especially graphics and math-oriented, the -ffast-math item will need to be removed. -O3 may have to be replaced with -O2 for some code, too.