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-march=pentium3 -mtune=generic -mfpmath=both ?

Obi Wan Kenobi · 2014-06-23T17:08:09

Im still confused what flags to use in mingw (gcc 4.7.1) to optymize binary as far as its possible - especially that this gcc dosumentation is partially weakly written for example i do understand (hopefully correct) 1) that -march says what instruction set i should restrict compiler to use (for example setting -march=pentium3 makes my binary onlu with instructions awaliable on pentium3) 2) also i understand that -mtune says to what target the previous instructions do optymize, for example i can get p3 instructions and optymize it for core2 confusingly the docs say "-march=cpu-type Generate instructions for the machine type cpu-type. The choices for cpu-type are the same as for -mtune. Moreover, specifying -march=cpu-type implies -mtune=cpu-type. " I doubt if this is true - does this mean that when choicing -march=pentium3 -mtune=generic the mtune setting is discarded and this is equiwalent of -march=pentium3 -mtune=pentium3 ? dont think so (this is confusing) other questions 1. i would like to chose resonable codeset that would be working on older machines but also working ok on more modern ones I chose -march=pentium3 as i doubt if someone uses something older than p3 and I didnt noticed noticable change when putting something newer here (like -march=core2 - i dint notice any speedup) 2. what in general i can yet add to this commandline to speed things up ? (or throw away some runtime or exception stuff bytes or something like that) c:\mingw\bin\g++ -O2 -Ofast -w -c transform_triangle_3d.c -funsafe-math-optimizations -mrecip -ffast-math -fno-rtti -fno-exceptions -march=pentium3 -mtune=generic -mfpmath=both (some flags may be redundant here but I added them to be sure, got no time to carefulle check up what is redundant ) im using here -O2 as i not noticed difference with -O3 i noticed that "-mfpmath=both " speeded things up (though docs say something that its dangerous didnt understand why) also (-ffast-math / -funsafe-math-optimizations also speeded things)