Red Hat Bugzilla – Bug 5143
Default optflags for some x86 machines could be better
Last modified: 2008-05-01 11:37:51 EDT
Default optflags for some x86 machines could be better. I would suggest the alignment be reduced for processors >i486. The default of 16 bytes is best for an i486 cache (one cache line), and wasteful on newer processors where 4 bytes will do just as well. This reduces executable size and improves performance slightly. optflags: i586 -O2 -m486 -malign-loops=2 -malign-jumps=2 -malign-functions=2 optflags: i686 -O2 -m486 -malign-loops=2 -malign-jumps=2 -malign-functions=2 Note the amount is specified in powers of 2 (2^2 = 4). The -fno-strength-reduce option probably isn't needed anymore; it was a kludge for a rare optimizer bug that I've only ever seen realized in the kernel, but people seem to have gotten used to using it. Adding the -pipe option speeds up compiles a bit, especially on SMP systems. The tradeoff is more memory used and less temporary disk space used, as the compiler output is piped directly to the assembler when this option is used.
Just noticed this... echo "int main(void){return 0;}" > test.c gcc -O2 -c -S -fverbose-asm test.c At the top of test.s you will find lists ("options passed:" and "options enabled:") of all the optimization options passed and enabled. This will be helpful for choosing which options in addition to -O2 to add (ie. avoid repeating options).
Yes, there will always be better optflags. FWIW, rpm can now do a better job of using different optflags for different platforms, if necessary, using per-platform macro configurations. Feel free to send me a patch with your favorite optflags instantiated in /usr/lib/rpm/<arch>-<vendor>-<os>/macros if you want rpm to carry your per-platform (i.e. per-vendor) optflags, see installplatform in rpm sources for where to do your dirty.