ohav chochmah <philomath868@xxxxxxxxx> writes: > On 6/1/12, Ian Lance Taylor <iant@xxxxxxxxxx> wrote: > >> When optimizing on x86 or x86_64, -fomit-frame-pointer is the default >> anyhow. -fomit-frame-pointer essentially implies >> -momit-leaf-frame-pointer. >> > > I was sure that's the answer, but then is GCC lying to me? > ~ $ gcc -S tst.c -O2 -march=native -Q --help=optimizers | grep omit > -fomit-frame-pointer [disabled] Basically, it's a bug. When you use the --help=optimizers option, GCC doesn't actually compile the file. That means that it doesn't run through the target specific code that causes -O2 to turn on -fomit-frame-pointer. And that means that the display is invalid. Please file a bug report at http://gcc.gnu.org/bugzilla if there isn't one open already. Thanks. >> For x86_64 -mfpmath=sse is the default. >> > here again: > ~ $ gcc -S tst.c -O2 -march=native -Q --help=target | grep 'mfpmath=' > -mfpmath= 387 Same answer. The -mfpmath setting is done in target specific code that is not run when using --help. >>> why is msse disabled by default even after march=native, while >>> msse[234], msse4.[12] and mssse3 are all enabled? >> >> Not sure--what makes you think this is the case? >> > sorry, 's/msse/mmmx/'. > ~ $ gcc -S tst.c -O2 -march=native -Q --help=target | grep mmx > -mmmx [disabled] > > ~ $ sudo x86info -a -v | grep -i mmx > mmx MMX support > >>> isn't the CRC32 instruction part of sse4.2?, why is mcrc32 disabled by >>> default even when sse4.2 is enabled? Same answer. Target specific code turns on -mmmx if -msse is on. Ian
