On 6/1/12, Ian Lance Taylor <iant@xxxxxxxxxx> wrote: > ohav chochmah <philomath868@xxxxxxxxx> writes: > >> first, is momit-leaf-frame-pointer bad for debugging (in the way >> fomit-frame-pointer can be)? > > It's not as bad. It may still trip up some debuggers. > but not GDB? >> if not, why is it disabled by default >> even when optimizing (as -Q --help=target | grep omit reveals)? > > 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] >> next, the manual mentions that fno-fat-lto-objects improves >> compilation time over plain LTO, but requires the whole toolchain to >> be aware of LTO and support plugins, which is why it's not (yet) the >> default. how can I know for certain if the toolchain I'm using meets >> the criteria (seems to be the case)? > > Use the most recent GNU binutils release. > good then, thanks. >> similarly, mtls-dialect=gnu2 is better then the default gnu, "but it >> may add compile- and run-time requirements that cannot be satisfied on >> all systems." how can I test for them? > > Build a program with it and see. Basically your GNU binutils need to be > fairly new and your glibc needs to be 2.9 or later. > good, thanks. >> is it true that mfpmath=sse can result in poor code when using glibc? >> (maybe I shouldn't ask that here...) > > For x86_64 -mfpmath=sse is the default. > here again: ~ $ gcc -S tst.c -O2 -march=native -Q --help=target | grep 'mfpmath=' -mfpmath= 387 >> 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? > > It's not. Don't be confused by the fact that -msse4.2 does not > automatically turn on -mcrc32. Everything that is available when using > -mcrc32 is also available when using -msse4.2. > thanks. > Ian > cheers.
