于 2012/6/1 15:59, ohav chochmah 写道:
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]
x86 or x86_64?
I remembered -fomit-frame-pointer enabled only on x86_64 by default.
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.
--
Best Regards,
xunxun
