some questions about GCC's options

[xunxun]

于 2012/6/1 15:59, ohav chochmah 写道:
> On 6/1/12, Ian Lance Taylor<iant@google.com>  wrote:
>> ohav chochmah<philomath868@gmail.com>  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