Re: some questions about GCC's options

[ohav chochmah <philomath868@xxxxxxxxx>]

On 6/1/12, xunxun <xunxun1982@xxxxxxxxx> wrote:
> 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.

x86_64.
according to the manual, "Starting with GCC version 4.6, the default
setting (when not optimizing for size) for 32-bit Linux x86 and 32-bit
Darwin x86 targets has been changed to ‘-fomit-frame-pointer’.   The
default can be reverted to ‘-fno-omit-frame-pointer’ by configuring
GCC with the ‘--enable-frame-pointer’ configure option."  it does not
mention x86_64.
the bottom-line seems to be, that -Q --help=<class> is sometimes inaccurate.


> Best Regards,
> xunxun

all the best.