"rep ret" even for Intel CPUs?

[kevin diggs]

Hi,

Uh ... "rep; ret"? Where do I look to see what this is?

Thanks!

kevin

On Tue, Mar 22, 2011 at 1:25 AM, Ian Lance Taylor <iant@google.com> wrote:
> Zuxy Meng <zuxy.meng@gmail.com> writes:
>
>> 2011/3/22 Ian Lance Taylor <iant@google.com>:
>>> "Zuxy Meng" <zuxy.meng@gmail.com> writes:
>>>
>>>> I found that gcc 4.5 generates "rep ret" even under -march=core2. Does
>>>> an Intel CPU benefit from this optimization too?
>>>
>>> As far as I know no Intel CPU benefits.  Note that -march selects the
>>> architecture but not the tuning.  The default tuning uses "rep; ret"
>>> when appropriate.  If you want to avoid it, use a -mtune option.
>>
>> 'info gcc' tells me that '-march=CPU-TYPE' implies '-mtune=CPU-TYPE',
>> at least for x86...
>
> You're quite right, sorry about that.
>
> gcc 4.5 will use "rep; ret" for K8, Athlon, AMDFam10, Core2, and
> generic.  Looking into the change history, I see that gcc has padded
> returns for Core2 ever since Core2 support was added here:
> http://gcc.gnu.org/ml/gcc-patches/2006-11/msg00860.html .  I suspect
> that Vlad just copied the generic tunings for Core2, and didn't notice
> that there was no particular reason to pad returns for Core2.
>
> Please consider opening a bug report per http://gcc.gnu.org/bugs/ with
> this information.  Thanks.
>
> Ian
>