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Re: [PATCH] Atom: Enabling unroll at O2 optimization level


On Thu, Apr 12, 2012 at 3:16 PM, Richard Guenther
<richard.guenther@gmail.com> wrote:
> On Thu, Apr 12, 2012 at 1:05 PM, Igor Zamyatin <izamyatin@gmail.com> wrote:
>> On Wed, Apr 11, 2012 at 12:39 PM, Richard Guenther
>> <richard.guenther@gmail.com> wrote:
>>> On Tue, Apr 10, 2012 at 8:43 PM, Igor Zamyatin <izamyatin@gmail.com> wrote:
>>>> Hi All!
>>>>
>>>> Here is a patch that enables unroll at O2 for Atom.
>>>>
>>>> This gives good performance boost on EEMBC 2.0 (~+8% in Geomean for 32
>>>> bits) with quite moderate code size increase (~5% for EEMBC2.0, 32
>>>> bits).
>>>
>>> 5% is not moderate. ?Your patch does enable unrolling at -O2 but not -O3,
>>> why? Why do you disable register renaming? ?check_imull requires a function
>>> comment.
>>
>> Sure, enabling unroll for O3 could be the next step.
>> We can't avoid code size increase with unroll - what number do you
>> think will be appropriate?
>> Register renaming was the reason of several degradations during tuning process
>> Comment for check_imull was added
>>
>>>
>>> This completely looks like a hack for EEMBC2.0, so it's definitely not ok.
>>
>> Why? EEMBC was measured and result provided here just because this
>> benchmark considers to be very relevant for Atom
>
> I'd say that SPEC INT (2000 / 2006) is more relevant for Atom (SPEC FP
> would be irrelevant OTOH). ?Similar code size for, say, Mozilla Firefox
> or GCC itself would be important.
>
>>> -O2 is not supposed to give best benchmark results.
>>
>> O2 is wide-used so performance improvement could be important for users.
>
> But not at a 5% size cost. ?Please also always check the compile-time effect
> which is important for -O2 as well.

What would be an acceptable number of size cost/compile-time increase
for O2 and O3 on EEMBC, SPEC INT 2000 and Mozilla?

Is it possible in common to put Atom-specific unroll heuristics under
some option which could be mentioned in GCC docs?

>
> Richard.
>
>>>
>>> Thanks,
>>> Richard.
>>>
>>>>
>>>> Tested for i386 and x86-64, ok for trunk?
>>
>> Updated patch attached
>>
>>>>
>>>> Thanks,
>>>> Igor
>>>>
>>>> ChangeLog:
>>>>
>>>> 2012-04-10 ?Yakovlev Vladimir ?<vladimir.b.yakovlev@intel.com>
>>>>
>>>> ? ? ? ?* gcc/config/i386/i386.c (check_imul): New routine.
>>>> ? ? ? ?(ix86_loop_unroll_adjust): New target hook.
>>>> ? ? ? ?(ix86_option_override_internal): Enable unrolling on Atom at -O2.
>>>> ? ? ? ?(TARGET_LOOP_UNROLL_ADJUST): New define.

Thanks,
Igor


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