Re: [PATCH 0/4 GCC11] IVOPTs consider step cost for different forms when unrolling

["Kewen.Lin" <linkw@linux.ibm.com>]

on 2020/2/11 上午5:29, Segher Boessenkool wrote:
> Hi!
> 
> On Mon, Feb 10, 2020 at 02:17:04PM +0800, Kewen.Lin wrote:
>> on 2020/1/20 下午8:33, Segher Boessenkool wrote:
>>> On Thu, Jan 16, 2020 at 05:36:52PM +0800, Kewen.Lin wrote:
>>>> As we discussed in the thread
>>>> https://gcc.gnu.org/ml/gcc-patches/2020-01/msg00196.html
>>>> Original: https://gcc.gnu.org/ml/gcc-patches/2020-01/msg00104.html,
>>>> I'm working to teach IVOPTs to consider D-form group access during unrolling.
>>>> The difference on D-form and other forms during unrolling is we can put the
>>>> stride into displacement field to avoid additional step increment. eg:
>>>
>>> <snip>
>>>
>>>> Imagining that if the loop get unrolled by 8 times, then 3 step updates with
>>>> D-form vs. 8 step updates with X-form. Here we only need to check stride
>>>> meet D-form field requirement, since if OFF doesn't meet, we can construct
>>>> baseA' with baseA + OFF.
>>>
>>> So why doesn't the existing code do this already?  Why does it make all
>>> the extra induction variables?  Is the existing cost model bad, are our
>>> target costs bad, or something like that?
>>>
>>
>> I think the main cause is IVOPTs runs before RTL unroll, when it's determining
>> the IV sets, it can only take the normal step cost into account, since its
>> input isn't unrolled yet.  After unrolling, the x-form indexing register has to
>> play with more UF-1 times update, but we can probably hide them in d-form
>> displacement field.  The way I proposed here is to adjust IV cost with
>> additional cost_step according to estimated unroll.  It doesn't introduce new
>> IV cand but can affect the final optimal set.
> 
> Yes, we should decide how often we want to unroll things somewhere before
> ivopts already, and just use that info here.

Agreed! If some passes are interested on this unroll factor estimation, we
can move backward there if it's before IVOPTs.  As patch 1/4, once it's set,
the later pass can just reuse that info.  As Richard B. suggested, we can
even skip the later RTL unroll factor determination.

> 
> Or are there advantage to doing it *in* ivopts?  It sounds like doing
> it there is probably expensive, but maybe not, and we need to do similar
> analysis there anyway.
> 

Good question.  I didn't consider that, the reason putting here is we need
this information in IVOPTs for some cases.  :)

BR,
Kewen