Re: [PATCH, vec-tails 04/10] Add masking cost

[Ilya Enkovich <enkovich.gnu@gmail.com>]

2016-05-20 14:15 GMT+03:00 Richard Biener <richard.guenther@gmail.com>:
> On Fri, May 20, 2016 at 11:44 AM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:
>> 2016-05-20 12:24 GMT+03:00 Richard Biener <richard.guenther@gmail.com>:
>>> On Thu, May 19, 2016 at 9:40 PM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:
>>>> Hi,
>>>>
>>>> This patch extends vectorizer cost model to include masking cost by
>>>> adding new cost model locations and new target hook to compute
>>>> masking cost.
>>>
>>> Can you explain a bit why you add separate overall
>>> masking_prologue/body_cost rather
>>> than using the existing prologue/body cost for that?
>>
>> When I make a decision I need vector loop cost without masking (what
>> we currently
>> have) and with masking (what I add).  This allows me to compute
>> profitability for
>> all options (scalar epilogue, combined epilogue, masked epilogue) and choose one
>> of them.  Using existing prologue/body cost would allow me compute masking
>> profitability with no fall back to scalar loop profitability.
>
> Yes, but for this kind of purpose you could simply re-start
> separate costing via the init_cost hook?

But that would require double scan through loop statements + double
profitability
estimations.  I compute masking cost during statements analysis
(see patch #05) in parallel with regular costs computations.  Note that masking
costs is a cost of masking only.  Thus cost of a vector masked iteration is
body cost + body masking cost.

>
>>> I realize that the current vectorizer cost infrastructure is a big
>>> mess, but isn't it possible
>>> to achieve what you did with the current add_stmt_cost hook?  (by
>>> inspecting stmt_info)
>>
>> Cost of a statement and cost of masking a statement are different things.
>> Two hooks called for the same statement return different values. I can
>> add vect_cost_for_stmt enum elements to cover masking but I thought
>> having stmt_masking_cost would me more clear.
>
> I agree we need some kind of overloading and I'm not against a separate hook
> for this.  On a related note what is "masking cost" here?  I could imagine
> that masking doesn't unconditionally add a cost to a stmt but its execution
> cost may now depend on whether an element is masked or not.
>
> Does the hook return the cost of the masked stmt or the cost of masking
> the stmt only (so you need to do add_stmt_cost as well on the same stmt)?

It returns the cost of masking the statement only.  Thus if a hardware has
no penalty for executing masked instruction then return value should be 0.

Thanks,
Ilya

>
> Thanks,
> Richard.
>
>> Thanks,
>> Ilya
>>
>>>
>>> Richard.
>>>
>>>> Thanks,
>>>> Ilya