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

[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?

>> 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)?

Thanks,
Richard.

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