Re: [PATCH v3 2/3] Add predict_doloop_p target hook

[Jeff Law <law at redhat dot com>]

On 5/21/19 4:20 AM, Richard Biener wrote:
> On Tue, 21 May 2019, Kewen.Lin wrote:
> 
>> on 2019/5/21 上午12:37, Segher Boessenkool wrote:
>>> On Mon, May 20, 2019 at 08:43:59AM -0600, Jeff Law wrote:
>>>>> I think we should have two hooks: one is called with the struct loop as
>>>>> parameter; and the other is called for every statement in the loop, if
>>>>> the hook isn't null anyway.  Or perhaps we do not need that second one.
>>>> I'd wait to see a compelling example from real world code where we need
>>>> to scan the statements.  Otherwise we're just dragging in more target
>>>> specific decisions which in fact we want to minimize target stuff.
>>>
>>> The ivopts pass will be too optimistic about what loops will end up as a
>>> doloop, and cost things accordingly.  The cases where we cannot later
>>> actually use a doloop are doing pretty much per iteration, so I think
>>> ivopts will still make good decisions.  We'll need to make the rtl part
>>> not actually do a doloop then, but we probably still need that logic
>>> anyway.
>>>
>>> Kewen, Bin, will that work satisfactorily do you think?
>>>
>>
>> If my understanding on this question is correct, IMHO we should try to make
>> IVOPTs conservative than optimistic, since once the predict is wrong from
>> too optimistic decision, the costing on the doloop use is wrong, it's very
>> possible to affect the global optimal set.  It looks we don't have any ways
>> to recover it in RTL then?  (otherwise, there should be better place to fix
>> the PR).  Although it's also possible to miss some good cases, it's at least
>> as good as before, I'm inclined to make it conservative.
> 
> I wonder if you could simply benchmark what happens if you make
> IVOPTs _always_ create a doloop IV (if possible)?  I doubt the
> cases where a doloop IV is bad (calls, etc.) are too common and
> that in those cases the extra simple IV hurts.
This had been in the back of my mind as well.  I wonder how the RTL IV
bits would respond to that.

Jeff