[PATCH] D28593: Update loop branch_weight metadata after loop rotation.

[Xin Tong via llvm-commits]

On Thu, Jan 12, 2017 at 10:29 AM, Xin Tong <trent.xin.tong at gmail.com> wrote:
> On Thu, Jan 12, 2017 at 10:26 AM, Dehao Chen <danielcdh at gmail.com> wrote:
>>
>>
>> On Thu, Jan 12, 2017 at 9:26 AM, Michael Kuperstein <mkuper at google.com>
>> wrote:
>>>
>>> I haven't looked at the patch yet, (and I'm also not at the office), but:
>>>
>>> 1) The new probabilities should be the opposite of the last example Dehao
>>> had. That is, the external if should be 1/7, and the one on the backedge
>>> 7/28.
>>
>>
>> For the 2nd case, the expected trip count is 0.2, thus it's not
>> straight-forward to me how to set this. But if we make the expected trip
>> count as 5, then looks to me the external if should be 5/1, and the one on
>> the backedge should be 4/1?
>>
>> And please make sure that when the input probability is the same, the output
>> probability should also be the same. E.g., if you annotate the branch as (5,
>> 1), and (10, 2), after the transformation, the probabilities should be
>> equivalent.
>>
>>>
>>> 2) I think this is just conditional probability - cond hasn't changed, but
>>> P(cond | cond has been true once) is lower than P(cond). This exactly
>>> corresponds to lowering the number of expected iterations by 1, given that
>>> we know we already executed one iteration.
>>
>>
>> Seem with this patch, when we compute the estimated trip count, we need to
>> first check if it's rotated or not: if yes, we use (BE.taken/BE.not_taken) +
>> 1, otherwise, we use (BE.taken/BE.not_taken). Is that true?
>

Sorry, Dehao I feel i talked past you, our current way of computing
estimated trip count requires loop to be rotated.
Yes, in the general case, the trip count of the loop depends on
whether the loop is rotated or not as you said.

> The way we compute EstimatedTripCount requires the loop to be rotated.
> i.e. we only look at the latch block and expect it to be bottom
> tested.
>>
>> Dehao
>>
>>>
>>>
>>>
>>> On Jan 12, 2017 09:21, "Xin Tong via Phabricator"
>>> <reviews at reviews.llvm.org> wrote:
>>>
>>> trentxintong added a comment.
>>>
>>> @danielcdh Its the true the cond is not changed, but the inputs to the
>>> condition has changed, before you were testing with index variable in the
>>> loop header, now u have 2 conditions and the SSA values you use in the
>>> conditions have changed, so the probability needs to be adjusted. I will
>>> explain more when i get to office.
>>>
>>>
>>> https://reviews.llvm.org/D28593
>>>
>>>
>>>
>>>
>>