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

[Xin Tong via llvm-commits]

On Thu, Jan 12, 2017 at 4:09 PM, Xin Tong <trent.xin.tong at gmail.com> wrote:
> On Thu, Jan 12, 2017 at 3:37 PM, Michael Kuperstein <mkuper at google.com> wrote:
>>
>>
>> On Thu, Jan 12, 2017 at 2:55 PM, Xin Tong <trent.xin.tong at gmail.com> wrote:
>>>
>>> On Thu, Jan 12, 2017 at 2:40 PM, Dehao Chen <danielcdh at gmail.com> wrote:
>>> > Are you suggesting changing it to:
>>> >
>>> > rotate_update_probability(float orig_prob) {
>>> >   if (orig_prob >= 0 && orig_prob < 0.5) {
>>> >     branch_prob_inside = 0;
>>> >     branch_prob_outside = orig_prob;
>>> >   } else {
>>> >     branch_prob_inside = 2 - 1/orig_prob;
>>> >     branch_prob_outside = 1;
>>> >   }
>>> > }
>>>
>>> This would not work for the case which the orig_prob is bigger than
>>> 0.5. branch_prob_outside tells me when the guard block is hit, it will
>>> always go to the exit.
>>> >
>>> > int estimate_trip_count(float latch_probability) {
>>> >    if (latch_probability == 0)
>>> >      return 0;
>>> >    else
>>> >      return 1/(1-latch_probability)
>>> > }
>>> >
>>> > My gut feeling tells me this is a little dangerous. Micheal, what do you
>>> > think?
>>> >
>>> > Dehao
>>>
>>> My general feeling is.
>>>
>>> 1. We do not have all the information to do a very precise
>>> distribution of the weight, and we will unlikely to be able to get
>>> such information in short term (we need value profiling at least I
>>> think or some other more clever approach). we made a decision to use
>>> average trip count for loop peeling, assuming the loop tripcounts are
>>> not polarized. Loop peeling will probably not be as effective if it
>>> is.  So we make the same assumption when we rotate the loops and
>>> update the !prof metadata.
>>
>>
>> I agree.
>>
>>>
>>> 2. We also consider the possibility of this assumption falling out, in
>>> the case of loop peel, we basically peeled for some gains when the
>>> iteration has a high trip count and some loss  (increase code size)
>>> when the iterations has low trip count. I feel this is acceptable.  In
>>> case of metadata update in loop rotation, if loop does have a very
>>> polarized distribution. i.e. cases where a few iterations takes all
>>> the trip counts and rest have close to zero. We basically
>>> underestimate the weight of the branch from guard block to exit block,
>>> as there should be more iterations of the loop that exits before even
>>> one iteration is executed. This will propagate into the CFG and result
>>> in us underestimating the average tripcount of the loops when it
>>> actually executes (i.e. we have fewer iterations responsible for all
>>> these body weight). This is OK, we become less aggressive in peeling,
>>> we left some performance opportunities on the table, but only if the
>>> loop distribution is polarized. And we may do a slightly worse job in
>>> block placement.
>>
>>
>> That's not exactly right. Underestimating can make us more aggressive, not
>> less.
>> Because we use a hard threshold for when to peel, underestimating the trip
>> count
>> can cause us to be more aggressive. I'm not sure this is a big problem,
>> because
>> if the threshold is chosen correctly, choosing to peel or not to peel when
>> we're very
>> close to the threshold should be a wash.
>> Admittedly, that's a pretty big if. :-) But I don't think it's a huge
>> problem.
>
> If the actual tripcount of the loop is 10 and we estimated to be 5, we
> will peel it 5 iterations.
> Its only when the
> https://github.com/llvm-mirror/llvm/blob/master/lib/Transforms/Utils/LoopUnrollPeel.cpp#L95
> is false,  we do not peel, then if the underestimation breaks this
> condition, the real number (bigger) will sure not cause the loop to be
> peeled.
>
> i.e. we will never peel more than this threshold. so I feel we are
> leaving opportunities on the table, instead of being too
> aggressive.
>
> -Xin

Ok, there is also the possiblity that we underestimate and peel a loop
which we would not peel given the right tripcount.  I think thats what
you meant.
-Xin
>>
>> Dehao, why do you think this is dangerous? It's certainly somewhat ugly,
>> but that's a separate issue.
>>
>>>
>>> 3. In case the loop distribution is close to each other, the patch
>>> handles it and distributes the weight in a reasonable way I feel.
>>>
>>> -Xin
>>> >
>>> > On Thu, Jan 12, 2017 at 2:33 PM, Xin Tong <trent.xin.tong at gmail.com>
>>> > wrote:
>>> >>
>>> >> On Thu, Jan 12, 2017 at 2:19 PM, Dehao Chen <danielcdh at gmail.com>
>>> >> wrote:
>>> >> >
>>> >> >
>>> >> > On Thu, Jan 12, 2017 at 2:14 PM, Xin Tong <trent.xin.tong at gmail.com>
>>> >> > wrote:
>>> >> >>
>>> >> >> On Thu, Jan 12, 2017 at 2:10 PM, Dehao Chen <danielcdh at gmail.com>
>>> >> >> wrote:
>>> >> >> >
>>> >> >> >
>>> >> >> > On Thu, Jan 12, 2017 at 2:02 PM, Xin Tong
>>> >> >> > <trent.xin.tong at gmail.com>
>>> >> >> > wrote:
>>> >> >> >>
>>> >> >> >> On Thu, Jan 12, 2017 at 1:34 PM, Dehao Chen <danielcdh at gmail.com>
>>> >> >> >> wrote:
>>> >> >> >> > If we assume that the trip count will always be the same, then
>>> >> >> >> > if
>>> >> >> >> > back
>>> >> >> >> > edge
>>> >> >> >> > taken probability falls in the range of (0, 0.5), then it
>>> >> >> >> > already
>>> >> >> >> > broke
>>> >> >> >> > this
>>> >> >> >> > assumption. So we do not have enough info to infer both branch
>>> >> >> >> > probabilities. So probably we should simply set the inner
>>> >> >> >> > probability
>>> >> >> >> > as
>>> >> >> >> > "0". And when we estimate loop trip count, we will always
>>> >> >> >> > return 0
>>> >> >> >> > if
>>> >> >> >> > the
>>> >> >> >> > latch probability is 0. Something like:
>>> >> >> >> >
>>> >> >> >> > rotate_update_probability(float orig_prob) {
>>> >> >> >> >   branch_prob_outside = orig_prob;
>>> >> >> >> >   if (orig_prob >= 0 && orig_prob < 0.5) {
>>> >> >> >> >     branch_prob_inside = 0;
>>> >> >> >> >   } else {
>>> >> >> >> >     branch_prob_inside = 2 - 1/orig_prob;
>>> >> >> >> >   }
>>> >> >> >> > }
>>> >> >> >> >
>>> >> >> >> > int estimate_trip_count(float latch_probability) {
>>> >> >> >> >    if (latch_probability == 0)
>>> >> >> >> >      return 0;
>>> >> >> >> >    else
>>> >> >> >> >      return 1/(1-latch_probability)
>>> >> >> >> > }
>>> >> >> >>
>>> >> >> >> We assume the trip count will be close to each other, they do not
>>> >> >> >> have
>>> >> >> >> to be identical =). And that assumption has implications, I
>>> >> >> >> addressed
>>> >> >> >> the implications when we have polarized loop tripcounts in an
>>> >> >> >> earlier
>>> >> >> >> response. I feel we need to make assumptions and also reason
>>> >> >> >> about
>>> >> >> >> the
>>> >> >> >> fallouts if that assumption turns out to be not true as Michael
>>> >> >> >> said.
>>> >> >> >
>>> >> >> >
>>> >> >> > Yes, the above code is trying to address one case where the
>>> >> >> > assumption
>>> >> >> > is
>>> >> >> > untrue.
>>> >> >> >
>>> >> >> >>
>>> >> >> >>
>>> >> >> >> Back to your example, I am not sure i understand it fully, is
>>> >> >> >> orig_prob the probability of branching outside before rotation ?
>>> >> >> >
>>> >> >> >
>>> >> >> > Yes, orig_prob is the original probability of the branch before
>>> >> >> > loop
>>> >> >> > rotation.
>>> >> >>
>>> >> >> If orig_prob == 0.5, the branch_prob_inside is 0.
>>> >> >> (branch_prob_inside
>>> >> >> is the probability that the loop body is branched to before rotation
>>> >> >> ?).
>>> >> >
>>> >> >
>>> >> > Sorry about the confusion. Let me use my original example:
>>> >> >
>>> >> > Before loop rotation
>>> >> >
>>> >> > L1:
>>> >> > if (cond) { // "orig_prob"
>>> >> >   stmt;
>>> >> >   goto L1;
>>> >> > }
>>> >> >
>>> >> > After loop rotation:
>>> >> >
>>> >> > if (cond) { // "branch_prob_outside"
>>> >> > L1:
>>> >> >   stmt;
>>> >> >   if (cond) { // "branch_prob_inside" // and also "latch_probability"
>>> >> >     goto L1;
>>> >> >   }
>>> >> > }
>>> >> >
>>> >> > Note that all probability are for when "cond" is true.
>>> >> >
>>> >> > Thanks,
>>> >> > Dehao
>>> >>
>>> >> Thanks for the code. One thing I notice is that branch_prob_outside
>>> >> probably should not be orig_prob. orig_prob before rotation is the (#
>>> >> of times the exit is taken)/(# of times the header branch is hit
>>> >> through forward and back edges). So if the loop has a cmp i32 %index,
>>> >> 127 and recurrence of +1 (index = index + 1), i.e. the loop has a trip
>>> >> count of 128, then branch_prob_outside = orig_prob = 1/128.  However,
>>> >> branch_prob_outside should really be 0 as the loop will always
>>> >> execute, i..e branch_prob_outside in the guard block will never be
>>> >> taken.
>>> >> >
>>> >> >
>>> >> >>
>>> >> >> Thanks,
>>> >> >> -Xin
>>> >> >> >
>>> >> >> >>
>>> >> >> >>
>>> >> >> >> Thanks,
>>> >> >> >> -Xin
>>> >> >> >>
>>> >> >> >> >
>>> >> >> >> >
>>> >> >> >> > On Thu, Jan 12, 2017 at 12:59 PM, Michael Kuperstein via
>>> >> >> >> > Phabricator
>>> >> >> >> > <reviews at reviews.llvm.org> wrote:
>>> >> >> >> >>
>>> >> >> >> >> mkuper added inline comments.
>>> >> >> >> >>
>>> >> >> >> >>
>>> >> >> >> >> ================
>>> >> >> >> >> Comment at: lib/Transforms/Scalar/LoopRotation.cpp:476
>>> >> >> >> >> +  // data.
>>> >> >> >> >> +  if (!updateLoopEstimatedBranchWeight(L, GBI, BI /*
>>> >> >> >> >> OrigHeader
>>> >> >> >> >> BR
>>> >> >> >> >> */)) {
>>> >> >> >> >> +    BI->setMetadata(LLVMContext::MD_prof, nullptr);
>>> >> >> >> >> ----------------
>>> >> >> >> >> Also, regardless of the rest of the discussion - I don't think
>>> >> >> >> >> we
>>> >> >> >> >> should
>>> >> >> >> >> drop the metadata on the floor if we fail.
>>> >> >> >> >>
>>> >> >> >> >> I don't think "No data is better than imprecise data" is right
>>> >> >> >> >> in
>>> >> >> >> >> the
>>> >> >> >> >> general case, but that's arguable. Specifically here, though,
>>> >> >> >> >> we're
>>> >> >> >> >> imprecise even if updateLoopEstimatedBranchWeight() succeeds,
>>> >> >> >> >> because
>>> >> >> >> >> of the
>>> >> >> >> >> assumptions we make on the distribution.
>>> >> >> >> >>
>>> >> >> >> >>
>>> >> >> >> >> https://reviews.llvm.org/D28593
>>> >> >> >> >>
>>> >> >> >> >>
>>> >> >> >> >>
>>> >> >> >> >
>>> >> >> >
>>> >> >> >
>>> >> >
>>> >> >
>>> >
>>> >
>>
>>