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

[Xin Tong via llvm-commits]

So there are really 3 cases.

case 1: est < real < threshold => we should peel more.
case 2: est < threshold < real => we would not peel with the real
tripcount, but we peeled with the underestimated tripcount. we are
being aggressive here. Michael, have you considered the possibility of
peeling just the threshold ?
case 3: threshold < est < real => we do not peel. this is fine.
-Xin

On Thu, Jan 12, 2017 at 4:14 PM, Xin Tong <trent.xin.tong at gmail.com> wrote:
> 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
>>>> >> >> >> >>
>>>> >> >> >> >>
>>>> >> >> >> >>
>>>> >> >> >> >
>>>> >> >> >
>>>> >> >> >
>>>> >> >
>>>> >> >
>>>> >
>>>> >
>>>
>>>