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

Thu Jan 12 15:37:23 PST 2017

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.

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
> >> >> >> >>
> >> >> >> >>
> >> >> >> >>
> >> >> >> >
> >> >> >
> >> >> >
> >> >
> >> >
> >
> >
>
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