[PATCH] D10825: Improvement on computing edge probabilities when choosing the best successor in machine block placement.

[Chandler Carruth]

On Tue, Jul 28, 2015 at 4:29 PM Cong Hou <congh at google.com> wrote:

> On Tue, Jul 28, 2015 at 4:20 PM, Chandler Carruth <chandlerc at gmail.com>
> wrote:
> > On Tue, Jul 28, 2015 at 3:58 PM Cong Hou <congh at google.com> wrote:
> >>
> >> I found it not straightforward to normalize weights in MBPI. Note that
> >> MBPI reads weights from MBB, which provides the interface
> >> addSuccessor(MachineBasicBlock *succ, uint32_t weight) to let users
> indicate
> >> arbitrary weights for new added edges. Because we don't know how users
> use
> >> this interface, it can be difficult to adjust weights in MBB. I think
> there
> >> may be two solutions:
> >>
> >> 1. We provide the interface normalizeWeights in MBB. Once we call this
> >> interface, the sum of weights is guaranteed to be less than UINT32_MAX.
> >> However, after the user add new successors with weights, we need to
> >> re-normalize it. This can still be useful when we want to calculate
> branch
> >> probabilities or do other adjustment after the weights are normalized.
> >>
> >> 2. We maintained both normalized and original weights in MBB, so that
> once
> >> the original weights is modified, we also adjust the normalized weights.
> >> This solutions requires more space and more frequent calculations
> comparing
> >> to the first one, but the normalized weights are always guaranteed.
> >>
> >> Do you have any other ideas? Or which one do you think is better?
> >
> >
> > First off, yuck, but thanks for digging into this.
> >
> > 3. Another alternative might be to keep the sum stored, and when adding a
> > weight, we could simply check if that would overflow, if so, re-normalize
> > everything. It would require auditing code that adds a successor to
> ensure
> > it doesn't rely on the weights after adding a successor being the same,
> but
> > that seems unlikely to come up.
> >
> > I'd particularly like to hear Duncan's thoughts on which of these options
> > would make most sense. I see pros and cons on all of these. I think I'd
> be
> > fine with either #1 or #3. I agree with your concern over the space
> > requirements of #2.
>
> Thanks for the suggestion! For #3 solution, what if the user first
> calculates weights for several successors and add them one by one? Do
> we need to notify the user how we normalize the weights so that the
> user can adjust the weights that are not added yet?
>

Yuck, possibly.

Are there such callers of this API?


>
> Cong
>
> >
> >>
> >>
> >>
> >> http://reviews.llvm.org/D10825
> >>
> >>
> >>
> >>
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