[PATCH] D103289: A post-processing for BFI inference

[Sergey Pupyrev via Phabricator via llvm-commits]

spupyrev marked an inline comment as done.
spupyrev added inline comments.


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Comment at: llvm/include/llvm/Analysis/BlockFrequencyInfoImpl.h:1489
+    }
+    if (OneMinusSelfProb != Scaled64::getOne())
+      NewFreq /= OneMinusSelfProb;
----------------
davidxl wrote:
> spupyrev wrote:
> > davidxl wrote:
> > > Does it apply to other backedges too?
> > not sure I fully understand the question, but we need an adjustment only for self-edges; blocks without self-edges don't need any post-processing
> > 
> > I added a short comment before the loop
> NewFreq /= OneMinusSelfProb looks like multiply the block freq (one iteration loop) with the average trip count -- that is why I asked if this applies to other backedges.
Here is the relevant math:

we want to find a new frequency for block I, `Freq[I]`, such that it is equal to `\sum Freq[J] * Prob[J][I]`, where the sum is taken over all (incoming) jumps (J -> I). These are "ideal" frequencies that BFI is trying to compute.

Clearly if I-th block has no self-edges, then we simply assign `Freq[I]:=\sum Freq[J] * Prob[J][I]` (that is, no adjustment). However, if there are self_edges, we need to assign `Freq[I]:=(\sum Freq[J] * Prob[J][I]) / (1 - Prob[I][I])` (the adjustment in the code)


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https://reviews.llvm.org/D103289