[PATCH] D109860: profi - a flow-based profile inference algorithm: Part I (out of 3)

[Sergey Pupyrev via Phabricator via llvm-commits]

spupyrev added a comment.

In D109860#3009910 <https://reviews.llvm.org/D109860#3009910>, @wmi wrote:

> Thanks Sergey. That looks like a good work to improve the consistency of the profile. Have you checked whether the new algorithm can infer the missing parts based on equivalence relationship on the CFG? If it can, could you add a test for it?

Do you mind to elaborate more on this? What kind of a test would you like to see? (i may not be familiar with the terminology)

In general, the algorithm builds a valid "flow" comprised of the block/jump counts. That is, the sum of incoming jump counts always equals the sum of outgoing jump counts (except for the source and sinks). Hence, if two blocks are guaranteed to have equal counts, the algorithm will always return equal counts. A few of the tests are (implicitly) verifying that.

> I will collect some performance number w/wo sample-profile-use-profi.

That would be awesome, thanks!
Tbh, I've seen some weird problems while using the algorithm with AutoFDO. There are a few issues that make profi to work not as expected:

- there are no "dangling" blocks in AutoFDO; the corresponding blocks seem to be reported as blocks with 0 counts;
- some blocks/counts seem to have been duplicated and there is no concept of "distribution factor"; profi has difficulties with such incorrect blocks.

Overall we see benefits of using the new inference with CSSPGO (where the above issues are resolved); however, AutoFDO weights/counts need to be polished before the inference.


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