[PATCH] D57805: [HotColdSplit] Move splitting after instrumented PGO use

Teresa Johnson via Phabricator via llvm-commits llvm-commits at lists.llvm.org
Thu Feb 14 16:12:10 PST 2019

tejohnson added a comment.

In D57805#1398718 <https://reviews.llvm.org/D57805#1398718>, @vsk wrote:

> In D57805#1393516 <https://reviews.llvm.org/D57805#1393516>, @tejohnson wrote:
> > ... I will try moving the splitting to after the ThinLTO backend (post-thinlink) inlining and see what effect there is. Theoretically we should be getting more accurate importing/inlining, it would be good to understand where this is going wrong if not!
> I have not yet tried the experiment you've described here. We've noticed that scheduling splitting early causes a regression for certain benchmarks in SPEC even without PGO data applied, however. As the heuristics for splitting are very conservative without PGO, this suggests that splitting before inlining may inadvertently hide important context from the optimizer.

I gave this a try with our internal benchmark that was slowing down with splitting before inlining. Moving it after inlining (the post link inlining in ThinLTO) caused the degradation to go away. Unfortunately no speedup though. I haven't had a chance to dig into the performance results more than that (and am heading out of town for the better part of the next couple weeks). I will review your new patch to move the pass later shortly though.




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