[PATCH] D39385: [BranchProbabilityInfo] Handle irreducible loops.

[Florian Hahn via Phabricator via llvm-commits]

fhahn accepted this revision.
fhahn added a comment.

In https://reviews.llvm.org/D39385#910911, @gberry wrote:

> In https://reviews.llvm.org/D39385#910489, @fhahn wrote:
>
> > This looks sensible to me. Do you have any numbers about the impact of this change? Not sure about the compile time impact of computing the SCC here (computing SCC should not be too expensive, O(N+E)), but at least BlockFrequencyInfoImpl uses SCC for a similar thing but it is only computed once it discovers irreducible control flow. It might be worth having this as a separate analysis to do the construction only once in the future.
>
>
> I observed the following performance improvements on AArch64 (Falkor):
>
>   spec2017/xz:train             1.00
>   spec2006/bzip2:lessnoise      7.24
>
>
>


Great thanks!

> As for compile time, the times for BPI do increase, but the increases are in the noise of the compile time as a whole.  I looked briefly at adding irreducible loop detection to LoopInfo, so I could check that here before computing SCCs (see FIXME comment around line 820), but it didn't seem simple enough to justify the effort given the level of compile time increase.

Yeah that's what I though too. LGTM


https://reviews.llvm.org/D39385