Hi Vedant, nice work!
> On Sep 6, 2016, at 12:36 PM, Vedant Kumar via swift-dev <swift-dev@swift.org> > wrote: > > Hi swift-dev, > > I've been working on some patches which add basic support for PGO to swift > [1]. > What I have so far is just a proof-of-concept. I'd like to get some feedback > on > the approach I've taken. > > I've added support for loading profile data, matching up execution counts to > the right parts of the AST, and attaching those execution counts to > conditional > branches. I added two fields to CondBranchInst (in SIL): > > /// The number of times the True branch was executed. > Optional<uint64_t> TrueBBCount; > > /// The number of times the False branch was executed. > Optional<uint64_t> FalseBBCount; > > I fixed up the SILCloner and a few other sites where conditional branches are > created to propagate the branch taken counts. I have a patch to propagate > branch taken counts through the SILOptimizer, but I didn't include it in [1] > because I don't know how to write tests for it. > > In IRGen, I added some logic to scale execution counts and create llvm > branch_weight metadata. > > Some questions: > > 1. Is it acceptable to make some SIL objects larger in order to store > execution counts (e.g CondBranchInst)? Yes, I don’t see a problem with that. But for saving some space, you might want to store the counts as a plain uint64_t instead of an Optional and use a special value (e.g. ~0) for the none-case. > If not, what's the best way to make > this information visible to SILOptimizer and IRGen? > > 2. Is it better to associate counts with SIL instructions, or with > SILSuccessor? I think storing the counts in SILSuccessor makes sense, because counts are needed for all kind of terminator instructions (like switch_enum), except for the unconditional br, but IMO we can live with wasting some bytes for this instructions. Another thing to consider is that the count information should be printed/parsed/serialized when writing and reading SIL. > > 3. Does anyone have tips on modifying swift/benchmark? I'd like to add a > benchmark driver which generates profile data, and another driver which > uses that data to turn on PGO. > > thanks, > vedant > > [1] https://github.com/vedantk/swift/tree/profile_use > _______________________________________________ > swift-dev mailing list > swift-dev@swift.org > https://lists.swift.org/mailman/listinfo/swift-dev _______________________________________________ swift-dev mailing list swift-dev@swift.org https://lists.swift.org/mailman/listinfo/swift-dev
