[LLVMdev] RFC - Improvements to PGO profile support

[Bob Wilson]

> On Feb 24, 2015, at 3:31 PM, Diego Novillo <dnovillo at google.com> wrote:
> 
> 
> We (Google) have started to look more closely at the profiling infrastructure in LLVM. Internally, we have a large dependency on PGO to get peak performance in generated code.
> 
> Some of the dependencies we have on profiling are still not present in LLVM (e.g., the inliner) but we will still need to incorporate changes to support our work on these optimizations. Some of the changes may be addressed as individual bug fixes on the existing profiling infrastructure. Other changes  may be better implemented as either new extensions or as replacements of existing code.
> 
> I think we will try to minimize infrastructure replacement at least in the short/medium term. After all, it doesn't make too much sense to replace infrastructure that is broken for code that doesn't exist yet.
> 
> David Li and I are preparing a document where we describe the major issues that we'd like to address. The document is a bit on the lengthy side, so it may be easier to start with an email discussion. This is a summary of the main changes we are looking at:
> Need to faithfully represent the execution count taken from dynamic profiles. Currently, MD_prof does not really represent an execution count. This makes things like comparing hotness across functions hard or impossible. We need a concept of global hotness.

The plan that we have discussed in the past (I don’t remember when) was to record simple function entry execution counts. Those could be combined with the  BlockFrequencyInfo to compare “hotness” across functions.
> When the CFG or callgraph change, there need to exist an API for incrementally updating/scaling counts. For instance, when a function is inlined or partially inlined, when the CFG is modified, etc. These counts need to be updated incrementally (or perhaps re-computed as a first step into that direction).
One of the main reasons that we chose to use branch weights to represent profile information within functions is that it makes this problem easier. Of course, we still need to update the branch weights when transforming the CFG, but I believe most of that work has already been done. Are you suggesting that we should work on incremental BlockFrequencyInfo updates? We have discussed that in the past, but so far, it has worked reasonably well to just re-run that analysis. (I wouldn’t be surprised if we’re missing some places where the analysis needs to be invalidated so that it gets re-run.)
> The inliner (and other optimizations) needs to use profile information and update it accordingly. This is predicated on Chandler's work on the pass manager, of course.
> Need to represent global profile summary data. For example, for global hotness determination, it is useful to compute additional global summary info, such as a histogram of counts that can be used to determine hotness and working set size estimates for a large percentage of the profiled execution.
> There are other changes that we will need to incorporate. David, Teresa, Chandler, please add anything large that I missed.
> 
> My main question at the moment is what would be the best way of addressing them. Some seem to require new concepts to be implemented (e.g., execution counts). Others could be addressed as simple bugs to be fixed in the current framework.
> 
> Would it make sense to present everything in a unified document and discuss that? I've got some reservations about that approach because we will end up discussing everything at once and it may not lead to concrete progress. Another approach would be to present each issue individually either as patches or RFCs or bugs.
> 
> I will be taking on the implementation of several of these issues. Some of them involve the SamplePGO harness that I added last year. I would also like to know what other bugs or problems people have in mind that I could also roll into this work.
> 
> 
> Thanks. Diego.
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