[LLVMdev] RFC - Improvements to PGO profile support

Bob Wilson bob.wilson at apple.com
Thu Mar 5 08:29:52 PST 2015

> On Mar 2, 2015, at 4:19 PM, Diego Novillo <dnovillo at google.com> wrote:
> On Thu, Feb 26, 2015 at 6:54 PM, Diego Novillo <dnovillo at google.com <mailto:dnovillo at google.com>> wrote:
> I've created a few bugzilla issues with details of some of the things I'll be looking into. I'm not yet done wordsmithing the overall design document. I'll try to finish it by early next week at the latest.
> The document is available at
> https://docs.google.com/document/d/15VNiD-TmHqqao_8P-ArIsWj1KdtU-ElLFaYPmZdrDMI/edit?usp=sharing  <https://docs.google.com/document/d/15VNiD-TmHqqao_8P-ArIsWj1KdtU-ElLFaYPmZdrDMI/edit?usp=sharing>
> There are several topics covered. Ideally, I would prefer that we discuss each topic separately. The main ones I will start working on are the ones described in the bugzilla links we have in the doc.
> This is just a starting point for us. I am not at all concerned with implementing exactly what is proposed in the document. In fact, if we can get the same value using the existing support, all the better.
> OTOH, any other ideas that folks may have that work better than this are more than welcome. I don't have really strong opinions on the matter. I am fine with whatever works.

Thanks for the detailed write-up on this. Some of the issues definitely need to be addressed. I am concerned, though, that some of the ideas may be leading toward a scenario where we have essentially two completely different ways of representing profile information in LLVM IR. It is great to have two complementary approaches to collecting profile data, but two representations in the IR would not make sense.

The first issue raised is that profile execution counts are not represented in the IR. This was a very intentional decision. I know it goes against what other compilers have done in the past. It took me a while to get used to the idea when Andy first suggested it, so I know it seems awkward at first. The advantage is that branch probabilities are much easier to keep updated in the face of compiler transformations, compared to execution counts. We are definitely missing the per-function execution counts that are needed to be able to compare relative “hotness” across functions, and I think that would be a good place to start making improvements. In the long term, we should keep our options open to making major changes, but before we go there, we should try to make incremental improvements to fix the existing infrastructure.

Many of the other issues you raise seem like they could also be addressed without major changes to the existing infrastructure. Let’s try to fix those first.
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