[cfe-dev] RFC: Clang-Misexpect Proposal

[Roman Lebedev via cfe-dev]

On Thu, Jul 25, 2019 at 1:06 AM Paul Kirth via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
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>
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> On Wed, Jul 24, 2019 at 6:31 AM Michael Kruse <cfe-dev at meinersbur.de> wrote:
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>> Sounds like a useful tool.
>>
>
> Thanks! I certainly hope it will be.
>
>>
>> Could it be enough for clang to emit warnings in PGO mode when it
>> detects a mismatch between collected profile data and
>> __builtin_expect() ?
>>
>
> You are right: it might be sufficient to simply gate these warnings behind a flag in the PGO pipeline. I think there is a good argument for doing it this way. In fact, it's how my prototype works right now, so we have certainly considered this option.
>
> However, I think it will still be desirable to have a standalone tool that doesn't require the full compilation pipeline. My understanding was that clang-tools are often aimed at this exact use-case: i.e. a custom driver and partial compilation pipeline.
>
> Another important point is that compiler warnings are, by and large, accurate. Our approach to identifying these mismatches is subject to some imprecision due to a deficiency in the collected profile.
Can those be simply reported as normal Remarks?
Those can later be visualized over the codebase via opt-viewer.

> When we issue a warning in clang-misexpect, it means one of two things: (i) your annotation is incorrect, or (ii) your profile data is insufficient. While it at least signals to the developer that investigation is warranted, it is a somewhat unsatisfactory feedback. I think our best effort warnings are a better fit for a clang-tool because of that. To me, warnings in the main compiler have a feeling of more significance and a higher burden of correctness. For something less precise or not generally applicable, a clang-tool appears to be the right place to implement this. This seems to be consistent with existing tools, like clang-tidy, that enable more warnings/diagnostics than are available in the main compiler.
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> That being said, we're happy to take feedback from the community. So if the general consensus is that this should be part of the PGO pipeline, then we can rescope our proposal to only add a new flag to clang.
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> Or maybe the right thing to do is to have both? I'm not exactly sure, which is one of the reasons we made this RFC.
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>>
>> Michael
>>
>>
>> Am Di., 23. Juli 2019 um 22:06 Uhr schrieb Paul Kirth via cfe-dev
>> <cfe-dev at lists.llvm.org>:
>> >
>> > We would like to propose adding a new clang based tool, clang-mispredict, for identifying potentially incorrect uses of __builtin_expect().
>> >
>> > --- TLDR ---
>> > What are we proposing?
>> > * Adding a new tool, clang-mispredict, to the LLVM project.
>> >
>> > What is its purpose?
>> > * Identifying potentially problematic uses of __builtin_expect().
>> >
>> > Why is this important?
>> > * __builtin_expect() has a high performance cost when it is used incorrectly.
>> >
>> > Where will the code live?
>> > * clang-mispredict will live in clang-tools-extra, and there will be some additions to clang.
>> >
>> > Open questions
>> > * What is the correct heuristic for determining a mismatch?
>> >
>> > Potential shortcomings
>> > * The approach is sensitive to both profiling input and configuration.
>> >
>> >
>> > --- Details ---
>> >
>> > We plant to build a new tool, clang-mispredict, on top of Clang’s PGO infrastructure. This tool will issue warnings about the use of _builtin_expect() when collected PGO profile data shows a mismatch with the use of __builtin_expect(). Existing solutions for validating these annotations generally follow this approach: leverage dynamic profiling to validate existing annotations. This is the approach used in the Linux kernel, for example. Unfortunately, existing solutions seem to be custom efforts specific to a particular code base. Supporting this in the LLVM ecosystem gives developers a general way to check the use of _builtin_expect() without creating custom instrumentation.
>> >
>> > Additionally, if _builtin_expect() is used incorrectly, then developers may notice a performance regression when switching to the new PM. This happens because the New PM follows performance annotations more closely than the legacy PM, and is therefore more likely to try and aggressively optimize these cases. Our proposed tool can help developers narrow their focus to potentially problematic areas during the transition.
>> >
>> > Finally, it may also be beneficial to suggest annotations when profiling suggests it would be beneficial. We consider this a desirable property not only from a performance standpoint, but also for identifying potential bugs (both in the profiling corpus and within the codebase).
>> >
>> > We think having clang-mispredict behave similar to clang-tidy is good approach. We can use a compile commands database in conjunction with profiling data to drive our validation tool. More concretely, we plan to use Clang’s existing PGO infrastructure to emit warnings where branch weights are assigned, but stop compilation before the IR is passed to the backend. This largely amounts to checking if the profile counters are outside of certain thresholds for 'hot' and 'cold' code. Similar to clang tidy, we plan to support suppressing warnings within the codebase through use of comment strings.
>> >
>> > The most obvious question about this approach is how to quantify what is ‘hot’ or ‘cold’. Initially, we plan to use the thresholds already present in LLVM; however, we are happy to use any suitable heuristic or empirically derived threshold. Input from the community about the correct heuristic are most welcome.
>> >
>> > Our strategy does have some shortcomings. The usefulness of the warnings are directly related to how representative the profiling input actually was when compared to its normal use. If the profile collected is not representative of typical use, then the warnings may not reflect the ground truth. Program and build configuration can also dramatically change which paths will be executed, thus affecting the quality of the profile data and the generated warnings.
>> >
>> > Ultimately, we are proposing adding a new standalone tool that takes a compile commands database and an instrumentation profile to emit warnings in a clang-tidy fashion.
>> >
>> > Please let us know if you have comments or concerns about this proposal.
>> >
>> > Thanks!
>> >
>> > --
>> > Paul Kirth
>> > _______________________________________________
>> > cfe-dev mailing list
>> > cfe-dev at lists.llvm.org
>> > https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
>
>
> --
> Paul Kirth
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Roman