[llvm-dev] PGO is ineffective for Rust - but why?

[Teresa Johnson via llvm-dev]

On Thu, Sep 12, 2019 at 8:18 AM Teresa Johnson <tejohnson at google.com> wrote:

> I just have a couple suggestions off the top of my head:
> - have you tried using the new pass manager
> (-fexperimental-new-pass-manager)? That has access to additional analysis
> info during inlining and is able to make more precise PGO based inline
> decisions.
>

(although note the above shouldn't make the difference between no
performance and a typical PGO performance boost)

Another thing I just thought of - are you using -ffunction-sections and
-fdata-sections? These will allow for PGO based function layout in the
linker (assuming you are using lld or gold).

- have you tried collecting profile data with and without PGO to see if you
> can compare where cycles are being spent? That's my usual way of debugging
> performance differences related to inlining or profile changes.
> - just a comment that it is odd you are getting better performance without
> the pre-inlining - which typically helps because you get better
> context-sensitive profile info. Maybe sanity check that the pre inlining is
> kicking in for both the profile gen and use passes?
>
> Teresa
>
> On Thu, Sep 12, 2019 at 2:18 AM Michael Woerister via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi everyone,
>>
>> As part of my work for Mozilla's Low Level Tools team I've
>> implemented PGO in the Rust compiler. The feature is
>> available since Rust 1.37 [1]. However, so far we have not
>> seen any actual performance gains from enabling PGO for
>> Rust code. Performance even seems to drop 1-3% with PGO
>> enabled. I wonder why that is and I'm hoping that someone
>> here might have experience debugging PGO effectiveness.
>>
>>
>> PGO in the Rust compiler
>> ------------------------
>>
>> The Rust compiler uses IR-level instrumentation (the
>> equivalent of Clang's `-fprofile-generate`/`-fprofile-use`).
>> This has worked pretty well and even enables doing PGO for
>> mixed Rust/C++ codebases when also using Clang.
>>
>> The Rust compiler has regression tests that make sure that:
>>
>> - instrumentation shows up in LLVM IR for the `generate` phase,
>>   and that
>>
>> - profiling data is actually used during the `use` phase, i.e.
>>   that cold functions get marked with `cold` and hot functions
>>   get marked with `inline`.
>>
>> I also verified manually that `branch_weights` are being set
>> in IR. So, from my perspective, the PGO implementation does
>> what it is supposed to do.
>>
>> However, as already mentioned, in all benchmarks I've seen so
>> far performance seems to stay the same at best and often even
>> suffers slightly. Which is suprising because for C++ code
>> using Clang's version of IR-level instrumentation & PGO brings
>> signifcant gains (up to 5-10% from what I've seen in
>> benchmarks for Firefox).
>>
>> One thing we noticed early on is that disabling the
>> pre-inlining pass (`-disable-preinline`) seems to consistently
>> improve the situation for Rust code. Doing that we sometimes
>> see performance wins of almost 1% over not using PGO. This
>> again is very different to C++ where disabling this pass
>> causes dramatic performance loses for the Firefox benchmarks.
>> And 1% performance improvement is still well below
>> expectations, I think.
>>
>> So my questions to you are:
>>
>> - Has anybody here observed something similar while
>>   wokring on or with PGO?
>>
>> - Are there certain known characteristics of LLVM IR code
>>   that inhibit PGO's effectiveness and that IR produced by
>>   `rustc` might exhibit?
>>
>> - Does anybody know of a good source that describes how to
>>   effectively debug a problem like this?
>>
>> - Does anybody know of a small example program in C/C++
>>   that is known to profit from PGO and that could be
>>   re-implemented in Rust for comparison?
>>
>> Thanks a lot for reading! Any help is appreciated.
>>
>> -Michael
>>
>> [1]
>> https://blog.rust-lang.org/2019/08/15/Rust-1.37.0.html#profile-guided-optimization
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>>
>
>
> --
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>


-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |
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