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

[Teresa Johnson via llvm-dev]

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.
- 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 |
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