[llvm-dev] 7-8% compile time slowdowns in LLVM 10

Tobias Hieta via llvm-dev llvm-dev at lists.llvm.org
Wed Apr 15 08:17:18 PDT 2020

Hi Neil,

That's unfortunate - I am in the process of updating to clang 10 here and
this slow down will impact our developers as well.

If I have some time this weekend I will try to profile clang and see if I
can figure out if there is a single regression or just many smaller things.


On Tue, Apr 14, 2020, 10:26 Neil Henning via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hey list,
> TL;DR - LLVM 10 is around 7-8% slower than LLVM 9 when compiling the same
> inputs.
> So here at Unity our Burst HPC# compiler uses LLVM to provide our users
> with some very optimal codegen. LLVM is used in two ways:
> 1. In the Unity editor we JIT compile user code.
> 2. We also have an AOT mode for when our users are building a full game.
> Particularly for 1., compile time really matters for us. Anything we can
> do to improve compile time will increase our users' experience when editing
> their gameplay code and seeing what effect it has on the scene they are
> using. We keep metrics of compile time as a result, and after hearing the
> concerns from the Rust folks about LLVM 10's slowdowns I had a look at our
> numbers with an upgraded LLVM 10 toolchain -  you can see a snap from the
> spreadsheet here that shows an overall 7-8% slowdown in the compiler
> https://twitter.com/sheredom/status/1247128694554087426.
> Also since we keep golden asm files for a huge range of important tests we
> need to preserve, I can also definitively say that the produced asm does
> not contain any significant improvements to warrant the extra compile time
> being used (some changes in register selection and a slightly better
> placement of instructions).
> I don't really have any answers to if or how this can be fixed, I just
> thought this was a useful data point to the community as a whole and I'd
> raise the visibility.
> Cheers,
> -Neil.
> --
> Neil Henning
> Senior Software Engineer Compiler
> unity.com
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200415/928687ad/attachment.html>

More information about the llvm-dev mailing list