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

[Neil Henning via llvm-dev]

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
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