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

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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200414/77d03893/attachment.html>