From 7c3ce02a1124b008addb9764173ad441f4741c00 Mon Sep 17 00:00:00 2001 From: Nicholas Nethercote Date: Fri, 9 Jun 2023 14:39:13 +1000 Subject: Introduce a minimum CGU size in non-incremental builds. Because tiny CGUs make compilation less efficient *and* result in worse generated code. We don't do this when the number of CGUs is explicitly given, because there are times when the requested number is very important, as described in some comments within the commit. So the commit also introduces a `CodegenUnits` type that distinguishes between default values and user-specified values. This change has a roughly neutral effect on walltimes across the rustc-perf benchmarks; there are some speedups and some slowdowns. But it has significant wins for most other metrics on numerous benchmarks, including instruction counts, cycles, binary size, and max-rss. It also reduces parallelism, which is good for reducing jobserver competition when multiple rustc processes are running at the same time. It's smaller benchmarks that benefit the most; larger benchmarks already have CGUs that are all larger than the minimum size. Here are some example before/after CGU sizes for opt builds. - html5ever - CGUs: 16, mean size: 1196.1, sizes: [3908, 2992, 1706, 1652, 1572, 1136, 1045, 948, 946, 938, 579, 471, 443, 327, 286, 189] - CGUs: 4, mean size: 4396.0, sizes: [6706, 3908, 3490, 3480] - libc - CGUs: 12, mean size: 35.3, sizes: [163, 93, 58, 53, 37, 8, 2 (x6)] - CGUs: 1, mean size: 424.0, sizes: [424] - tt-muncher - CGUs: 5, mean size: 1819.4, sizes: [8508, 350, 198, 34, 7] - CGUs: 1, mean size: 9075.0, sizes: [9075] Note that CGUs of size 100,000+ aren't unusual in larger programs. --- src/doc/rustc/src/codegen-options/index.md | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'src') diff --git a/src/doc/rustc/src/codegen-options/index.md b/src/doc/rustc/src/codegen-options/index.md index 1041d502669..8de638dde4f 100644 --- a/src/doc/rustc/src/codegen-options/index.md +++ b/src/doc/rustc/src/codegen-options/index.md @@ -31,8 +31,8 @@ Supported values can also be discovered by running `rustc --print code-models`. ## codegen-units -This flag controls how many code generation units the crate is split into. It -takes an integer greater than 0. +This flag controls the maximum number of code generation units the crate is +split into. It takes an integer greater than 0. When a crate is split into multiple codegen units, LLVM is able to process them in parallel. Increasing parallelism may speed up compile times, but may -- cgit 1.4.1-3-g733a5