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b01326ab03
Make `ensure_sufficient_stack()` non-generic, using cargo-llvm-lines Inspired by [this blog post](https://blog.mozilla.org/nnethercote/2020/08/05/how-to-speed-up-the-rust-compiler-some-more-in-2020/) from `@nnethercote,` I used [cargo-llvm-lines](https://github.com/dtolnay/cargo-llvm-lines/) on the rust compiler itself, to improve it's compile time. This PR contains only one low-hanging fruit, but I also want to share some measurements. The function `ensure_sufficient_stack()` was monomorphized 1500 times, and with it the `stacker` and `psm` crates, for a total of 1.5% of all llvm IR lines. With some trickery I convert the generic closure into a dynamic one, and thus all that code is only monomorphized once. # Measurements Getting these numbers took some fiddling with CLI flags and I [modified](https://github.com/Julian-Wollersberger/cargo-llvm-lines/blob/master/src/main.rs#L115) cargo-llvm-lines to read from a folder instead of invoking cargo. Commands I used: ``` ./x.py clean RUSTFLAGS="--emit=llvm-ir -C link-args=-fuse-ld=lld -Z self-profile=profile" CARGOFLAGS_BOOTSTRAP="-Ztimings" RUSTC_BOOTSTRAP=1 ./x.py build -i --stage 1 library/std # Then manually copy all .ll files into a folder I hardcoded in cargo-llvm-lines in main.rs#L115 cd ../cargo-llvm-lines cargo run llvm-lines ``` The result is this list (see [first 500 lines](https://github.com/Julian-Wollersberger/cargo-llvm-lines/blob/master/llvm-lines-rustc-before.txt) ), before the change: ``` Lines Copies Function name ----- ------ ------------- 16894211 (100%) 58417 (100%) (TOTAL) 2223855 (13.2%) 502 (0.9%) rustc_query_system::query::plumbing::get_query_impl::{{closure}} 1331918 (7.9%) 1287 (2.2%) hashbrown::raw::RawTable<T>::reserve_rehash 774434 (4.6%) 12043 (20.6%) core::ptr::drop_in_place 294170 (1.7%) 499 (0.9%) rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl 245410 (1.5%) 1552 (2.7%) psm::on_stack::with_on_stack 210311 (1.2%) 1 (0.0%) rustc_target::spec::load_specific 200962 (1.2%) 513 (0.9%) rustc_query_system::query::plumbing::get_query_impl 190704 (1.1%) 1 (0.0%) rustc_middle::ty::query::<impl rustc_middle::ty::context::TyCtxt>::alloc_self_profile_query_strings 180272 (1.1%) 468 (0.8%) rustc_query_system::query::plumbing::load_from_disk_and_cache_in_memory 177396 (1.1%) 114 (0.2%) rustc_query_system::query::plumbing::force_query_impl 161134 (1.0%) 445 (0.8%) rustc_query_system::dep_graph::graph::DepGraph<K>::with_anon_task 141551 (0.8%) 186 (0.3%) rustc_query_system::query::plumbing::incremental_verify_ich 110191 (0.7%) 7 (0.0%) rustc_middle::ty::context::_DERIVE_rustc_serialize_Decodable_D_FOR_TypeckResults::<impl rustc_serialize::serialize::Decodable<__D> for rustc_middle::ty::context::TypeckResults>::decode::{{closure}} 108590 (0.6%) 420 (0.7%) core::ops::function::FnOnce::call_once 88488 (0.5%) 21 (0.0%) rustc_query_system::dep_graph::graph::DepGraph<K>::try_mark_previous_green 86368 (0.5%) 1 (0.0%) rustc_middle::ty::query::stats::query_stats 85654 (0.5%) 3973 (6.8%) <&T as core::fmt::Debug>::fmt 84475 (0.5%) 1 (0.0%) rustc_middle::ty::query::Queries::try_collect_active_jobs 81220 (0.5%) 862 (1.5%) <hashbrown::raw::RawIterHash<T> as core::iter::traits::iterator::Iterator>::next 77636 (0.5%) 54 (0.1%) core::slice::sort::recurse 66484 (0.4%) 461 (0.8%) <hashbrown::raw::RawIter<T> as core::iter::traits::iterator::Iterator>::next ``` All `.ll` files together had 4.4GB. After my change they had 4.2GB. So a few percent less code LLVM has to process. Hurray! Sadly, I couldn't measure an actual wall-time improvement. Watching YouTube while compiling added to much noise... Here is the top of the list after the change: ``` 16460866 (100%) 58341 (100%) (TOTAL) 1903085 (11.6%) 504 (0.9%) rustc_query_system::query::plumbing::get_query_impl::{{closure}} 1331918 (8.1%) 1287 (2.2%) hashbrown::raw::RawTable<T>::reserve_rehash 777796 (4.7%) 12031 (20.6%) core::ptr::drop_in_place 551462 (3.4%) 1519 (2.6%) rustc_data_structures::stack::ensure_sufficient_stack::{{closure}} ``` Note that the total was reduced by 430 000 lines and `psm::on_stack::with_on_stack` has disappeared. Instead `rustc_data_structures::stack::ensure_sufficient_stack::{{closure}}` appeared. I'm confused about that one, but it seems to consist of inlined calls to `rustc_query_system::*` stuff. Further note the other two big culprits in this list: `rustc_query_system` and `hashbrown`. These two are monomorphized many times, the query system summing to more than 20% of all lines, not even counting code that's probably inlined elsewhere. Assuming compile times scale linearly with llvm-lines, that means a possible 20% compile time reduction. Reducing eg. `get_query_impl` would probably need a major refactoring of the qery system though. _Everything_ in there is generic over multiple types, has associated types and passes generic Self arguments by value. Which means you can't simply make things `dyn`. --------------------------------------- This PR is a small step to make rustc compile faster and thus make contributing to rustc less painful. Nonetheless I love Rust and I find the work around rustc fascinating :) |
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| rustc | ||
| rustc_apfloat | ||
| rustc_arena | ||
| rustc_ast | ||
| rustc_ast_lowering | ||
| rustc_ast_passes | ||
| rustc_ast_pretty | ||
| rustc_attr | ||
| rustc_builtin_macros | ||
| rustc_codegen_llvm | ||
| rustc_codegen_ssa | ||
| rustc_data_structures | ||
| rustc_driver | ||
| rustc_error_codes | ||
| rustc_errors | ||
| rustc_expand | ||
| rustc_feature | ||
| rustc_fs_util | ||
| rustc_graphviz | ||
| rustc_hir | ||
| rustc_hir_pretty | ||
| rustc_incremental | ||
| rustc_index | ||
| rustc_infer | ||
| rustc_interface | ||
| rustc_lexer | ||
| rustc_lint | ||
| rustc_llvm | ||
| rustc_macros | ||
| rustc_metadata | ||
| rustc_middle | ||
| rustc_mir | ||
| rustc_mir_build | ||
| rustc_parse | ||
| rustc_parse_format | ||
| rustc_passes | ||
| rustc_plugin_impl | ||
| rustc_privacy | ||
| rustc_query_system | ||
| rustc_resolve | ||
| rustc_save_analysis | ||
| rustc_serialize | ||
| rustc_session | ||
| rustc_span | ||
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| rustc_traits | ||
| rustc_ty | ||
| rustc_typeck | ||