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Auto merge of #76680 - Julian-Wollersberger:nongeneric_ensure_sufficient_stack, r=jyn514

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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 :)
This commit is contained in:
bors 2020-09-21 17:32:57 +00:00
commit b01326ab03
2 changed files with 3 additions and 3 deletions
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4
Cargo.lock
View File

@ -4468,9 +4468,9 @@ checksum = "a8f112729512f8e442d81f95a8a7ddf2b7c6b8a1a6f509a95864142b30cab2d3"
[[package]]
name = "stacker"
version = "0.1.11"
version = "0.1.12"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "a92bc346006ae78c539d6ab2cf1a1532bc657b8339c464877a990ec82073c66f"
checksum = "21ccb4c06ec57bc82d0f610f1a2963d7648700e43a6f513e564b9c89f7991786"
dependencies = [
"cc",
"cfg-if",

2
compiler/rustc_data_structures/Cargo.toml
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@ -26,7 +26,7 @@ rustc_index = { path = "../rustc_index", package = "rustc_index" }
bitflags = "1.2.1"
measureme = "0.7.1"
libc = "0.2"
stacker = "0.1.11"
stacker = "0.1.12"
tempfile = "3.0.5"
[dependencies.parking_lot]