Merge pull request #21 from SparrowLii/parallel-front-end

Add a project idea about promoting parallel front end

Author: Kobzol

README.md

@@ -28,6 +28,7 @@ We use the GSoC project size parameters for estimating the expected time complex
     - [Bootstrap of rustc with rustc_codegen_gcc](#Bootstrap-of-rustc-with-rustc_codegen_gcc)
     - [Refactoring of rustc_codegen_ssa to make it more convenient for the GCC codegen](#Refactoring-of-rustc_codegen_ssa-to-make-it-more-convenient-for-the-GCC-codegen)
     - [ABI/Layout handling for the automatic differentiation feature](#abilayout-handling-for-the-automatic-differentiation-feature)
+    - [Improving parallel frontend](#improving-parallel-frontend)
 - **Infrastructure**
     - [Implement merge functionality in bors](#implement-merge-functionality-in-bors)
     - [Improve bootstrap](#Improve-bootstrap)
@@ -294,6 +295,58 @@ Medium to hard.
 **Zulip streams**
 - [Idea discussion](https://rust-lang.zulipchat.com/#narrow/channel/390790-wg-autodiff)
 
+### Improving parallel frontend
+
+**Description**
+
+Improving compiler performance has always been a focus of the Rust community and one of the main tasks of the compiler team. [Parallelization](https://rustc-dev-guide.rust-lang.org/parallel-rustc.html) of rust compiler is an important and effective approach.
+Currently, the backend end (codegen part) of the compiler has been parallelized, which has brought a huge improvement in the performance of the compiler. However, there is still much room for improvement in the parallelization of the rust frontend.
+
+The most important and valuable work in this area are two aspects:
+
+A) Diagnosing and fixing deadlock [issues](https://github.com/rust-lang/rust/issues?q=is%3Aopen+label%3AWG-compiler-parallel+deadlock) caused by the execution order of compiler queries in a multithreaded environment.
+[Queries](https://rustc-dev-guide.rust-lang.org/query.html) is a unique design of the Rust compiler, which is used to achieve incremental compilation process. It divides the compiler 
+process into various parts and caches the execution results of each part. However, queries caching dependencies between multiple threads may cause deadlock. 
+[`Work-stealing`](https://en.wikipedia.org/wiki/Work_stealing), a method used to improve parallelization performance, is the core reason.
+
+To solve these problems, we need to find the part of the compiler process that causes deadlock through diagnosing coredumps in issues, and adjusting the execution order 
+of this part of code so that there will be no circular dependencies on the query caches between multiple threads. This [PR](https://github.com/rust-lang/rust/pull/118488) is a good example of solving a deadlock problem.
+
+B) Improving the performance of the parallel frontend
+The parallel frontend has implemented parallelization in type checking, MIR borrow checking and other parts of the compiler. However, there is still a lot of room for improvement:
+- HIR lowering. Modifying the array structure of `tcx.untracked.definitions` so that it can be accessed efficiently in multiple threads is likely to be the key.
+- Macro expansion. How to deal with the order problem of name resolution during macro expansion is a difficult problem.
+- Lexing and/or parsing.
+
+Achieving the above goals is of big significance to improving the performance of the Rust compiler.
+
+The project could choose either one of these two areas, or try to tackle both of them together.
+
+**Expected result**
+
+Parallel frontend will not cause deadlock issues. We can ensure usability through [UI testing](https://github.com/rust-lang/rust/pull/132051).
+
+The performance of the compiler will be improved, ideally at least by a couple of percentage points.
+
+**Desirable skills**
+
+Intermediate knowledge of Rust. A basic understanding of the implementation of the compiler process (such as typeck, hir_lowering, macro expansion) would be ideal.
+
+**Project size**
+
+Medium to hard (depending on the chosen scope).
+
+**Difficulty**
+
+Medium to hard.
+
+**Mentor**
+- Sparrow Li ([GitHub](https://github.com/SparrowLii), [Zulip](https://rust-lang.zulipchat.com/#narrow/dm/353056-Sparrow-Li))
+
+**Zulip streams**
+- Idea discussion (TODO)
+- [Parallel frontend working group](https://rust-lang.zulipchat.com/#narrow/channel/187679-t-compiler.2Fwg-parallel-rustc)
+
 ## Infrastructure
 
 ### Implement merge functionality in bors