Stabilize parallel front end

[nikomatsakis]

Metadata
Owner(s)	@SparrowLii
Team(s)	compiler
Goal document	2024h2/parallel-front-end

Summary

We will move rustc's support for parallel front end closer to stability by resolving [ICE] and [deadlock] issues, completing the [test] suit for multithreaded scenario and integrating parallel front end into bootstrap. This fits into our larger goal of improving rustc build times by 20% by leveraging multiple cores and enhance its robustness.

Tasks and status

• Implementation (@SparrowLii)
• Author tests (@SparrowLii)
• Discussion and moral support (compiler )

[nikomatsakis]

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

[SparrowLii]

• Key developments: *
  Use ui test to locate and solve the ICE problem of parallelfront end
  Discussed strategies for customizing and enhancing the compiler testing suit to address hard-to-reproduce bugs
• Blockers: 
  Many deadlock issues are difficult to reproduce and resolve
• Help wanted:
  Help test the deadlock code in the issue list and try to reproduce the issue

[SparrowLii]

• Key developments: *
  Add the cases in the issue list to the UI test to reproduce the bug or verify the non-reproducibility
• Blockers: 
  null
• Help wanted:
  Help test the deadlock code in the issue list and try to reproduce the issue

[SparrowLii]

• Key developments: *
  Still in the process of determining the cause of the deadlock through local testing and compiler code analysis.
• Blockers: 
  null
• Help wanted:
  Help test the deadlock code in the issue list and try to reproduce the issue

[nikomatsakis]

@SparrowLii opened a “proposed 2025H1 project goal](https://rust-lang.github.io/rust-project-goals/2025h1/parallel-front-end.html) to continue this work. The plans for the next 6 months are

• Solve reproducible deadlock issues via tests.
• Enable parallel frontends in bootstrap.
• Continue to improve parallel compilation performance, with the average speed increase from 20% to 25% under 8 cores and 8 threads.
• Communicate with Cargo team on the solution and plan to support parallel front end.

[SparrowLii]

• Key developments: *
  We discussed solutions to avoid potential deadlocks in mechanism, and so far disabling work-stealing in rayon's subloops is an effective solution. We will try to incorporate related modification PR: Only work-steal in the main loop rustc-rayon#12
• Blockers: 
  null
• Help wanted:
  Help test the deadlock code in the issue list and try to reproduce the issue