T Compiler Meeting Agenda 2022 02 10

T-compiler Meeting Agenda 2022-02-10

Tracking Issue

Announcements

MCPs/FCPs

  • New MCPs (take a look, see if you like them!)
    • No new proposals this time.
  • Old MCPs (not seconded, take a look)
    • “Accept pc in place of unknown and unknown in place of pc for x86_64 and i?86 targets” compiler-team#441 (last review activity: 7 months ago)
    • “Make -Z binary-dep-depinfo the default behavior” compiler-team#464 (last review activity: 4 months ago)
    • “Tier 3 target proposal: riscv64gc-linux-android (Android target for riscv64gc)” compiler-team#472 (last review activity: 2 months ago)
    • “-Dwarnings to cover all warnings” compiler-team#473 (last review activity: 2 months ago)
    • “Build-time execution sandboxing” compiler-team#475 (last review activity: about 44 days ago)
    • “Dealing with type/const ambiguities” compiler-team#480 (last review activity: about 20 days ago)
  • Pending FCP requests (check your boxes!)
  • Things in FCP (make sure you’re good with it)
  • Accepted MCPs
    • No new accepted proposals this time.
  • Finalized FCPs (disposition merge)
    • “Tracking issue for #[cfg(target_has_atomic = …)]” rust#32976
    • “Stabilize -Z instrument-coverage as -C instrument-coverage” rust#90132

WG checkins

  • rust#92306 fixed a major blocker (there are only diagnostic blockers left) for stabilizing nll
  • @_WG-rustc-dev-guide by @Santiago Pastorino and @Yuki Okushi|217081 (previous checkin)

Most notable changes

  • Clean up section about FCPs #1287
  • Streamline “Getting Started” some more. #1286

Most notable WIPs

  • Describe Type Alias Impl Trait (TAIT) Inference Algorithm #1297
  • method-lookup.md improvements #1296
  • Extend debugging llvm section #1290
  • Added detail to codegen section #1216
  • Update build instructions for rustdoc #1117
  • Document inert vs active attributes #1110
  • Explain the new valtree system for type level constants. #1097

Backport nominations

T-compiler beta / T-compiler stable

  • :beta: “Fix invalid special casing of the unreachable! macro” rust#93179
    • Fixes T-libs p-high issue rust#92137 about the unreachable!() macro behaviour in edition 2021
    • also nominated for stable backport
    • @simulacrum remarks that the perf regression is not due to this PR but to the codegen unit (see comment)
  • :beta: “backport llvm fix for issue 91671.” rust#93426
  • :beta: “Resolve lifetimes for const generic defaults” rust#93669
    • opened by @Michael Goulet (compiler-errors)
    • fixes P-critical rust#93647
  • :beta: “Complete removal of #[main] attribute from compiler” rust#93753
    • Removes the old #[main] and prevents future crates using it by mistake (more context in this comment)
    • a crater run should indicate if there are crate sin the wild still using this attribute
  • :stable: “Fix invalid special casing of the unreachable! macro” rust#93179
    • discussed in beta-backport

T-rustdoc beta / T-rustdoc stable

  • No backport nominations for T-rustdoc this time.

:back: / :shrug: / :hand:

PRs S-waiting-on-team

T-compiler

  • No PRs waiting on T-compiler this time.

Oldest PRs waiting for review

T-compiler

  • “Mir-Opt for copying enums with large discrepancies” rust#85158 (last review activity: 3 months ago)
  • “Switch to normalized triples for Fuchsia” rust#90510 (last review activity: 2 months ago)
    • PR assignee @Esteban Küber, reviewed also by @tmandry
    • though last comment seems to indicate it’s a bit stale
  • “Improve unsafe diagnostic” rust#91133 (last review activity: 2 months ago)
    • Seems PR author is asking a new review from @Esteban Küber (assigned reviewer)
  • “Remove eval_always for privacy.” rust#90581 (last review activity: 2 months ago)
    • PR author pushed after the last perf report (with mixed results)
    • Perhaps this is waiting on someone to trigger @rust-timer again?
  • “Remove eval_always for privacy_access_levels.” rust#91295 (last review activity: 2 months ago)
    • rust-highfive assigned to @Aaron Hill
    • maybe waiting for a review after last perf run (with regressions)

Issues of Note

Short Summary

P-critical

T-compiler

  • “Const generic defaults ICE: No bound vars found” rust#93647
    • @Michael Goulet (compiler-errors) provided a fix in PR rust#93669 (beta-backport nominated)
  • “#92007 appears to break inference for previously compiling code” rust#93821
  • “Internal Compiler Error when compiling rocket = “0.5.0-rc.1"” rust#93831
  • No P-critical issues for T-rustdoc this time.

P-high regressions

P-high beta regressions

  • No P-high beta regressions this time.

Unassigned P-high nightly regressions

  • No unassigned P-high nightly regressions this time.

Performance logs

triage logs for 2022-02-02

This was a relatively quiet week with regressions and improvements relatively equally each other out. The big exception is with a somewhat large regression in rustdoc which was needed for a large and important architectural change. This could lead to users see somewhat slower doc build times with more memory used especially in projects with large crate dependency graphs.

Triage done by @rylev. Revision range: c54dfee65126a0ac385d55389a316e89095a0713..1ea4851715893ee3f365a8ef09d47165e9a7864f

2 Regressions, 4 Improvements, 2 Mixed; 1 of them in rollups 29 Untriaged Pull Requests 35 comparisons made in total

Regressions

rustdoc: Pre-calculate traits that are in scope for doc links #88679

  • Average relevant regression: 5.4%
  • Largest regression in instruction counts: 9.1% on full builds of regression-31157 doc
  • All the regressions are localized to rustdoc and are quite substantial but exacerbated by an architectural issue which needs to be solved.
  • The regressions are well known and extensively discussed in the PR with the next steps being tracked in an issue.

Store def_id_to_hir_id as variant in hir_owner. #93373

  • Average relevant regression: 1.2%
  • Average relevant improvement: -0.3%
  • Largest regression in instruction counts: 5.5% on incr-unchanged builds of match-stress-enum check
  • This is the first commit (originally bundled with #93301) of a larger PR (#89278) which saw larger PR regressions than the two sub PRs combined.
  • A more detailed justification for this can be found here, but in general this allows for even more incremental compilation and is thus worth the small immediate compilation performance penalty.

Improvements

  • Fix the unsoundness in the early_otherwise_branch mir opt pass #91840
  • Only traverse attrs once while checking for coherence override attributes #93343
  • Rollup of 10 pull requests #93427
  • Disable drop range analysis #93284

Mixed

Store hir_id_to_def_id in OwnerInfo. #93301

  • Average relevant regression: 0.6%
  • Average relevant improvement: -0.5%
  • Largest improvement in instruction counts: -0.6% on full builds of unused-warnings check
  • Largest regression in instruction counts: 2.2% on incr-unchanged builds of match-stress-enum check
  • This is the first commit (originally bundled with #93373) of a larger PR (#89278) which saw larger PR regressions than the two sub PRs combined.

Create core::fmt::ArgumentV1 with generics instead of fn pointer #90891

  • Average relevant regression: 1.2%
  • Average relevant improvement: -1.2%
  • Largest improvement in instruction counts: -6.7% on full builds of tokio-webpush-simple opt
  • Largest regression in instruction counts: 2.1% on full builds of keccak check
  • The improvements outweigh the regressions considerably and this was considered a fine trade off.

Nominated Issues

T-compiler

  • No I-compiler-nominated at this time

RFC

  • No nominated RFCs for T-compiler this time.