T-compiler Meeting Agenda 2022-05-26

Announcements

• Tomorrow time:2022-05-27T13:00:00+02:00 Types Team: Formality roadmap calendar link and issue
• Reminder: if you see a PR/issue that seems like there might be legal implications due to copyright/IP/etc, please let the Core team know (or at least message @pnkfelix or @Wesley Wiser so we can pass it along).

Other upcoming WG meetings (calendar link)

• time:2022-05-23T15:00:00+02:00 wg-rls-2.0 weekly sync-up (calendar link)
• time:2022-05-30T20:00:00+02:00 [wg-traits] GATs Sync (calendar link)
• time:2022-06-01T13:00:00+02:00 [Types team] Hack session: Advanced subtyping (calendar link)
• time:2022-06-03T15:00:00+02:00 Types Team: Planning/Deep-Dive meeting (calendar link)

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: about 4 days ago)
    • :loudspeaker: Stale MCP: candidate for closing in a week
    • Zulip discussion
    • @pnkfelix checked the pulse of the discussion in September 2021
    • the core proposal of the MCP seem to have received mixed comments
  • “Make -Z binary-dep-depinfo the default behavior” compiler-team#464 (last review activity: about 4 days ago)
    • :loudspeaker: Stale MCP: candidate for closing in a week
    • Zulip discussion
    • MCP discussion received some favorable comments, no unresolved points
  • “Tier 3 target proposal: riscv64gc-linux-android (Android target for riscv64gc)” compiler-team#472 (last review activity: 5 months ago)
  • “-Dwarnings to cover all warnings” compiler-team#473 (last review activity: 5 months ago)
  • “Dealing with type/const ambiguities” compiler-team#480 (last review activity: 4 months ago)
  • “Removing codegen logic for nvptx-nvidia-cuda (32-bit target)” compiler-team#496 (last review activity: 2 months ago)
  • “Change compiletest declarations parsing” compiler-team#512 (last review activity: about 17 days ago)
  • “Arbitrary annotations in compiletest” compiler-team#513 (last review activity: about 17 days ago)
• Pending FCP requests (check your boxes!)
  • “Stabilize -Zgcc-ld=lld as -Clink-self-contained=linker -Clinker-flavor=gcc-lld” compiler-team#510
  • “Increase the minimum linux-gnu versions” rust#95026
• Things in FCP (make sure you’re good with it)
  • “Build-time execution sandboxing” compiler-team#475
  • “Tracking issue for Consistent no-prelude attribute (RFC 501)” rust#20561
  • “Tracking Issue for -Z terminal-width” rust#84673
  • “Stabilize the bundle native library modifier” rust#95818
  • “Remove label/lifetime shadowing warnings” rust#96296
• Accepted MCPs
  • “Add attribute to run specific tests in an isolated process” compiler-team#508
  • “Add an EarlyBinder type” compiler-team#514
• Finalized FCPs (disposition merge)
  • “Neither require nor imply lifetime bounds on opaque type for well formedness” rust#95474

WG checkins

• @_WG-diagnostics by @Esteban Küber and @oli

• Introduction of translation machinery in the rustc codebase, with a handful of errors being migrated to this new system for testing.
• New fn argument diagnostic that detects additions, removals and substitutions in the call site, with appropriate suggestions.
• Tons of diagnostics improvements and fixes.

• @_WG-rustc-dev-guide by @Santiago Pastorino and @Yuki Okushi|217081

Most notable changes

• Add docs for logging of queries. #1350
• mention WithOptConstParam #1346
• Update docs for deprecated attribute #1338

Most notable WIPs

• Update suggested config to expand proc macros #1341
• rewrite bootstrapping stages #1327
• Describe Type Alias Impl Trait (TAIT) Inference Algorithm #1297
• 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 stable / T-compiler beta

• :beta: “Fix some typos in arg checking algorithm” rust#97303
  • Fixes rust#97197, P-medium ICE
  • patch authored and nominated for backport by @Michael Goulet (compiler-errors)
• :beta: “rustc: Fix ICE in native library error reporting” rust#97328
  • Fixes rust#97299, building Rust broken for MSYS2/Mingw (tier 1)
• :stable: “rustc: Fix ICE in native library error reporting” rust#97328
  • backport nomination of previous regression

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: 2 months ago)
  • PR author asks for comment cc @Wesley Wiser
• “Only compile #[used] as llvm.compiler.used for ELF targets” rust#93718 (last review activity: about 41 days ago)
  • PR seems ready for another round of review cc: @pnkfelix
• “Micro-optimize InferCtxt::replace_bound_vars_with_placeholders by avoiding unnecessary allocation” rust#96021 (last review activity: about 35 days ago)
  • rust highfive bot assigned to @nnethercote, reroll dice?
• “libcore: Add iter::from_generator which is like iter::from_fn, but for coroutines instead of functions” rust#96298 (last review activity: about 32 days ago)
  • part of PR may be of interest for T-compiler, assign a reviewer for that?
• “TypeId: use a (v0) mangled type to remain sound in the face of hash collisions.” rust#95845 (last review activity: about 31 days ago)
  • last review from @lcnr end of april (comment)
  • after that, lengthy discussion - unsure if still in flux or any actionable available (maybe add a summary?)

Issues of Note

Short Summary

• 0 T-compiler P-critical issues
  • 0 of those are unassigned
• 60 T-compiler P-high issues
  • 45 of those are unassigned
• 0 P-critical, 0 P-high, 3 P-medium, 0 P-low regression-from-stable-to-beta
• 0 P-critical, 0 P-high, 2 P-medium, 0 P-low regression-from-stable-to-nightly
• 0 P-critical, 38 P-high, 80 P-medium, 10 P-low regression-from-stable-to-stable

P-critical

T-compiler

• No P-critical issues for T-compiler this time.

T-rustdoc

• 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 2022-05-24

Overall a positive week for non-incremental performance (roughly 0.5% faster), with >1% regressions on incremental benchmarks, primarily due to #95563, which will hopefully be investigated in the coming weeks.

Triage done by @simulacrum. Revision range: 7355d971a954ed63293e4191f6677f60c1bc07d9..43d9f3859e0204e764161ee085a360274b5f3e9a

2 Regressions, 5 Improvements, 4 Mixed; 0 of them in rollups 57 artifact comparisons made in total

Regressions

Transition to valtrees pt1 #97019 (Comparison Link)

@oli-obk has suggested applying some targeted inlining as a likely resolution to the regressions; work to follow up needs to be done. It seems like the regression is likely to be resolved (or at least effort applied to do so).

Make write/print macros eagerly drop temporaries #96455 (Comparison Link)

Change is relatively minor and limited to just a single benchmark, so marking the regression as triaged; further investigation is not warranted at this time.

Improvements

Add PID to PGO profile data filename #97110 (Comparison Link)

An expected improvement in our PGO scripts, with a small magnitude effect across many benchmarks.

–remap-path-prefix: Fix duplicated path components in debuginfo #96867 (Comparison Link)

Improvements likely to be primarily driven by reducing the amount of codegen due to simpler/smaller debuginfo.

rustdoc: Reduce clean::Type size #93963 (Comparison Link)

rustc_parse: Move AST -> TokenStream conversion logic to rustc_ast #97251 (Comparison Link)

Refactor call terminator to always include destination place #96098 (Comparison Link)

Mixed

Add a query for checking whether a function is an intrinsic. #97012 (Comparison Link)

Mixed results, but overall looks to be pretty neutral. The preparation for future maintainability outweighs the possible small cost to keeping this change. Regression was already marked as triaged by @oli-obk (thank you!).

Cache more queries on disk #95418 (Comparison Link)

This is overall an improvement, with regressions primarily limited to secondary benchmarks. Mixed results are fairly expected from a change like this; marking the regression as triaged. @cjgillot (PR author) also posted a good summary:

This PR caches many more queries' results on disk. The queries have been chosen to be: (1) outputs of typechecking, and (2) parts of metadata. This caching allows for wide and large compile time savings, up to 36%. The tradeoff is a compile time increase (4%) when these caches cannot be usefully used, and a increased on-disk size around 30%.

Move the extended lifetime resolution into typeck context #95563 (Comparison Link)

Looks like a fairly large regression across a number of benchmarks, though limited to the incremental scenarios. Non-incremental benchmarks show an improvement (hence the mixed results). Left a comment on the PR asking for clarification and further investigation.

Update rustc PGO benchmark list #97120 (Comparison Link)

Overall an improvement, some slight regressions are expected from a larger change like this which bumps PGO inputs.

Nominated Issues

T-compiler

• “Change enum->int casts to not go through MIR casts.” rust#96862
  • nominated by @RalfJ (T-compiler specifically for first point in comment)

RFC

• “New rustc and Cargo options to allow path sanitisation by default” rfcs#3127
  • nominated by @matklad in response to this comment from @Andy Wang asking T-compiler an opinion about the implementation

Next week’s WG checkins

• @_WG-traits (impl trait) by @nikomatsakis @oli
• @_WG-llvm by @nagisa and @Nikita Popov