T-compiler Meeting Agenda 2023-07-20

Announcements

• Proposal from @apiraino: removing T-rustdoc backport nominations from T-compiler meeting’s agenda, T-rustdoc now has their own meetings and can take on that. Is everybody happy with that?
• 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 WG meetings (calendar link)

• wg-async reading club: Tree-Structured Concurrency at time:2023-07-20T13:00:00-04:00
• AFIT/RPITIT Impl Triage at time:2023-07-20T16:00:00-04:00
• Types team meeting: generator locals at time:2023-07-24T10:00:00-04:00
• [Types] Rotating new solver / formality / polonius deep dive at time:2023-07-24T11:00:00-04:00

MCPs/FCPs

• New MCPs (take a look, see if you like them!)
  • “Expose default_hidden_visibility as a rustc command line option” compiler-team#656
• Old MCPs (not seconded, take a look)
  • “Cell Broadband Engine SPU support” compiler-team#614 (last review activity: 3 months ago)
  • “Add support for Zephyr OS” compiler-team#629 (last review activity: about 28 days ago)
  • “Consistently use “region” terminology in later stages of the compiler” compiler-team#634 (last review activity: about 55 days ago)
  • “Add a new --build-id flag to rustc” compiler-team#635 (last review activity: about 55 days ago)
  • “Simplify and improve explicitness of the check-cfg syntax” compiler-team#636 (last review activity: about 35 days ago)
  • “[MCP] proposing a macros working group” compiler-team#637 (last review activity: about 34 days ago)
  • “Add support for visionOS targets” compiler-team#642 (last review activity: about 20 days ago)
  • “Add illumos Tier3 targets” compiler-team#644 (last review activity: about 20 days ago)
  • “Migrate away from u32 as an offset/length type” compiler-team#647 (last review activity: about 13 days ago)
  • “New tier-3 targets for TEEOS” compiler-team#652 (last review activity: about 6 days ago)
• Pending FCP requests (check your boxes!)
  • “Retire the mailing list and make all decisions on zulip” compiler-team#649
  • “Support overriding warnings level for a specific lint via command line” rust#113307
• Things in FCP (make sure you’re good with it)
  • “Revise error code documentation standard” compiler-team#615
  • “Add a blanket flag to enable/disable codegen UB checks” compiler-team#625
  • “Report all lints, even if other errors already occurred.” compiler-team#633
  • “Use FieldIdx all the way down” compiler-team#639
  • “Add support for the riscv64-linux-android target” compiler-team#640
  • “Implement alignment and size niches for references” compiler-team#641
  • “Changes to StatementKind::Coverage” compiler-team#645
  • “KRabcake SANitization (KRSAN) mode” compiler-team#646
  • “Drop MIPS to tier 3” compiler-team#648
  • “Disallow non-identifier-valid –extern crate names " compiler-team#650
  • “Windows support schedule 2024” compiler-team#651
  • “Add option to pass environment variables” compiler-team#653
  • “Amend target policy to require codegen support” compiler-team#655
  • “Add documentation on v0 symbol mangling.” rust#97571
• Accepted MCPs
  • No new accepted proposals this time.
• Finalized FCPs (disposition merge)
  • No new finished FCP (disposition merge) this time.

WG checkins

• @_WG-rls2.0 by @Lukas Wirth (last checkin)

  Since last time we’ve had a lot of MIR improvements from hkalbasi (we can actually interpret a few of the r-a tests in MIR form). Also notable was upgrading chalk, which should have fixed a bunch of long-standing issues and the start of a memory layout viewer.

• @_WG-self-profile by @mw and @Wesley Wiser (last checkin)

  check-in text

Backport nominations

T-compiler stable / T-compiler beta

• :beta: [1.72] “make MCP510 behavior opt-in to avoid conflicts between the CLI and target flavors” rust#113631
  • nominated by @lqd (comment)
  • Fixes #113597 (now in beta) and bypasses the new behavior introduced in #112910
• :beta: [1.72] “allow opaques to be defined by trait queries, again” rust#113690
  • reverts #112963 and fixes #113689, nominated by @oli

• No stable nominations for T-compiler this time.

:back: / :shrug: / :hand:

PRs S-waiting-on-team

T-compiler

• “Tracking issue for dyn upcasting coercion” rust#65991
  • issue is progressing, new comment from Niko
• Other issues in progress or waiting on other teams

Issues of Note

Short Summary

• 0 T-compiler P-critical issues
  • 0 of those are unassigned
• 56 T-compiler P-high issues
  • 25 of those are unassigned
• 0 P-critical, 0 P-high, 4 P-medium, 1 P-low regression-from-stable-to-beta
• 0 P-critical, 0 P-high, 4 P-medium, 2 P-low regression-from-stable-to-nightly
• 0 P-critical, 34 P-high, 100 P-medium, 18 P-low regression-from-stable-to-stable

P-critical

T-compiler

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

T-types

• No P-critical issues for T-types at this time.

T-rustdoc

• No P-critical issues for T-rustdoc at 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 2023-07-18

A lot of spurious results in the regressions this week. However, we did see some real gains with PR #113609, with nearly 40 real-world benchmarks improving their check-build performance by >=1%.

Triage done by @pnkfelix. Revision range: 1d4f5aff..6b9236ed

Summary:

5 Regressions, 5 Improvements, 5 Mixed; 2 of them in rollups 57 artifact comparisons made in total 30 Untriaged Pull Requests

Regressions

miri: protect Move() function arguments during the call #113569 (Comparison Link)

• RalfJ is investigating; has potential fix up in PR #113630, …
• … but its not totally certain that PR is a real fix (i.e. the regression may already have been masked or otherwise resolved independently).
• But meanwhile, I am hypothesizing that the regression reported here is spurious (discussion)
• marking as triaged

Ignore flaky clippy tests. #113621 (Comparison Link)

• already marked as triaged (its noise)

Rollup of 6 pull requests #113673 (Comparison Link)

• addressed by PR #113697, already marked as triaged.

Add even more GHA log groups #113514 (Comparison Link)

• PR author says “this doesn’t affect how the compiler was built at all, the perf regression must be spurious.”
• I agree, marking as triaged

Rollup of 3 pull requests #113738 (Comparison Link)

Improvements

Rewrite UnDerefer, again #113316 (Comparison Link)

(re-)tighten sourceinfo span of adjustments in MIR #112945 (Comparison Link)

Bump bootstrap to 1.72 beta #113637 (Comparison Link)

Add a cache for maybe_lint_level_root_bounded #113609 (Comparison Link)

Remove LLVMRustCoverageHashCString #113430 (Comparison Link)

Mixed

Eliminate ZST allocations in Box and Vec #113113 (Comparison Link)

• regressions here were anticipated and unavoidable. This is a bug fix.
• Marking as triaged.

Enable MIR reference propagation by default #109025 (Comparison Link)

• This is turning on a MIR pass at lower optimizations levels, so its expected that it would cause the compiler to do more work.
• its clear from the perf runs on the PR itself that the PR author already put in much effort to make the pass faster than it had started out.
• marking as triaged.

Resurrect: rustc_target: Add alignment to indirectly-passed by-value types, correcting the alignment of byval on x86 in the process. #112157 (Comparison Link)

• The six primary regressions were all to variants of diesel (all of check/debug/opt) for variious full and incr-full scenarios.
• It isn’t noise, there seems to be a clear cliff that starts with this PR when looking at the graph starting from 2023-0702.
• not marking as triaged yet, but was tempted to do so, because this PR is a prerequiste for unlocking various memcpy optimizations added by pcwalton to LLVM

Remove unneeded handling for ExternalLocation::Unknown in rustdoc render context #113697 (Comparison Link)

• this was mentioned up above when I was talking about PR #113673
• the secondary regressions are all to ctfe-stress-5, which lqd says was noisy at this time.
• marked as triaged.

Add support for allocators in Rc & Arc #89132 (Comparison Link)

• primary regression was to image-0.24.1 opt full by 0.79%
• I think this is just noise. From the graph, it seems like image has unpredictably jumped up and down between two plateaus since PR #113113 (a PR discussed up above that changed low level allocation procotol code in Box and Vec, and thus might be expected to have some weird follow-on effects).
• marked as triaged

Nominated Issues

T-compiler

• “MSVC and rustc disagree on minimum stack alignment on x86 Windows” rust#112480
  • discussed last week (notes), needs further discussion.
• ““Legacy” tier 2 targets have misplaced or absent maintainer docs” rust#113739
  • nominated by @Jubilee, Zulip discussion happening here
  • summary: some tier 2 compile targets are basically unsupported, we should probably refresh our list of support

RFC

• No I-compiler-nominated RFCs this time.

Oldest PRs waiting for review

• “Only unpack tupled args in inliner if we expect args to be unpacked” rust#110833 (last review activity: 2 months ago)
  • cc @cjgillot
• “add a Callable trait that is implemented for unsafe functions, too” rust#107123
  • @Michael Goulet (compiler-errors) / @olireminder about triaging the crater report, thanks!
• “Only run MaybeInitializedPlaces dataflow once to elaborate drops” rust#111555
  • cc: @Wesley Wiser
• “Fix target_feature 1.1 should print the list of missing target features” rust#109710 (last review activity: about 41 days ago)
  • cc: @est31
• “Tweak spans for self arg, fix borrow suggestion for signature mismatch” rust#112508 (last review activity: about 38 days ago)
  • cc: @Wesley Wiser

Next week’s WG checkins

• Types team by @nikomatsakis and @Jack Huey
• @_WG-mir-opt by @oli

Next week’s agenda draft: https://hackmd.io/IPpNv80sQAmD2ZTF6h56ng