T-compiler Meeting Agenda 2023-06-15

Announcements

• Input from @apiraino (echoing a comment from @Jack Huey): rethinking WGs checkins in t-compiler meetings. Is the current rotation reflecting the reality? Suggests opening a topic under #t-compiler and discuss with the leads
• Types team meeting at time:2023-06-19T10:00:00-04:00
• 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: Open discussion at time:2023-06-15T16:30:00-04:00
• wg-rls-2.0 weekly sync-up at time:2023-06-19T11:00:00-04:00
• wg-rls-2.0 steering meeting at time:2023-06-19T11:00:00-04:00

MCPs/FCPs

• New MCPs (take a look, see if you like them!)
  • “[MCP] proposing a macros working group” compiler-team#637
• Old MCPs (stale MCP might be closed as per MCP procedure)
  • None at this time
• Old MCPs (not seconded, take a look)
  • “2024: Decrease debuginfo generated by -Cdebuginfo=1” compiler-team#613 (last review activity: about 54 days ago)
  • “Cell Broadband Engine SPU support” compiler-team#614 (last review activity: 2 months ago)
  • “Revise error code documentation standard” compiler-team#615 (last review activity: about 54 days ago)
  • “Runtime checks for occupied niches” compiler-team#624 (last review activity: about 19 days ago)
  • “Add support for Zephyr OS” compiler-team#629 (last review activity: about 19 days ago)
  • “Add mips64r6 option to target_arch” compiler-team#632 (last review activity: about 19 days ago)
  • “Consistently use “region” terminology in later stages of the compiler” compiler-team#634 (last review activity: about 19 days ago)
  • “Add a new --build-id flag to rustc” compiler-team#635 (last review activity: about 19 days ago)
  • “Simplify and improve explicitness of the check-cfg syntax” compiler-team#636 (last review activity: about 0 days ago)
• Pending FCP requests (check your boxes!)
  • No pending FCP requests this time.
• Things in FCP (make sure you’re good with it)
  • “Disallow incoherent cfgs” compiler-team#610
  • “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
• Accepted MCPs
  • “Take MIR Analysis by &mut” compiler-team#598
• Finalized FCPs (disposition merge)
  • “Make pointer_structural_match normal and warn” rust#110166
  • “rustdoc: Add search result item types after their name” rust#110688
  • “[mir-opt] SimplifyLocals should also clean up debuginfo” rust#110702
  • “Stabilize inline asm for LoongArch64” rust#111235
  • “Uplift clippy::undropped_manually_drops lint” rust#111530
  • “Uplift clippy::invalid_utf8_in_unchecked lint” rust#111543
  • “Uplift clippy::cast_ref_to_mut lint” rust#111567
  • “Uplift clippy::fn_null_check lint” rust#111717
  • “Uplift clippy::cmp_nan lint” rust#111818
  • “rustdoc: search for slices and arrays by type with []” rust#111958

WG checkins

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

  @andjo403 modified the event file format to support ASCII newline characters in https://github.com/rust-lang/measureme/pull/208. This resolved a panic that @jyn reported when profiling the compiler.

Backport nominations

T-compiler stable / T-compiler beta

• :beta: 1.71.0 “Make struct layout not depend on unsizeable tail” rust#112062
  • fixes #112048, P-critical unsoundness
  • it is now merged, perf. run triage neutral

• :beta: 1.71.0 “Update to LLVM 16.0.5” rust#112312
  • Fixes a number of regressions: #111823, #112061, #112170
  • perf. bench triaged and approved (see comment)

• No stable nominations for T-compiler this time.

T-rustdoc stable / T-rustdoc beta

• No stable nominations for T-rustdoc this time.
• No beta nominations for T-rustdoc this time.

:back: / :shrug: / :hand:

PRs S-waiting-on-team

T-compiler

• Issues in progress or waiting on other teams

Issues of Note

Short Summary

• 0 T-compiler P-critical issues
  • 0 of those are unassigned
• 62 T-compiler P-high issues
  • 29 of those are unassigned
• 0 P-critical, 1 P-high, 1 P-medium, 0 P-low regression-from-stable-to-beta
• 1 P-critical, 0 P-high, 1 P-medium, 2 P-low regression-from-stable-to-nightly
• 0 P-critical, 35 P-high, 100 P-medium, 17 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 new P-high regressions

Unassigned P-high nightly regressions

• No unassigned P-high nightly regressions this time.

Nominated Issues

T-compiler

• “MSVC and rustc disagree on minimum stack alignment on x86 Windows” rust#112480
  • unsoundness for windows-msvc 32bit target, caused by (quote) “the x86 ABI on Windows doesn’t guarantee the stack alignment above 4”.
  • @Chris Denton suggests skipping the check (comment) but what to do in the long term?
  • example at comment
  • nominated to figure out at which level (t-lang, t-compiler, t-else) should this be discussed and addressed
• “Resurrect: rustc_target: Add alignment to indirectly-passed by-value types, correcting the alignment of byval on x86 in the process.” rust#112157
  • nominated by @Nikita Popov as a FIY for the T-compiler (comment)
  • more context from @_erikdesjardins at this comment
• “Add internal_features lint” rust#108955
  • nominated by Felix: does it need an FCP or not?

RFC

• No I-compiler-nominated RFCs this time.

Performance logs

triage logs 2023-06-13

Our build pipeline got sped up by PR #112012, which side-steps one of the rustc rebuilds we were suffering with before. (There is further potential speed-up by caching LLVM, as noted by on that PR.) Other than that, various small regressions that are largely noise, as well as one unexpected increase in binary sizes from PR #109005 that we should follow up on.

Triage done by @pnkfelix. Revision range: adc719d7..4bd4e2ea

Summary:

7 Regressions, 3 Improvements, 5 Mixed; 5 of them in rollups 46 artifact comparisons made in total 30 untriaged Pull Requests

Regressions

Misc HIR typeck type mismatch tweaks #112116 (Comparison Link)

• only primary benchmark to regress was helloworld (3 incr check variants), and not by all that much (relatively speaking)
• secondary regressions were mainly to unify-linearly, await-call-tree, token-stream-stress.
• impact seems acceptable, marking as triaged.

Uplift clippy::undropped_manually_drops lint #111530 (Comparison Link)

• only primary benchmark to regress was helloworld (3 incr check variants), and not by all that much (relatively speaking)
• secondary regressions were solely to unify-linearly, await-call-tree, token-stream-stress.
• impact seems acceptable (one expects new lint to add some amount of extra work, and I wouldn’t be surprised if this is actually noise,. given #112116 above).
• marking as triaged.

Rollup of 3 pull requests #112465 (Comparison Link)

• already marked as triaged (expected regressions to doc benchmarks)

increase the accuracy of effective visibilities calculation #112426 (Comparison Link)

• primary regressions are to serde (check, debug, opt) and also cargo + webrender (both check)
• This PR added some extra work to the compiler to ensure some lazily-filled in tables are constructed correctly.
• Therefore, this seems like extra work that is largely unavoidable.
• marking as triaged.

rustdoc: re-elide cross-crate default trait-object lifetime bounds #107637 (Comparison Link)

• already marked as triaged (expected regressions to doc benchmarks)

Rollup of 3 pull requests #112530 (Comparison Link)

• One of the rolled up PRs, PR #112528, is itself a partial revert of PR #110221, which yields the large compile-time losses noted here (solely for helloworld, it is worth noting).
• the point is, the gains were 1. accidental 2. isolated to a toy and 3. due to an unintended change (which was now reverted).
• marking as triaged.

Private-in-public lints implementation #111801 (Comparison Link)

• primary regressions were to stm32f4-0.14.0 check+opt incr, but by a really small amount (0.18%).
• marking as triaged.

Improvements

Avoid one rustc rebuild in the optimized build pipeline #112012 (Comparison Link)

• Interesting case where removing PGO data improved a class of benchmarks
• Specifically, nearly all the improvements were to debug builds
• this makes sense, because the PGO data we were gathering was during rustc bootstrap, which does not exercise the debug build paths.

Rollup of 5 pull requests #112450 (Comparison Link)

[rustdoc] Fix infinite loop when retrieving impls for type alias #112543 (Comparison Link)

Mixed

Use load+store instead of memcpy for small integer arrays #111999 (Comparison Link)

• changes are all to secondary benchmarks
• only notable was coercions debug incr-full regressing by 5.7%, but that’s acceptable given what we expect benefits to be here w.r.t. codegen.
• marking as triaged.

Update to LLVM 16.0.5 #112312 (Comparison Link)

• effects are all to secondary benchmarks
• effects are small enough that they would not block an LLVM upgrade
• marking as triaged.

Rollup of 7 pull requests #112344 (Comparison Link)

• sole regression is to secondary coercions debug incr-full.
• not worth dissecting this rollup for that.
• marking as triaged.

Remember names of cfg-ed out items to mention them in diagnostics #109005 (Comparison Link)

• instruction counts regressed (expected) and binary sizes regressed (which may have been unexpected).
• specifically the binary sizes for libc and syn both regressed on the order of 3%.
• not marking as triaged until we get confirmation that it is expected for the metadata in question to leak into the binary sizes being measured.

Rollup of 4 pull requests #112420 (Comparison Link)

• currently guessing that the regression here might be due to PR #109953.
• doing a specific rust-timer run now to check that, not marking as triaged for now.

Oldest PRs waiting for review

T-compiler

• “Validate fluent variable references in tests” rust#111269 (last review activity: about 31 days ago)
  • cc @davidtwco
• “Implement a global value numbering MIR optimization” rust#109597 (last review activity: about 22 days ago)
  • cc: @Jak{e,ob} Degen maybe also @RalfJ (since participant in the review)
• “Add casting suggestion when assigning negative 2’s complement bin or hex literal to a size compatible signed integer” rust#111212
  • ~2 months old PR is a diag improvement
  • author (new contributor) rerolls the review assignment, are the changes straightforward enough?

Next week’s WG checkins

• Generic Associated Types initiative by @Jack Huey
• @_WG-diagnostics by @Esteban Küber and @oli

Agenda draft: https://hackmd.io/EpILk3k_Q_SMcZKucMEAOQ