T-compiler Meeting Agenda 2022-06-16

Announcements

• T-compiler steering: path sanitisation changes rfc#3127 at time:2022-06-17T10:00:00-04:00
• 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)

• Async WG triage meeting at time:2022-06-20T11:30:00-04:00
• Types Team: Planning/Deep-Dive meeting at time:2022-06-17T09:00:00-04:00
• wg-debugging triage meeting at time:2022-06-20T10:00:00-04:00
• Types team Hack session: Advanced subtyping at time:2022-06-22T09:00:00-04:00
• wg-traits GATs Sync at time:2022-06-20T16:00:00-04:00

MCPs/FCPs

• New MCPs (take a look, see if you like them!)
  • “Add support for the LoongArch architecture” compiler-team#518
• Old MCPs (not seconded, take a look)
  • “-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)
  • “Arbitrary annotations in compiletest” compiler-team#513 (last review activity: about 40 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
  • “session: stabilize split debuginfo on linux” rust#98051
• Things in FCP (make sure you’re good with it)
  • “Tier 3 target proposal: riscv64gc-linux-android (Android target for riscv64gc)” compiler-team#472
• Accepted MCPs
  • No new accepted proposals this time.
• Finalized FCPs (disposition merge)
  • “Remove migrate borrowck mode” rust#95565
  • “Modify MIR building to drop repeat expressions with length zero” rust#95953
  • “Lang: Stabilize usage of rustc_nonnull_optimization_guaranteed on -1” rust#97122

WG checkins

• @_wg-polymorphization Polymorphization by @davidtwco (previous checkin)

Checkin text

• @_WG-rls2.0 RLS 2.0 by @matklad (previous checkin)

Checkin text

Backport nominations

T-compiler beta / T-compiler stable

• :beta: “Update LLVM submodule” rust#97690

  • Discussed last week
  • backport decision postponed to better evaluate the entity of the LLVM patch being backported

• :beta: “Aarch64 call abi does not zeroext (and one cannot assume it does so)” rust#97800

  • Discussed last week
  • backport decision postponed (more work is being done on #97800)

• :beta: “Revert “remove num_cpus dependency” in rustc and update cargo” rust#97911

  • patch authored by @David Tolnay
  • Fixes rust#97549, P-high regression (increased memory usage)
  • nominated by @simulacrum cc: @Michael Goulet (compiler-errors) so users won’t wait until 1.63 to receive this patch

• :beta: “debuginfo: Fix NatVis for Rc and Arc with unsized pointees.” rust#98137

  • patch authored by @mw
  • nominated by @Wesley Wiser for these reasons

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.
  • (4 PRs omitted since either drafts or waiting on other teams / RFC process)

Oldest PRs waiting for review

T-compiler

• “Add option to pass environment variables” rust#94387 (last review activity: about 33 days ago)
  • cc latest reviewers: @Esteban Küber and @bjorn3
• “Clone suggestions” rust#95115 (last review activity: about 26 days ago)
  • assigned reviewer @Esteban Küber, suggests an additional reviewer (comment)
  • also cc: @oli and @Jak{e,ob} Degen (as they left comments, too)
  • any actionable for the PR author at this time?
• “Add Finalize statement to make deaggregation “reversible” by storing all information in MIR” rust#96043 (last review activity: about 25 days ago)
  • unsure about status: maybe a comment on latest perf run results?
  • cc: @Wesley Wiser assigned reviewer
• “Print type of every call in a method call chain” rust#96918 (last review activity: about 20 days ago)
  • unsure about current status
  • cc: @Michael Goulet (compiler-errors) maybe to comment on latest notes left by @Esteban Küber?
• “Add round_ties_even to f32 and f64” rust#95317 (last review activity: about 41 days ago)
  • r’ed by Felix comment
  • anything else from T-compiler?

Issues of Note

Short Summary

• 2 T-compiler P-critical issues
  • 0 of those are unassigned
• 63 T-compiler P-high issues
  • 44 of those are unassigned
• 0 P-critical, 0 P-high, 4 P-medium, 0 P-low regression-from-stable-to-beta
• 1 P-critical, 0 P-high, 1 P-medium, 0 P-low regression-from-stable-to-nightly
• 0 P-critical, 40 P-high, 78 P-medium, 8 P-low regression-from-stable-to-stable

P-critical

T-compiler

• “Wrong cast of u16 to usize on aarch64” rust#97463
  • Issue is being addressed by @pnkfelix in #97800
• “NLL: unsound verification of higher ranked outlives bounds” rust#98095
  • discussion in comments, cc: @Michael Goulet (compiler-errors) and @nikomatsakis
  • Also I-compiler-nominated

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

2022-06-14 Triage Log

A mixed week. I suppose it is best to focus on the fact we made some big improvements to a large number of primary benchmarks, at the cost of some smaller regressions to a smaller number of primary benchmarks.

Triage done by @pnkfelix. Revision range: bb55bd449e65e611da928560d948982d73e50027..edab34ab2abbafc16a78daedf71dbacd2eb0b7bf

Summary:

4 Regressions, 3 Improvements, 5 Mixed; 4 of them in rollups 47 artifact comparisons made in total 30 Untriaged Pull Requests

Regressions

Rollup of 5 pull requests #97825 (Comparison Link)

• stm32f4-0.14.0 regressed for check+debug+opt on incr-patched: negate. diesel-1.4.8 regressed for check on incr-patched: println
• Given that all the regressions are to incremental, I’m assuming this is because of #97058
• skimming over the perf details for stm32f4 check, it seems like the bulk of the time delta is coming from expand_crate. A total time delta of 0.66, and the biggest contributors to that delta are expand_crate (0.033), incr_comp_load_dep_graph (0.015), misc_checking_1 (0.007), hir_owner_nodes (0.005), generate_crate_metadata (0.005), incr_comp_encode_dep_graph (0.004), and wf_checking (0.004). The remainder are <= 0.003, most of them <= 0.000.
• given the relatively small size and scope of the regression, and the fact that it was in a rollup, I do not think this is worth investigating further. marked as triaged.

Rollup of 6 pull requests #97968 (Comparison Link)

• diesel-1.4.8 regressed on check full, opt full, and check incr-full, by about 0.3% each time.
• rylev says that diesel has started to become more noisy in its behavior, perhaps since we turned on PGO.
• I do not think this is worth investigating further. marked as triaged.

Handle def_ident_span like def_span. #95880 (Comparison Link)

• Already triaged by the PR author.
• “we encode more spans into metadata and invoke more queries, so the slight regression is to be expected.”

Rollup of 5 pull requests #98025 (Comparison Link)

• All six regressions are variations of issue-88862: {opt,debug} x { incr-full, full, incr-unchanged }. The two incr-unchanged cases regressed by 0.94% and 1.72% . The full cases regressed by 7.13% and 7.57% . The incr-full cases regressed by 7.86% and 7.80%.
• None of these PRs strike me as something that would cause a problem for the code exercised by #88862. (I briefly thought it might be #98012, but issue-88862 doesn’t exercise HKT’s…)
• but also, given that issue-88862 is a canary that is trying to catch a catastrophic regression, I think we can accept a 7-8% regression here.

Improvements

Re-use the type op instead of calling the implied_outlives_bounds query directly #97081 (Comparison Link)

Revert part of #94372 to improve performance #97905 (Comparison Link)

Tidy up miscellaneous bounds suggestions #97778 (Comparison Link)

Mixed

Folding revamp #97447 (Comparison Link)

• already triaged by its author
• “The perf effects are fairly small and there are more improvements than regressions.”

Make Encodable and Encoder infallible. #94732 (Comparison Link)

• PR author already investigated.
• “Good news: #97905 fixed the regressions here. That PR plus this PR combined gave a clear performance win.”
• marked as triaged.

cleanup bound variable handling #97648 (Comparison Link)

Rollup of 10 pull requests #98066 (Comparison Link)

• diesel-1.4.8 regressed by 0.5% for variations on check/debug/opt and incr-full/full. That’s the only primary regression, and its within the noise level that we’re currently associating with diesel, I think.
• The other regressions are wf-projection-stress-65510, projection-caching, regression-31157, and wg-grammar.
• wf-projection-stress-65510 and regression-31157 are canaries where we are trying to catch a massive regression, not a minor one like the ones presented here.
• given that this is a rollup and the remaining regressions are well under 0.5%, I think that’s the limit to the amount of investigation I want to do here.

Remove RegionckMode in favor of calling new skip_region_resolution #98041 (Comparison Link)

• Skimming over the comparison (and even just from the table), it seems clear that the improvements far far far outweighed the gains here.
• Marking as triaged.

Nominated Issues

T-compiler

• “sess: stabilize --terminal-width” rust#95635
  • nominated by @oli for a mention for T-compiler
• “Update Mac Catalyst support for Clang 13” rust#96392
  • nominated by @Esteban Küber in comment to find someone more conversant with that platform
  • @_Thom Chiovoloni pinged a list of names that could help (comment)
• “NLL: unsound verification of higher ranked outlives bounds” rust#98095
  • nominated by @Jack Huey, see P-critical

RFC

• No new RFC for T-compiler

Next week’s WG checkins

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