T-compiler Meeting Agenda 2022-07-21

Announcements

• Types Team: Planning/Deep-Dive meeting at time:2022-07-22T09:00:00-04:00
• 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

• next weeks' meetings from google calendar

MCPs/FCPs

• New MCPs (take a look, see if you like them!)
  • “Add import_name_type parameter to #[link]” compiler-team#525
  • “Add #[alias] attribute to allow symbol aliasing” compiler-team#526
  • “move hir typeck into separate crate” compiler-team#529
• Old MCPs (not seconded, take a look)
  • “Arbitrary annotations in compiletest” compiler-team#513 (last review activity: 2 months ago)
  • “Add support for the LoongArch architecture” compiler-team#518 (last review activity: about 33 days ago)
• Pending FCP requests (check your boxes!)
  • “Increase the minimum linux-gnu versions” rust#95026
• Things in FCP (make sure you’re good with it)
  • “Removing codegen logic for nvptx-nvidia-cuda (32-bit target)” compiler-team#496
  • “Stabilize -Zgcc-ld=lld as -Clink-self-contained=linker -Clinker-flavor=gcc-lld” compiler-team#510
  • “Introduce -Z flag to control how proc-macros are run” compiler-team#528
  • “Create #[rustc_on_type_error] mimicking #[rustc_on_unimplemented] for E0308 customization” compiler-team#530
  • “Split Predicate into Goal and Clause” compiler-team#531
  • “Deref is not a projection” compiler-team#532
  • “session: stabilize split debuginfo on linux” rust#98051
• Accepted MCPs
  • “-Dwarnings to cover all warnings” compiler-team#473
  • “Track licensing metadata with REUSE” compiler-team#519
  • “Revise how MIR variants are distinguished” compiler-team#522
• 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
  • “Remove a back-compat hack on lazy TAIT” rust#97346
  • “Make outlives::{components,verify} agree” rust#97406
  • “make cenum_impl_drop_cast deny-by-default” rust#97652
  • “make const_err show up in future breakage reports” rust#97743
  • “lub: don’t bail out due to empty binders” rust#97867
  • “allow unions with mutable references and tuples of allowed types” rust#97995

WG checkins

• @_WG-mir-opt by @oli (previous checkin)

  • we now run the MIR inliner by default in release mode
  • const prop lints now run before optimizations so that they are no longer influenced by other mir opts (like inlining)
  • optimization MIR and codegen MIR no longer have Deref projections in the middle of a place projection, only at the start, which simplified a few backend things and optimizations.

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

  Nothing from wg-polymorphization this cycle.

Backport nominations

T-compiler beta / T-compiler stable

• :beta: “[beta] Do not call check_expr twice in check_compatible” rust#99397
  • fixes #98894 and #98897
  • approved by @Jack Huey, needs merge
• :beta: “Revert “Stabilize $$ in Rust 1.63.0"” rust#99435
  • reverts commit 9edaa7
• :beta: “Do not resolve associated const when there is no provided value” rust#99449
  • fixes rust#98629, P-high regression
  • pending comments from @lqd and merge
• No stable nominations for T-compiler this time.

T-rustdoc beta / T-rustdoc stable

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

:back: / :shrug: / :hand:

PRs S-waiting-on-team

T-compiler

• No new PRs waiting on team

Oldest PRs waiting for review

T-compiler

• “Support #[global_allocator] without the allocator shim” rust#86844 (last review activity: 3 months ago)
  • cc Felix
• “Add [f32]::sort_floats and [f64]::sort_floats” rust#93397 (last review activity: 3 months ago)
  • assigned to @kennytm , reroll assignment?
• “Add special_module_name lint” rust#94467 (last review activity: 3 months ago)
  • cc Felix
• “Simplify some code that depend on Deref” rust#97077 (last review activity: about 46 days ago)
  • cc @oli (maybe not S-blocked anymore)
• “[RFC] Support .comment section like GCC/Clang (!llvm.ident)” rust#97550 (last review activity: about 35 days ago)
  • needs an FCP but @bjorn3 or @Miguel Ojeda have no permission to start one: can someone assist them?

Issues of Note

Short Summary

• 4 T-compiler P-critical issues
  • 2 of those are unassigned
• 55 T-compiler P-high issues
  • 35 of those are unassigned
• 1 P-critical, 1 P-high, 2 P-medium, 0 P-low regression-from-stable-to-beta
• 2 P-critical, 0 P-high, 1 P-medium, 0 P-low regression-from-stable-to-nightly
• 1 P-critical, 37 P-high, 86 P-medium, 7 P-low regression-from-stable-to-stable

P-critical

T-compiler

• “Recent nightly started ICEing with “No counters provided the source_hash for used function”” rust#98833
  • PR to resolve rust#98868 authored by @tm|352985
  • needs reviewer

T-rustdoc

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

P-high regressions

P-high beta regressions

• “evaluation error hides “impl item missing” error” rust#98629
  • @Michael Goulet (compiler-errors) authored PR rust#99449, nominated for beta backport

Unassigned P-high nightly regressions

• No unassigned P-high nightly regressions this time.

Performance logs

triage logs for 2022-07-19

A rather rough week for compiler performance with regressions outweighing improvements by a considerable margin, in particular in real world crates. To add insult to injury, the biggest regressions came in rollups which make it difficult to trace the cause.

Triage done by @rylev. Revision range: b3f4c3119957aa0a250cab08ab586b7a9a680ef1..8bd12e8cca3f28f302b9cc0f1f47bb64bd1f98fd

Summary:

7 Regressions, 5 Improvements, 3 Mixed; 4 of them in rollups 48 artifact comparisons made in total

Regressions

Rollup of 6 pull requests #99177 (Comparison Link)

• Relatively small regression in secondary benchmarks. Nothing stands out as an obvious culprit, so we can mark as triaged.

Pull Derefer before ElaborateDrops #98145 (Comparison Link)

• Slowdowns are happening in MIR borrow checking which this PR directly changes, so regressions are not surprising.
• An issue should be opened to look into whether this can be improved.
• A comment was left asking for a follow up.

Rollup of 5 pull requests #99210 (Comparison Link)

• Seems the issue comes from calling an LLVM API more often due to needing to have unstable feature information kept around longer for use with the asm! macro.
• Ran a cachegrind diff and found that the likely culprit is #99155 calling LLVMRustHasFeature more often.
• Left a comment asking for a follow up.

Rollup of 5 pull requests #99231 (Comparison Link)

• It’s very hard to tell which PR might be responsible. Luckily, there are only 5 PRs in total so it should be possible to track the issue down.
• Left a comment suggesting as much.

Support unstable moves via stable in unstable items #95956 (Comparison Link)

• The change seems to be localized to incremental compilation scenarios.
• It seems queries like incr_comp_encode_dep_graph are being hit more often. I’m unsure whether this is simply because of the additional pass that’s happening or something else.
• Left a comment asking for more insight.

Rollup of 10 pull requests #99346 (Comparison Link)

• Regression was found in #98582 and will be reverted in #99368 or a follow up.

Use ICF (identical code folding) for building rustc #99062 (Comparison Link)

• Small performance changes and some wins for binary size.
• Asked in the PR whether this can be marked as triaged.

Improvements

gather body owners #98203 (Comparison Link)

interpret: get rid of MemPlaceMeta::Poison #99013 (Comparison Link)

stop using FnCtxt outside of hir typeck #99015 (Comparison Link)

Use constant eval to do strict mem::uninit/zeroed validity checks #99033 (Comparison Link)

replace_bound_vars fast path: check predicates, don’t check consts #99232 (Comparison Link)

Mixed

Stop keeping metadata in memory before writing it to disk #96544 (Comparison Link)

• The perf here is a wash.

Final derive output improvements #99046 (Comparison Link)

• Improvements much outweigh the regressions as noted here.

proc_macro/bridge: stop using a remote object handle for proc_macro Ident and Literal #98189 (Comparison Link)

• Still more improvements than regressions, and this is an active area of performance work as noted here.

Nominated Issues

T-compiler

• “Revert #94158, “Apply noundef metadata to loads of types that do not permit raw init”” rust#98966
  • nominated by @Ben Kimock (Saethlin)

    In #66151 we have decided against turning technically UB code into a panic on account of the amount of code that would be affected, so it doesn’t seem right to turn technically into directly visible to LLVM. If the breakage from adding a panic for this case is too high, the danger from turning this case into visible UB should be as well.

  • cc @Thom Chiovoloni
  • Seems to be replaced by rust#99182? (see comment)

RFC

• No nominated RFCs for T-compiler this time.

Next week’s WG checkins

• @_WG-rls2.0 by @matklad
• @_WG-self-profile by @mw and @Wesley Wiser