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T-compiler Meeting Agenda 2021-08-19

Tracking Issue

Announcements

MCPs/FCPs

  • New MCPs (take a look, see if you like them!)
  • Old MCPs (not seconded, take a look)
    • "Add different entry points for x.py" compiler-team#396 (last comments: GH 7m ago, Zulip 12d ago)
    • "rustdoc is using rustc_ast_pretty, would it be possible to make it somewhat "stable"?" compiler-team#403 (last comments: GH 6m ago, Zulip 50d ago)
    • "Don't steal the resolver when lowering HIR; instead store an immutable resolver in TyCtxt" compiler-team#437 (last comments: GH 2m ago, Zulip 49d ago)
    • "CI should exercise (subset of) tests under –stage 1" compiler-team#439 (last comments: GH 49d ago, Zulip none)
    • "Accept pc in place of unknown and unknown in place of pc for x86_64 and i?86 targets" compiler-team#441 (last comments: GH 48d ago, Zulip 59d ago)
    • "Non exhaustive reachable patterns lint" compiler-team#445 (last comments: GH 28d ago, Zulip 20d ago)
    • "Make resolve_instance fallible" compiler-team#449 (last comments: GH 18d ago, Zulip none)
    • "Add TyKind::Const and remove GenericArgKind::Const" compiler-team#453 (last comment: GH none, Zulip yesterday)
  • Pending FCP requests (check your boxes!)
    • "Reproducible command line + determinism" compiler-team#450
    • "Write text output files to stdout if options like -o - or --emit asm=- are provided" compiler-team#431
    • "Extend -Cdebuginfo with new options and named aliases" rust#83947
  • Things in FCP (make sure you're good with it)
  • Accepted MCPs
  • Finalized FCPs (disposition merge)
    • No new finished FCP (disposition merge) this time.

WG checkins

@WG-self-profile by @mw @Wesley Wiser (previous checkin)

@rylev and @mw are slowly working towards being able to record non-timestamp data like the sizes of files that the compiler emits – e.g. object files and incr. comp. on-disk data. @Wesley Wiser is doing the reviewing. We should have something to show by the next checkin :)

Backport nominations

T-compiler stable / T-compiler beta

  • :beta: "Always preserve sub-obligations in the projection cache" rust#85868
    • @pnkfelix self-assigned to find another reviewer
    • feedback from @Jack Huey on it to @Aaron Hill. But Jack also said they would not be comfortable reviewing without at least some support from e.g. @nikomatsakis (perhaps during one of Niko's office times)
  • No stable nominations for T-compiler this time.

T-rustdoc stable / T-rustdoc beta

  • No backport nominations for T-rustdoc this time.

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PRs S-waiting-on-team

T-compiler

  • "Extend -Cdebuginfo with new options and named aliases" rust#83947
  • "Encode spans relative to the enclosing item" rust#84373
    • opened by @cjgillot
    • assigned to @Vadim Petrochenkov
    • pending wg-incr-comp discussion
  • "Remove all json handling from rustc_serialize" rust#85993
    • discussed last week, we'd like to have @Vadim Petrochenkov and @cjgillot in the loop

Issues of Note

Short Summary

P-critical

T-compiler

  • "LLVM crash: Basic Block does not have terminator!" rust#88043
    • opened by @David Tolnay
    • seems caused by rust#83417 cc: @oli, suggests a quick revert and then look at this again without pressure
    • @david tolnay also points out relation with rust#88043

T-rustdoc

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

P-high regressions

P-high beta regressions

  • "ICE: unexpected concrete region in borrowck: ReStatic" rust#87455
  • "ICE: unexpected concrete region in borrowck: ReEarlyBound(0, 'a)" rust#83190
    • assigned same P-high as sibling rust#87455
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Unassigned P-high nightly regressions

  • No unassigned P-high nightly regressions this time.

Performance logs

triage logs for 2021-08-17

A fairly busy week which was to expected given that we've adjusted our algorithm for whether we label a change as a regression or not. Most regressions were relatively small, and only one has not yet been addressed in some way.

Triage done by @rylev.
Revision range: 6b20506d17f4e5e5bf5bcad7e94add4d754b0ae3..aa8f27bf4d980023a8b245ceb25a490a18041eb2
59 comparisons made in total

3 Regressions, 2 Improvements, 2 Mixed; 0 of them in rollups

Regressions

encode generics_of for fields and ty params #87815

  • Moderate regression in instruction counts (up to 1.2% on incr-unchanged builds of diesel)
  • While the regressions are fairly small, the largest ones seem to be happening in real-world crates like diesel, serde, and futures.
  • The largest regressions seem to be in the explicit_predicates_of query which would seem to be directly impacted by this change.
  • Left a comment to see how we should address this issue.

Various refactorings of the TAIT infrastructure #87587

  • Moderate regression in instruction counts (up to 1.5% on full builds of inflate)
  • The issue is expected and is being monitored with a possible solution in the works.

Name the captured upvars for closures/generators in debuginfo #85020

  • Large regression in instruction counts (up to 4.9% on incr-patched: println builds of webrender-wrench)
  • This is a large regression in one benchmark, but it seems to be in codegen which this change would impact.
  • There is a comment justifying this as concerning but worth it given it only impacts one benchmark. I still believe this might be one worth keeping an eye on.

Improvements

  • Avoid using the copy_nonoverlapping wrapper through mem::replace. #87827
  • Run RemoveZsts pass at mir-opt-level=1 #83417

Mixed

Introduce hir::ExprKind::Let - Take 2 #80357

  • Large improvement in instruction counts (up to -2.1% on incr-patched: println builds of webrender)
  • Moderate regression in instruction counts (up to 0.7% on full builds of cranelift-codegen)

BTree: merge the complication introduced by #81486 and #86031 #87696

  • Small improvement in instruction counts (up to -0.6% on full builds of issue-46449)
  • Moderate regression in instruction counts (up to 1.7% on full builds of ripgrep)
  • This is mostly a wash in terms of performance (with performance shifting somewhat equally across all the benchmarks), with the largest changes happening in stress tests.

Untriaged Pull Requests

Nominated Issues

T-compiler

  • "Remove all json handling from rustc_serialize" rust#85993
    • discussed in S-waiting-on-team
  • "Binary size is significant increased from 1.46.0 to 1.51.0" rust#86610
    • Has action item on HackMD to add binary size tracking to perf.rlo, needs an owner

RFC

  • "[RFC2603] Extend <const> to include str and structural constants." rfcs#3161
    • discussed last week
    • @mw suggested to merge the current implementation, not the RFC, and gather real-world data
    • still needs discussion or I-nominated can be removed?
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