T Compiler Meeting Agenda 2023 10 19

T-compiler Meeting Agenda 2023-10-19


  • Compiler Team Planning meeting at time:2023-10-20T10:00:00-04:00
  • @apiraino proposal to rename the WG-prioritization to T-compiler/ops team (Zulip thread). Happy to receive feedback!
  • Reminder: if you see a PR/issue that seems like there might be legal implications due to copyright/IP/etc, please let us know (or at least message @davidtwco or @Wesley Wiser so we can pass it along).

Other WG meetings (calendar link)


  • New MCPs (take a look, see if you like them!)
    • No new proposals this time.
  • Old MCPs (stale MCP might be closed as per MCP procedure)
    • None at this time
  • Old MCPs (not seconded, take a look)
    • “Consistently use “region” terminology in later stages of the compiler” compiler-team#634 (Zulip) (last review activity: 4 months ago)
    • “Add a new --build-id flag to rustc” compiler-team#635 (Zulip) (last review activity: 4 months ago)
    • “Add support for visionOS targets” compiler-team#642 (Zulip) (last review activity: about 47 days ago)
    • “Match the behavior of strip and split-debuginfo across platforms” compiler-team#669 (Zulip) (last review activity: about 44 days ago)
  • Pending FCP requests (check your boxes!)
  • Things in FCP (make sure you’re good with it)
  • Accepted MCPs
  • Finalized FCPs (disposition merge)
    • “rustdoc: hide #[repr(transparent)] if it isn’t part of the public ABI” rust#115439
    • “const-eval: make misalignment a hard error” rust#115524
    • “const_eval: allow function pointer signatures containing &mut T in const contexts” rust#116015
    • “Bump COINDUCTIVE_OVERLAP_IN_COHERENCE to deny + warn in deps” rust#116493

WG checkins

Backport nominations

T-compiler stable / T-compiler beta

  • :beta: “Fix a performance regression in obligation deduplication.” rust#116826
    • authored by @nnethercote
    • Fixes #116780 #116797, noteworthy perf. regressions
  • :beta: “Make #[repr(Rust)] and #[repr(C)] incompatible with one another” rust#116829
    • authored by @León Orell Liehr (fmease)
    • Fixes #116825, where rustc didn’t reject an incorrect [repr(C, Rust)] block.
    • T-lang ok’d these changes (comment)
  • :beta: “Update to LLVM 17.0.3” rust#116840
    • authored by @Nikita Popov
    • Fixes #116582 (unsoundness) and #104862 (miscompilation), introduced with LLVM 17 upgrade
  • :beta: “Disable effects in libcore again” rust#116856
    • authored by @oli
    • reverts #114776, accidentally enabled libcore without feature gate
  • No stable nominations for T-compiler this time.

PRs S-waiting-on-team


Issues of Note

Short Summary



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


  • “regression when relating two opaque types in canonical queries” rust#116877
    • That will be closed by the beta backport of #116879


  • 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

triage logs for 2023-10-18

Overall an interesting week performance wise, with small improvements to a vast number of benchmarks seeming to outweigh an isolated set of (slightly) larger regressions. It included a number of PRs regressed instruction counts but did not matter for cycle times, plus one mysterious regression to check_match and mir_borrowck from reworking constructor splitting (see report on PR 116391 for details), and an awesome broad set of improvements from automatically inlining small functions across crates (see report on PR 116505 for details).

Triage done by @pnkfelix. Revision range: 84d44dd1..b9832e72


(instructions:u) mean range count
Regressions (primary) 3.0% [0.3%, 12.2%] 7
Regressions (secondary) 0.7% [0.3%, 1.2%] 15
Improvements (primary) -1.1% [-17.9%, -0.2%] 131
Improvements (secondary) -2.4% [-39.6%, -0.2%] 121
All (primary) -0.9% [-17.9%, 12.2%] 138

4 Regressions, 1 Improvements, 4 Mixed; 3 of them in rollups 84 artifact comparisons made in total 30 Untriaged Pull Requests


Rollup of 7 pull requests #116605 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) 0.4% [0.2%, 0.6%] 7
Regressions (secondary) 0.3% [0.3%, 0.4%] 3
Improvements (primary) - - 0
Improvements (secondary) - - 0
All (primary) 0.4% [0.2%, 0.6%] 7
  • solely rustdoc regression
  • believed to be caused by PR 109422 “rustdoc-search: add impl disambiguator to duplicate assoc items”
  • already marked as triaged

Optimize librustc_driver.so with BOLT #116352 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) 2.3% [0.2%, 5.7%] 10
Regressions (secondary) 1.9% [0.3%, 5.0%] 60
Improvements (primary) - - 0
Improvements (secondary) -0.3% [-0.3%, -0.3%] 4
All (primary) 2.3% [0.2%, 5.7%] 10
  • primary instruction-count regressions were restricted to helloworld and html5ever
  • As noted in comment by Kobzol, the instruction counts regressed for many benchmarks, but the cycle counts solely improved, significantly so, and bootstrap time improved (628.052s -> 623.517s (-0.72%)).
  • already marked as triaged

Rollup of 3 pull requests #116742 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) 0.3% [0.3%, 0.4%] 3
Regressions (secondary) - - 0
Improvements (primary) - - 0
Improvements (secondary) - - 0
All (primary) 0.3% [0.3%, 0.4%] 3
  • Regressions are solely to bitmaps full scenarios.
  • Looks like a blip (i.e. noise) based on the graph over time.
  • marking as triaged.

don’t UB on dangling ptr deref, instead check inbounds on projections #114330 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) - - 0
Regressions (secondary) 0.7% [0.5%, 1.0%] 17
Improvements (primary) - - 0
Improvements (secondary) - - 0
All (primary) - - 0
  • From skimming the PR, one can see that the PR author (RalfJung) iterated on this to identify a solution that would minimize regressions.
  • As noted by the PR author, only secondary benchmarks were affected.
  • Also, while instruction-counts regressed, the cycle-counts did not, at least not enough to pass our noise threshold.
  • marking as triaged.


optimize zipping over array iterators #115515 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) - - 0
Regressions (secondary) - - 0
Improvements (primary) -0.3% [-0.4%, -0.2%] 3
Improvements (secondary) - - 0
All (primary) -0.3% [-0.4%, -0.2%] 3
  • A small win from a PR addressing user-filed performance regression, namely issue #115339, “Performance regression of array::IntoIter vs slice::Iter”


Also consider call and yield as MIR SSA. #113915 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) 3.9% [3.9%, 3.9%] 1
Regressions (secondary) 0.1% [0.1%, 0.1%] 2
Improvements (primary) -0.4% [-0.9%, -0.2%] 26
Improvements (secondary) -0.4% [-0.6%, -0.3%] 5
All (primary) -0.2% [-0.9%, 3.9%] 27
  • The try perf run had sole primary regression of unicode-normalization-0.1.19 opt-full (1.19%), while the perf run against master had sole primary regression of exa-0.10.1 opt-full (3.90%).
  • The exa regression has persisted forward (i.e. it is not transient noise).
  • It was already been marked as triaged, as the performance changes were deemed a wash, apart from object code sizes which saw “small but clear” improvement.

Rollup of 5 pull requests #116640 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) - - 0
Regressions (secondary) 1.1% [1.1%, 1.1%] 1
Improvements (primary) -0.3% [-0.4%, -0.2%] 4
Improvements (secondary) -0.4% [-0.5%, -0.4%] 6
All (primary) -0.3% [-0.4%, -0.2%] 4
  • sole regression was to secondary benchmark coercions debug incr-patched: add static arr item
  • Looks like a blip (i.e. noise) based on the graph over time.
  • marking as triaged

exhaustiveness: Rework constructor splitting #116391 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) 0.2% [0.2%, 0.3%] 4
Regressions (secondary) 3.9% [0.5%, 5.8%] 9
Improvements (primary) -0.4% [-0.4%, -0.4%] 1
Improvements (secondary) - - 0
All (primary) 0.1% [-0.4%, 0.3%] 5
  • the primary regressions were to cranelift-codegen-0.82.1 and cargo-0.60.0 in various incremental settings (mostly check builds)
  • the large (>5%) secondary regressions are all to match-stress.
  • the above cases were regressions for instruction-counts, but the cycle-counts didn’t get marked as regressed in any of the same cases.
  • in all cases, the performance loss from these regressions was subsequently recovered (or masked) by PR 116505 “Automatically enable cross-crate inlining for small functions”. (I don’t know if that’s actually related or just an awesome change that bought so much performance that it masked this problem).
  • Since the match-stress one was relatively large, I looked at the self-profile results in the details which indicates a change in the delta(time) for match-stress might be due to new overheads in check_match and mir_borrowck.
  • But this is strange; I cannot tell how this PR could have affected codegen, which would be the only way I could imagine those functions being impacted.
  • Not marking as triaged for now; this mystery might be worth looking into a bit more. (But then again, the only significant regression was to a secondary stress test, so maybe its not worth spending time on.)

Automatically enable cross-crate inlining for small functions #116505 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) 2.3% [0.3%, 13.0%] 8
Regressions (secondary) 0.5% [0.2%, 0.8%] 2
Improvements (primary) -1.2% [-18.1%, -0.1%] 148
Improvements (secondary) -2.2% [-39.8%, -0.2%] 209
All (primary) -1.0% [-18.1%, 13.0%] 156
  • Already marked as triaged
  • This was clearly awesome and amazing (all the more amazing if you review the history)
  • ‘Nuff said.

Nominated Issues


  • “Code no longer compiles after -Zdrop-tracking-mir was enabled by default” rust#116242
    • waiting for the crater run results. See comment
    • (@apiraino looked for the results but didn’t find them)


  • No I-compiler-nominated RFCs this time.

Oldest PRs waiting for review

  • “Save liveness results for DestinationPropagation” rust#115291
    • cc @Jak{e,ob} Degen
  • “Fix duplicated crate keyword in suggested path” rust#115876 (last review activity: about 28 days ago)
    • cc @Esteban Küber, roll this one off your dish?
  • “Introduce -C instrument-coverage=branch to gate branch coverage” rust#116094 (last review activity: about 25 days ago)
    • cc @Wesley Wiser, roll this one off your dish?

Next week’s WG checkins

  • @_WG-diagnostics by @Esteban Küber and @oli
  • @_WG-rustc-dev-guide by @Santiago Pastorino and @Yuki Okushi|217081

Next meetings’ agenda draft: hackmd link