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T-compiler Meeting Agenda 2024-01-25

Announcements

  • Steering meeting: Tracking Issue Review time:2024-01-25T16:00:00:00+01
  • 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

MCPs/FCPs

WG checkins

  • @_WG-async-foundations by @nikomatsakis and @tmandry

    Checkin text

  • @_WG-diagnostics by @Esteban Küber and @oli

    • new activity on annotate-snippets (for now just for use in cargo, but moving towards using it in rustc, too)
    • removing the reliance of the compiler on the global error counter by instead tainting or bubbling up errors as appropriate

Backport nominations

T-compiler beta / T-compiler stable

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

T-types stable / T-types beta

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

Note: 2 beta backport nominations for T-libs

PRs S-waiting-on-team

T-compiler

Issues of Note

Short Summary

P-critical

T-compiler

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

T-types

  • "Pointer casts allow switching trait parameters for trait objects, which doesn’t interact soundly with trait upcasting" rust#120222
    • I-types-nominated
    • T-types approved the revert of #118133 with rust#120233 (comment)
    • Revert also backported

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 2024-01-23

This week saw a bunch of regressions caused by correctness fixes and in general doing more work
in the compiler. These were offset by many improvements (especially around hashing in the compiler)
that improved performance by ~2% across a large number of benchmarks. Don't get too excited about the
large 45+% wins though, these were only for tiny benchmarks like helloworld. They were caused by a
change in Cargo which introduces stripping of debug
symbols from Rust release binaries by default, and in turn also improves compilation time for small
crates.

Triage done by @kobzol.
Revision range: f9c2421a..d6b151fc

Summary:

(instructions:u) mean range count
Regressions (primary) 0.7% [0.2%, 1.5%] 11
Regressions (secondary) 2.2% [0.2%, 9.9%] 26
Improvements (primary) -3.2% [-47.5%, -0.2%] 191
Improvements (secondary) -7.9% [-46.5%, -0.1%] 123
All (primary) -3.0% [-47.5%, 1.5%] 202

4 Regressions, 4 Improvements, 9 Mixed; 4 of them in rollups
48 artifact comparisons made in total

Regressions

fix fn/const items implied bounds and wf check (rebase) #120019 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) 0.3% [0.2%, 0.5%] 26
Regressions (secondary) 1.3% [0.3%, 1.6%] 9
Improvements (primary) - - 0
Improvements (secondary) - - 0
All (primary) 0.3% [0.2%, 0.5%] 26
  • This was a correctness fix that had an expected performance hit.

Rollup of 8 pull requests #120187 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) 0.5% [0.3%, 1.0%] 8
Regressions (secondary) 0.9% [0.3%, 1.8%] 16
Improvements (primary) - - 0
Improvements (secondary) - - 0
All (primary) 0.5% [0.3%, 1.0%] 8
  • This was a mixture of several PRs that had a perf. effect.
  • #119461 and #118811
    slightly regressed the deep-vector benchmark, which was however quite noisy recently, so it was
    probably just a blip.
  • #116090 caused a real regression on doc
    benchmarks, which was caused simply by adding a bunch of new methods to the standard library.

Rollup of 8 pull requests #120196 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) 1.3% [1.3%, 1.3%] 1
Regressions (secondary) - - 0
Improvements (primary) - - 0
Improvements (secondary) - - 0
All (primary) 1.3% [1.3%, 1.3%] 1
  • Caused by #120145, which was a correctness fix,
    so marked as triaged.

Rollup of 10 pull requests #120242 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) - - 0
Regressions (secondary) 3.1% [3.1%, 3.1%] 1
Improvements (primary) - - 0
Improvements (secondary) - - 0
All (primary) - - 0
  • Caused by #120137, which added more validation
    to the compiler, and the perf. hit was deemed acceptable, so marked as triaged.

Improvements

Cache local DefId-keyed queries without hashing #119977 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) 0.7% [0.7%, 0.7%] 1
Regressions (secondary) - - 0
Improvements (primary) -1.2% [-8.2%, -0.2%] 161
Improvements (secondary) -1.5% [-3.7%, -0.3%] 64
All (primary) -1.2% [-8.2%, 0.7%] 162
  • Incredible wins across many benchmarks, caused by improving hashing in the compiler.
  • The wall-time results of this PR were even more impressive, resulting in a ~4% mean improvement across more than a hundred benchmark
    configurations!

Get rid of the hir_owner query. #120006 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) - - 0
Regressions (secondary) 0.3% [0.2%, 0.4%] 3
Improvements (primary) -1.8% [-5.5%, -0.3%] 129
Improvements (secondary) -2.1% [-4.4%, -0.4%] 46
All (primary) -1.8% [-5.5%, -0.3%] 129
  • Again, great wins across a lot of benchmarks.

Use UnhashMap for a few more maps #120076 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) - - 0
Regressions (secondary) - - 0
Improvements (primary) -0.3% [-0.3%, -0.2%] 13
Improvements (secondary) -0.3% [-0.4%, -0.2%] 4
All (primary) -0.3% [-0.3%, -0.2%] 13
  • Continuation of hashing improvements.

Always use RevealAll for const eval queries #119821 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) - - 0
Regressions (secondary) - - 0
Improvements (primary) -0.3% [-0.3%, -0.3%] 3
Improvements (secondary) -1.4% [-1.8%, -1.2%] 7
All (primary) -0.3% [-0.3%, -0.3%] 3
  • This PR improved mainly the performance of const evaluation in the compiler.

Mixed

Sandwich MIR optimizations between DSE. #119672 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) 0.7% [0.2%, 1.4%] 14
Regressions (secondary) 0.5% [0.2%, 2.7%] 14
Improvements (primary) -1.0% [-2.2%, -0.2%] 31
Improvements (secondary) -0.9% [-2.2%, -0.2%] 10
All (primary) -0.4% [-2.2%, 1.4%] 45
  • Several nice wins on larger benchmarks, overall the wins outweighed the losses.

large_assignments: Lint on specific large args passed to functions #116520 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) 0.4% [0.2%, 0.8%] 45
Regressions (secondary) 0.6% [0.2%, 2.1%] 8
Improvements (primary) - - 0
Improvements (secondary) -6.0% [-6.0%, -6.0%] 1
All (primary) 0.4% [0.2%, 0.8%] 45
  • This new lint introduced more tracking of spans in the compiler, which expectedly
    slightly regressed compile times. It is expected that the new spans will be used
    by future lints/errors to provide better diagnostics.

Update cargo #120036 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) 0.4% [0.4%, 0.5%] 2
Regressions (secondary) 1.4% [1.4%, 1.4%] 1
Improvements (primary) -16.8% [-47.6%, -0.4%] 13
Improvements (secondary) -20.5% [-46.9%, -2.2%] 37
All (primary) -14.5% [-47.6%, 0.5%] 15
  • A few tiny regressions, but there's not much to be done about these since this
    was a Cargo update.
  • The large instruction count improvements came from https://github.com/rust-lang/cargo/pull/13257,
    which introduced stripping of --release binaries by default. This coincidentally also reduces
    linking time on Linux quite a lot for tiny programs, which caused these improvements on
    helloworld-like crates.

error on incorrect implied bounds in wfcheck except for Bevy dependents #118553 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) 0.7% [0.2%, 1.6%] 30
Regressions (secondary) 0.4% [0.2%, 0.6%] 13
Improvements (primary) - - 0
Improvements (secondary) -1.0% [-1.2%, -0.8%] 6
All (primary) 0.7% [0.2%, 1.6%] 30
  • Regressions have outweighted the improvements here, however, this was a correctness fix,
    so we will have to take the hit. A part of the regressions reverted wins from
    #120123.

Rollup of 9 pull requests #120112 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) - - 0
Regressions (secondary) 3.2% [0.3%, 4.1%] 7
Improvements (primary) -0.7% [-0.7%, -0.7%] 1
Improvements (secondary) - - 0
All (primary) -0.7% [-0.7%, -0.7%] 1
  • This was caused by #120037, which
    seems to have just undone a previous perf. win on the same benchmarks caused by
    code being reshuffled around.

use implied bounds compat mode in MIR borrowck #120123 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) - - 0
Regressions (secondary) 0.9% [0.9%, 1.0%] 4
Improvements (primary) -0.7% [-0.9%, -0.6%] 6
Improvements (secondary) - - 0
All (primary) -0.7% [-0.9%, -0.6%] 6
  • This PR had more wins than losses, however its effects were later mostly reverted in
    #118553.

LLVM 18 x86 data layout update #116672 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) 0.7% [0.7%, 0.7%] 1
Regressions (secondary) 1.0% [0.3%, 3.3%] 6
Improvements (primary) -0.4% [-0.6%, -0.3%] 8
Improvements (secondary) -0.6% [-1.1%, -0.4%] 11
All (primary) -0.3% [-0.6%, 0.7%] 9
  • The wins have outweighted the losses.
  • This change is required to update to LLVM 18.
  • It introduced a few Max-RSS regressions, which were later fixed in
    #120080.

Rollup of 6 pull requests #120170 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) 0.6% [0.6%, 0.6%] 1
Regressions (secondary) - - 0
Improvements (primary) -0.9% [-1.4%, -0.5%] 2
Improvements (secondary) - - 0
All (primary) -0.4% [-1.4%, 0.6%] 3
  • Tiny regression on a single benchmark, marked as triaged.

Pack u128 in the compiler to mitigate new alignment #120080 (Comparison Link)

(instructions:u) mean range count
Regressions (primary) 0.3% [0.2%, 0.5%] 12
Regressions (secondary) 1.2% [0.4%, 2.1%] 2
Improvements (primary) -0.7% [-0.7%, -0.7%] 1
Improvements (secondary) -0.6% [-0.7%, -0.3%] 5
All (primary) 0.3% [-0.7%, 0.5%] 13
  • This PR mitigates the Max-RSS hits caused by #116672,
    at the cost of a few small instruction count regressions.

Nominated Issues

T-compiler

  • No I-compiler-nominated issues this time.

RFC

  • No I-compiler-nominated RFCs this time.

Oldest PRs waiting for review

T-compiler

  • None this week

Next week's WG checkins

  • @_WG-rustc-dev-guide by @Santiago Pastorino and @Yuki Okushi|217081
  • Impl Trait initiative by @oli

Next meetings' agenda draft: hackmd link

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