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2021-11-23 Triage Log

This week, there were a number of cases where the incr-unchanged variants of inflate went up or down by 5% to 6%; we believe these are instances of increased noise in benchmarks documented on rustc-perf#1105. I was tempted to remove these from the report, but its non-trivial to re-construct the report "as if" some benchmark were omitted.

Otherwise, there were some nice wins for performance. For example, PR #90996 more than halved the compilation time for full builds of diesel by revising how we hash ObligationCauseData. If anyone is interested, it might be good to follow-up on the effects of PR #90352, "Simplify for loop desugar", where we have hypothesized that the increased compilation time is due to more LLVM optimizations being applied.

Triage done by @pnkfelix. Revision range: 934624fe..22c2d9dd

1 Regressions, 3 Improvements, 8 Mixed; 3 of them in rollups 34 comparisons made in total

Regressions

rustdoc: Make two small cleanups #91073

  • Very large regression in instruction counts (up to 5.3% on incr-unchanged builds of inflate)
  • This was previously triaged as spurious by the PR author.
  • (Only inflate debug incr-unchanged was affected significantly.)

Improvements

Rollup of 7 pull requests #90966

  • Very large improvement in instruction counts (up to -5.9% on incr-unchanged builds of inflate)
  • (This is bouncing around in the same manner that the above regression bounced around; i.e., this is a spurious improvement.)

Optimize impl Hash for ObligationCauseData by not hashing ObligationCauseCode variant fields #90996

Avoid documenting top-level private imports #91094

  • Very large improvement in instruction counts (up to -45.1% on full builds of webrender-wrench)
  • This appears to counter-act a regression that was introduced in PR #90769 a rollup where the root cause is believed to be PR #88447, "Use computed visibility in rustdoc".

rustdoc: Cleanup DocFragment #91034

  • Moderate improvement in instruction counts (up to -0.7% on full builds of serde)
  • It has been pointed out that this was actually a nice -0.8% to -1.4% win on max-rss as well.
  • (This is solely for rustdoc benchmarking, to be clear.)

libcore: assume the input of next_code_point and next_code_point_reverse is UTF-8-like #89611

Mixed

Remove DropArena. #90919

std: Get the standard library compiling for wasm64 #90382

  • Large improvement in instruction counts (up to -4.3% on incr-unchanged builds of clap-rs)
  • Moderate regression in instruction counts (up to 1.0% on incr-unchanged builds of issue-88862)
  • There were a lot of various changes in this PR, such as updates to dependencies (compiler_builtins and dlmalloc). We probably shoud have done a pre-merge rust-timer run on this PR.
  • The flagged regressions of magnitude greater than 0.5% are isolated to "issue-88862 check" and "style-servo check". The improvements tended to outweigh the regressions. For the most part almost all significant performance effects are isolated to check builds.

Rollup of 8 pull requests #91019

  • Moderate improvement in instruction counts (up to -0.6% on incr-unchanged builds of style-servo)
  • Moderate regression in instruction counts (up to 0.8% on full builds of await-call-tree)
  • The regression is entirely associated with doc builds, which led to the PR author to flag PR #90750 as the root cause.
  • It seems to me like the extra work injected by PR #90750 may be unavoidable; but was it expected to be significant?

Implement clone_from for State #90535

  • Small improvement in instruction counts (up to -0.2% on full builds of many-assoc-items)
  • Very large regression in instruction counts (up to 6.1% on incr-unchanged builds of inflate)
  • This was evaluated for its effect on performance prior to merge; that run returned no relevant changes.
  • As noted elsewhere, for this report we should probably treat "inflate" as noisy.

Update stdarch #91052

  • Very large improvement in instruction counts (up to -5.8% on incr-unchanged builds of inflate)
  • Small regression in instruction counts (up to 0.3% on incr-patched: println builds of html5ever)
  • The only benchmark that regressed is "html5ever debug" ("incr-patched: println" and "incr-unchanged"), and only by a relatively small amount. This seems acceptable to me, compared to the effort involved in figuring out how this change could be related to that effect.

Point at source of trait bound obligations in more places #89580

  • Large improvement in instruction counts (up to -5.0% on incr-unchanged builds of inflate)
  • Small regression in instruction counts (up to 1.1% on incr-unchanged builds of wg-grammar)
  • (Again, we can probably ignore the change to inflate.)
  • Other than that, there were a broad set of small regressions. Putting aside the ones tagged with ? ("noisy"), there are 19 benchmarks that regressed by 0.10% to 0.42%. This seems like an acceptable cost.

Simplify for loop desugar #90352

  • Very large improvement in instruction counts (up to -6.2% on incr-full builds of clap-rs)
  • Large regression in instruction counts (up to 2.3% on full builds of regex)
  • This was triaged in the PR comments both before it landed and after it landed with the justification "The regressions seem to all be in -opt builds and solely part of the time spent in LLVM, so I'm hoping it's that more optimizations apply now (and worst case some optimizations require more work but don't result in better code)."

Manually outline error on incremental_verify_ich #89883

  • Small improvement in instruction counts (up to -0.8% on incr-unchanged builds of clap-rs)
  • Large regression in instruction counts (up to 1.1% on incr-unchanged builds of coercions)
  • The pre-merge rustc-timer run did not predict such a significant impact on coercions.
  • The driving force for this change was to reduce the critical path in bootstrap time, so the most important thing to look at is the bootstrap timing data. Specifically: while there is a big mix of ups and downs on the percentages column, the crate that takes the longest to compile (rustc_query_impl, the laggard at over 80 seconds of compilation time).
  • This PR brings the compilation time of rustc_query_impl from 87.1 seconds down to 85.6 seconds, a -1.8% improvement.
  • That is consistent with the predicted effect of the PR, and justifies the isolated impact on instruction counts.

Untriaged Pull Requests