triage for this week

Author: Mark-Simulacrum

triage/2021-02-16.md

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+# 2021-02-16 Triage Log
+
+A mostly quiet week, though with an excellent improvement in bootstrap times,
+shaving off a couple percent off the total and 10% off of rustc_middle due to
+changes in the code being compiled.
+
+Triage done by **@simulacrum**.
+Revision range: [ea098255f74923d69ea234ee526df6b9cecc3b9b..f1c47c79fe8438ed241630f885797eebef3a6cab](https://perf.rust-lang.org/?start=ea098255f74923d69ea234ee526df6b9cecc3b9b&end=f1c47c79fe8438ed241630f885797eebef3a6cab&absolute=false&stat=instructions%3Au)
+
+1 Regressions, 2 Improvements, 1 Mixed
+
+#### Regressions
+
+Initialize BTree nodes directly in the heap [#81494](https://github.com/rust-lang/rust/issues/81494)
+- Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=7e0241c63755ea28045d512b742f50b307874419&end=3c10a880eca60379343a6c6d19dd5bda38ead55d&stat=instructions:u) (up to 1.3% on `full` builds of `cargo-debug`)
+- Not really expected, but this is aiming for optimizing initialization of large
+  k/v pairs, so perhaps worth it. Left a nag.
+
+#### Improvements
+
+[experiment] remove `#[inline]` from rustc_query_system::plumbing [#81892](https://github.com/rust-lang/rust/issues/81892)
+- Moderate improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=185de5f41a64e1b790524d5873dc1f7e368f64ab&end=097bc6a84f2280a889b9ab4b544f27851a978927&stat=instructions:u) (up to -1.0% on `full` builds of `ctfe-stress-4-opt`)
+
+directly expose copy and copy_nonoverlapping intrinsics [#81238](https://github.com/rust-lang/rust/issues/81238)
+- Moderate improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=3c10a880eca60379343a6c6d19dd5bda38ead55d&end=8e54a21139ae96a2aca3129100b057662e2799b9&stat=instructions:u) (up to -2.2% on `incr-patched: u32 3072` builds of `issue-46449-debug`)
+- Surprisingly large improvement from essentially removing a function wrapper,
+  seems pretty unfortunate that this is the case. May have something to do with
+  MIR opts not triggering on these intrinsics when they were previously wrapped.
+
+#### Mixed
+
+Check the result cache before the DepGraph when ensuring queries [#81855](https://github.com/rust-lang/rust/issues/81855)
+- Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=9503ea19edbf01b9435e80e17d60ce1b88390116&end=d1206f950ffb76c76e1b74a19ae33c2b7d949454&stat=instructions:u) (up to 2.3% on `full` builds of `unused-warnings-check`)
+- Moderate improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=9503ea19edbf01b9435e80e17d60ce1b88390116&end=d1206f950ffb76c76e1b74a19ae33c2b7d949454&stat=instructions:u) (up to -1.0% on `incr-unchanged` builds of `clap-rs-check`)
+- Major improvement in rustc_middle compile times (10% win), but benchmarks are
+  regressed across the board pretty much, and this is surprising as PR perf
+  showed a decisive win. Left a nag to ask for reasons why this might be.
+
+#### Nags requiring follow up
+
+Investigations pending:
+- https://github.com/rust-lang/rust/pull/81494#issuecomment-779938322
+- https://github.com/rust-lang/rust/pull/81855#issuecomment-779950478
+
+From last week:
+- Waiting on more investigation in [#81476](https://github.com/rust-lang/rust/issues/81476).
+- Need to follow up on the regressions reported this week.