Perf triage for 2020-10-27 week

Author: Mark-Simulacrum

triage/2020-10-27.md

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+# 2020-10-27 Triage Log
+
+Relatively quiet week for performance.
+
+Triage done by **@simulacrum**.
+Revision range: [22e6b9c68941996daa45786b4145e6196e51f0f4..824f900a96d752da2d882863c65f9736e5f2b347](https://perf.rust-lang.org/?start=22e6b9c68941996daa45786b4145e6196e51f0f4&end=824f900a96d752da2d882863c65f9736e5f2b347&absolute=false&stat=instructions%3Au)
+
+0 Regressions, 2 Improvements, 3 Mixed
+1 in rollups.
+
+#### Regressions
+
+Some mixed results, but no PRs with solely regressions this week.
+
+#### Improvements
+
+[#78077](https://github.com/rust-lang/rust/issues/78077)
+- Very large improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=1d2726726f8f3128e98191e4c6cb94bd76d0ddd4&end=1eaadebb3dee31669c7649b32747381d11614fae&stat=instructions:u) (up to -9.9% on `incr-full` builds of `deeply-nested-async-check`)
+
+[#77476](https://github.com/rust-lang/rust/issues/77476)
+- Large improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=3e0dd24a6c0812eedbb02182a75c352f8a7e184a&end=5171cc76c264fd46f32e140c2e460c77ca87d5e5&stat=instructions:u) (up to -6.2% on `incr-full` builds of `clap-rs-check`)
+- Interesting case where instruction counts show major improvement across the
+  board, but wall times are either a slight regression or largely unchanged
+  (within noise bound).
+
+#### Mixed
+
+[#78334](https://github.com/rust-lang/rust/issues/78334)
+- Moderate improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=ffa2e7ae8fbf9badc035740db949b9dae271c29f&end=f58ffc93815f76576eb56df4bdeec2fe8f12b766&stat=instructions:u) (up to -1.5% on `full` builds of `match-stress-enum-check`)
+- Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=ffa2e7ae8fbf9badc035740db949b9dae271c29f&end=f58ffc93815f76576eb56df4bdeec2fe8f12b766&stat=instructions:u) (up to 1.2% on `full` builds of `unicode_normalization-check`)
+- Unclear as to causes, left nag; will folow-up in a future week.
+
+[#77187](https://github.com/rust-lang/rust/issues/77187)
+- Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=0da6d42f297642a60f2640ec313b879b376b9ad8&end=fd542592f08ca0d1f7255600115c2eafdf6b5da7&stat=instructions:u) (up to 2.9% on `full` builds of `regression-31157-debug`)
+- Moderate improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=0da6d42f297642a60f2640ec313b879b376b9ad8&end=fd542592f08ca0d1f7255600115c2eafdf6b5da7&stat=instructions:u) (up to -1.1% on `incr-patched: b9b3e592dd cherry picked` builds of `style-servo-debug`)
+- Roughly neutral (or an improvement) on wall times; and a long-desired change.
+  Not proposing a revert at this time.
+
+[#77876](https://github.com/rust-lang/rust/issues/77876)
+- Large improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=fd542592f08ca0d1f7255600115c2eafdf6b5da7&end=a4d30a7b490065f0aa56f58e508a11546445aea9&stat=instructions:u) (up to -8.1% on `full` builds of `ctfe-stress-4-check`)
+- Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=fd542592f08ca0d1f7255600115c2eafdf6b5da7&end=a4d30a7b490065f0aa56f58e508a11546445aea9&stat=instructions:u) (up to 4.4% on `full` builds of `cranelift-codegen-opt`)
+- Regression has unclear cause; left a nag for next week.
+
+#### Nags requiring follow up
+
+* [#78334](https://github.com/rust-lang/rust/pull/78334)
+* [#77876](https://github.com/rust-lang/rust/pull/77876#issuecomment-717326989)