Add triage for this week

Author: Mark-Simulacrum

triage/2020-12-08.md

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+# 2020-12-08 Triage Log
+
+Quiet week.
+
+Triage done by **@simulacrum**.
+Revision range: [c7cff213e937c1bb301be807ce04fcf6092b9163..4fd4a98d4788bc987d7f7add9df5f5ead6a1c15e](https://perf.rust-lang.org/?start=c7cff213e937c1bb301be807ce04fcf6092b9163&end=4fd4a98d4788bc987d7f7add9df5f5ead6a1c15e&absolute=false&stat=instructions%3Au)
+
+1 Regressions, 3 Improvements, 1 Mixed
+0 of them in rollups
+
+#### Regressions
+
+#### Improvements
+
+[#79594](https://github.com/rust-lang/rust/issues/79594)
+- Moderate improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=c7cff213e937c1bb301be807ce04fcf6092b9163&end=d015f0d92144f0e72735a918aee8510b0fe2cff5&stat=instructions:u) (up to -1.7% on `incr-full` builds of `ctfe-stress-4-debug`)
+- Likely due to the stress test not reflecting benefits of memoizing CTFE, which
+  this partially removed (due to the addition of heap allocation, which should
+  not be deduplicated).
+
+[#79680](https://github.com/rust-lang/rust/issues/79680)
+- Very large improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=e6225434fff7d493baac0aa91c57f2da923ea196&end=2218520b8adf8b8e64b817537d9eb0a84840e2f1&stat=instructions:u) (up to -20.6% on `full` builds of `match-stress-enum-check`)
+- Fixes a regression from last week by adding a `#[inline]` attribute on some
+  hot code. Improvements are likely not significant outside stress tests.
+
+#### Mixed
+
+[#78373](https://github.com/rust-lang/rust/issues/78373)
+- Very large improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=551a2c6cbcf239f662a18233cf647bf67e5a74ed&end=9122b769c8306b1cb3c8d1f96fca3ea3e208e22e&stat=instructions:u) (up to -28.4% on `incr-patched: println` builds of `clap-rs-debug`)
+- Large regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=551a2c6cbcf239f662a18233cf647bf67e5a74ed&end=9122b769c8306b1cb3c8d1f96fca3ea3e208e22e&stat=instructions:u) (up to 9.9% on `incr-patched: println` builds of `regression-31157-opt`)
+- Soundness fix ("Don't leak return value after panic in drop") and the perf
+  results are mixed, largest ones mostly in incremental and generally likely
+  just "generating more (necessary) LLVM IR".
+
+#### Nags requiring follow up
+
+- stdarch expansion causing a 40% libcore compile time regression is still not
+  resolved, and resolution is unclear. It seems likely that this could be one of
+  our key elements for improving std compile times at least, though.
+- No new nags.