Add note about regressions to perf gains in a fix from last week

Author: rylev

triage/2020-12-15.md

@@ -1,6 +1,6 @@
 2020-12-15 Triage Log
 
-A week dominated by small regressions with only 1 modest yet clear performance gain. None of the regressions are large enough to cause concern, but there should be a followup to ensure that those regressions are at least examined.
+A week dominated by small regressions with only 1 modest yet clear performance gain. None of the regressions are large enough to cause concern, but there should be a followup to some to ensure that those regressions are at least examined.
 
 Triage done by **@rylevick**.
 Revision range: [4fd4a98d4788bc987d7f7add9df5f5ead6a1c15e..e1cce06e4ff5206daf397e1dcf91ed53653be171](https://perf.rust-lang.org/?start=4fd4a98d4788bc987d7f7add9df5f5ead6a1c15e&end=e1cce06e4ff5206daf397e1dcf91ed53653be171&absolute=false&stat=instructions%3Au)
@@ -18,6 +18,7 @@ Also generate `StorageDead` in constants[#78679](https://github.com/rust-lang/ru
 Properly re-use def path hash in incremental mode[#79721](https://github.com/rust-lang/rust/issues/79721)
 - Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=cc03ee6702053ded253c3656cbd02f0bfdf25c73&end=fa55f668e5ea5388ec98b9340969527252239151&stat=instructions:u) (up to 1.5% on `incr-unchanged` builds of `deeply-nested-opt`)
 - Fixes a p-high [ICE issue](https://github.com/rust-lang/rust/issues/79661), and so the perf regressions were deemed acceptable.
+- This is a fix for [#74967](https://github.com/rust-lang/rust/pull/74967) which saw [large perf gains](https://perf.rust-lang.org/compare.html?start=db79d2f63780613e700cb58b4339c48287555ae0&end=bf8e95436e60effbeb46a32e17df8ab7fcb0c6ad).
 
 Accept arbitrary expressions in key-value attributes at parse time[#78837](https://github.com/rust-lang/rust/issues/78837)
 - Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=1cc410710993d036730c11556039e40109f6ab41&end=58d2bad9f7ab0971495247b6c94978848760ca9d&stat=instructions:u) (up to 1.4% on `incr-unchanged` builds of `match-stress-exhaustive_patterns-check`)
@@ -50,8 +51,7 @@ Use `def_path_hash_to_def_id` when re-using a `RawDefId`[#79915](https://github.
 - Moderate improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=a2e29d67c26bdf8f278c98ee02d6cc77a279ed2e&end=19eb1c4c526071c430c05fffc64da71ac057a3d5&stat=instructions:u) (up to -3.5% on `incr-unchanged` builds of `clap-rs-check`)
 - Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=a2e29d67c26bdf8f278c98ee02d6cc77a279ed2e&end=19eb1c4c526071c430c05fffc64da71ac057a3d5&stat=instructions:u) (up to 2.7% on `incr-patched: dummy fn` builds of `unused-warnings-check`)
 - The amount of regressions outweighs the improvements (which were just in the clap benchmark).
-- This is a followup fix to [#79721](https://github.com/rust-lang/rust/issues/79721) which also introduced perf regressions.
-- [Pinged on the issue](https://github.com/rust-lang/rust/pull/79915#issuecomment-745409364)
+- This is a followup fix to [#79721](https://github.com/rust-lang/rust/issues/79721). Overall these regressions still represent a perf gain when compared to before the changes introduced in [#74967](https://github.com/rust-lang/rust/pull/74967).
 
 Lower `discriminant_value` intrinsic[#79922](https://github.com/rust-lang/rust/issues/79922)
 - Moderate improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=8b3ee82eb68cb35030bb745c23f8aa76d9de5bee&end=5d77fc8d0db3b69f3a3691d86eba23e4cdc390e1&stat=instructions:u) (up to -3.9% on `full` builds of `match-stress-enum-check`)