Perf triage, 2020-07-21.

Author: nnethercote

triage/2020-07-21.md

@@ -0,0 +1,59 @@
+# 2020-07-21 Triage Log
+
+See the [README](README.md) for details on how this log was gathered.
+
+This week was a disaster, perf-wise. 28 revisions checked. At least 7
+regressions, several of them ranging from large to huge, many in rollups. Some
+additional regressions may have occurred in rollups that were masked by other
+regressions/improvements. 3 improvements, one of which was a reversion of a
+regression. Unpicking all this is going to take days and I'm not confident
+we'll actually identify every regression.
+
+In better news, rustdoc performance is now being benchmarked, thanks to the
+efforts of [Joshua Nelson](https://github.com/rust-lang/rustc-perf/pull/675).
+
+Triage done by njn. Revision range: [9d09331e00b02f81c714b0c41ce3a38380dd36a2..71384101ea3b030b80f7def80a37f67e148518b0](https://perf.rust-lang.org/?start=9d09331e00b02f81c714b0c41ce3a38380dd36a2&end=71384101ea3b030b80f7def80a37f67e148518b0&absolute=false&stat=instructions%3Au).
+
+Regressions
+- [Rollup of 18 pull requests #74461](https://github.com/rust-lang/rust/pull/74461#issuecomment-660492867)
+  ([instructions](https://perf.rust-lang.org/compare.html?start=39d5a61f2e4e237123837f5162cc275c2fd7e625&end=d3df8512d2c2afc6d2e7d8b5b951dd7f2ad77b02&stat=instructions:u)):
+  Up to 73.0% losses on many benchmarks.
+  [#74416](https://github.com/rust-lang/rust/pull/74416#issuecomment-660545238)
+  is at least partly at fault; it was reverted in
+  [#74478](https://github.com/rust-lang/rust/pull/74478), see below.
+- [std: Switch from libbacktrace to gimli #73441](https://github.com/rust-lang/rust/pull/73441#issuecomment-660881380)
+  ([instructions](https://perf.rust-lang.org/compare.html?start=7d31ffc1ac9e9ea356e896e63307168a64501b9d&end=1fa54ad9680cc82e7301f8ed4e9b7402dfd6ce0e&stat=instructions:u),
+   [max-rss](https://perf.rust-lang.org/compare.html?start=7d31ffc1ac9e9ea356e896e63307168a64501b9d&end=1fa54ad9680cc82e7301f8ed4e9b7402dfd6ce0e&stat=max-rss)):
+  Up to 44.8% instruction increases on many benchmarks, and 5-10% max-rss increases on many benchmarks. I have requested it be reverted.
+- [Rollup of 11 pull requests #74468](https://github.com/rust-lang/rust/pull/74468#issuecomment-661371815)
+  ([instructions](https://perf.rust-lang.org/compare.html?start=d3df8512d2c2afc6d2e7d8b5b951dd7f2ad77b02&end=7d31ffc1ac9e9ea356e896e63307168a64501b9d&stat=instructions:u)):
+  Up to 10.2% losses on many benchmarks. Unclear which PR is responsible.
+- [Support const args in type dependent paths (Take 2) #74113](https://github.com/rust-lang/rust/pull/74113#issuecomment-661470398)
+  ([instructions](https://perf.rust-lang.org/compare.html?start=d9e8d6290745a65025a3e082aea72fbe372292c6&end=7e11379f3b4c376fbb9a6c4d44f3286ccc28d149&stat=instructions:u)):
+  Up to 1.5% losses on numerous benchmarks, which was expected.
+- [Reduce the amount of interning and `layout_of` calls in const eval. #74202](https://github.com/rust-lang/rust/pull/74202#issuecomment-661475728)
+  ([instructions](https://perf.rust-lang.org/compare.html?start=4cd0ee9343da86d9770bf0a514a682d240e0dce8&end=125c58caebc67c32ec45ac6c0581b596fd532082&stat=instructions:u)):
+  Up to 2% losses on one benchmark.
+- [Rollup of 9 pull requests #74543](https://github.com/rust-lang/rust/pull/74543#issuecomment-661507295)
+  ([instructions](https://perf.rust-lang.org/compare.html?start=2c21a6f3a8b1c75c444b87fde5116853383b3fbd&end=891e6fee572009ff2be4d4057fb33483610c36a7&stat=instructions:u)):
+  Up to 12% losses on rustdoc for many benchmarks.
+- [Rollup of 4 pull requests #74518](https://github.com/rust-lang/rust/pull/74518#issuecomment-661498214)
+  ([instructions](https://perf.rust-lang.org/compare.html?start=48036804d2bc461b243c5d291b850e44bcca68ef&end=d7f94516345a36ddfcd68cbdf1df835d356795c3&stat=instructions:u)):
+  Up to 6.4% losses on rustdoc for two benchmarks.
+
+Improvements
+- [Rollup of 7 pull requests #74493](https://github.com/rust-lang/rust/pull/74493#issuecomment-661521298)
+  ([instructions](https://perf.rust-lang.org/compare.html?start=1fa54ad9680cc82e7301f8ed4e9b7402dfd6ce0e&end=0701419e96d94e5493c7ebfcecb66511ab0aa778&stat=instructions:u)):
+  Up to 33.5% improvements, at least partly due to the backout of
+  [#74416](https://github.com/rust-lang/rust/pull/74416) in
+  [#74478](https://github.com/rust-lang/rust/pull/74478), but it's also
+  possible that another PR in that rollup [caused a regression that was masked
+  by the improvement from the backout](https://github.com/rust-lang/rust/pull/74493#issuecomment-661521298).
+- [Change `SymbolName::name` to a `&str`. #74214](https://github.com/rust-lang/rust/pull/74214#issuecomment-661459141)
+  ([instructions](https://perf.rust-lang.org/compare.html?start=c714eae0e3b4ba263c193d54f6e46bb9a1feb2cc&end=d9e8d6290745a65025a3e082aea72fbe372292c6&stat=instructions:u)):
+  Up to 2.5% wins on numerous benchmarks.
+- [Don't run `everybody_loops` for rustdoc; instead ignore resolution errors #73566](https://github.com/rust-lang/rust/pull/73566#issuecomment-661484787)
+  ([instructions](https://perf.rust-lang.org/compare.html?start=6ee1b62c811a6eb68d6db6dfb91f66a49956749b&end=5c9e5df3a097e094641f16dab501ab1c4da10e9f&stat=instructions:u)):
+  Wins of up to 62.6% and losses of up to 8.5%, all on rustdoc builds. Overall,
+  the improvements greatly outweigh the losses. (Landed in rollup
+  [#74408](https://github.com/rust-lang/rust/pull/74408).)