Perf triage 2020-08-17

Author: ecstatic-morse

triage/2020-08-17.md

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+# 2020-08-17 Triage Log
+
+See the [README](README.md) for details on how this log was gathered.
+
+This week was quite noisy, with many small regressions and improvements.
+Overall, instruction counts have increased somewhat, while max RSS remains the
+same. There were several updates to core data structures (`HashMap`,
+`IndexMap`), which contributed to some of the noise. 
+
+Triage done by @ecstaticmorse.
+Revision range: [8b84156c6ee2b4e707dc32f5a516e3143ab924d3..67e7b9b8cf776222825dbbd4cb1e39b7765ef27c](https://perf.rust-lang.org/?start=8b84156c6ee2b4e707dc32f5a516e3143ab924d3&end=67e7b9b8cf776222825dbbd4cb1e39b7765ef27c&absolute=false&stat=instructions%3Au)
+
+4 Regressions, 3 Improvements, 4 Mixed, 0 of them in rollups.
+
+#### 4 Regressions
+
+[#73851](https://github.com/rust-lang/rust/pull/73851) Remove most specialization use in serialization
+* [Very small regressions across the board](https://perf.rust-lang.org/compare.html?start=95879ad96104afa584e7aec7806cec1d0bd84116&end=668a34e0f438d4a950b9440239656d6755ad963c&stat=instructions:u)
+
+[#75048](https://github.com/rust-lang/rust/pull/75048) Prevent `__rust_begin_short_backtrace` frames from being tail-call optimised away
+* [Regressions in several benchmarks](https://perf.rust-lang.org/compare.html?start=c2d1b0d9800d922b0451921d2ce17e6ae665d5b4&end=f3a9de9b08659e20ce7c282ed77bc43ddd149107&stat=instructions:u)
+* Mostly for crates with very little codegen
+
+[#75306](https://github.com/rust-lang/rust/pull/75134) Hash parent ExpnData
+* [Very small regressions across the board](https://perf.rust-lang.org/compare.html?start=8bc801b05019cd3e0ef19e6c4c028d55baa645d2&end=543f03d24118d3af784aa98c507c00e30c796a0e&stat=instructions:u)
+* Necessary for correctness
+
+[#73656](https://github.com/rust-lang/rust/pull/73656) move Deaggregate pass to post\_borrowck\_cleanup 
+* [Very small regressions across the board](https://perf.rust-lang.org/compare.html?start=4b9ac5161781ca6a376daab3d3b2f2623d8f3789&end=cbe7c5ce705896d4e22bf6096590bc1f17993b78&stat=instructions:u)
+
+#### 3 Improvements
+
+[#74512](https://github.com/rust-lang/rust/pull/74512) Put panic code path from `copy_from_slice` into cold function
+* [Small improvements across the board, albeit mostly for incremental builds](https://perf.rust-lang.org/compare.html?start=576d27c5a6c80cd39ef57d7398831d8e177573cc&end=847ba835ce411d47364a93ddf0b4a5c0f27928a9&stat=instructions:u)
+
+[#74877](https://github.com/rust-lang/rust/pull/74877) Implement the `min_const_generics` feature gate
+* [A small improvement on `wf-projection-stress`](https://perf.rust-lang.org/compare.html?start=f3a9de9b08659e20ce7c282ed77bc43ddd149107&end=f9c2177ddc605f9c75ca1a3e6ddb33835b8a178d&stat=instructions:u)
+
+[#75121](https://github.com/rust-lang/rust/pull/75121)  Avoid `unwrap_or_else` in str indexing
+* [A small improvement on `encoding-opt`](https://perf.rust-lang.org/compare.html?start=63e34422bbaf4ae4ed5ae7309183185aa2aa13a4&end=98922795f68e86b0bca5aea8cfc66499d58eba1a&stat=instructions:u)
+
+
+
+#### 4 Mixed
+
+[#70052](https://github.com/rust-lang/rust/pull/70052)  Update hashbrown to 0.8.1
+* [Mixed results across the board](https://perf.rust-lang.org/compare.html?start=d4c940f0821754a98491b2d23fbb5323c14a2bf5&end=8b26609481c956a666f9189738f1ba611078e1ab&stat=instructions:u)
+
+[#75278](https://github.com/rust-lang/rust/pull/75278)  Upgrade indexmap and use it more
+* [Very small regressions for most builds, notably docs](https://perf.rust-lang.org/compare.html?start=39e593ab14c53fda63c3f2756716c5ad3cbb6465&end=18f3be7704a4ec7976fcd1272c728974243d29bd&stat=instructions:u)
+* Offset by larger wins on a few benchmarks: `encoding`, `keccak`, `inflate`.
+* Author cites improved CPU cycle counts.
+
+[#75306](https://github.com/rust-lang/rust/pull/75306) Update hashbrown to 0.8.2 
+* [Mixed results across the board](https://perf.rust-lang.org/compare.html?start=dcf107728c4e545b9fee6b0e6a929837429275cf&end=aced185592b6f99a21190965a7fecfcd72d954dc&stat=instructions:u)
+* Mostly improvements, but `incr-patched: println` has taken a rather large hit.
+
+[#75382](https://github.com/rust-lang/rust/pull/75382) First iteration of simplify match branches
+* [Very small improvements across the board](https://perf.rust-lang.org/compare.html?start=b6396b75e782954acb085447fb836c4e0ff5281d&end=5e3f1b148db5bfa27fee52464ae1f5d34c49d77b&stat=instructions:u)
+* However, a few moderate regressions on synthetic benchmarks
+
+Nags requiring follow up
+
+- [#73656](https://github.com/rust-lang/rust/pull/73656)
+- [#73851](https://github.com/rust-lang/rust/pull/73851)
+- [#75048](https://github.com/rust-lang/rust/pull/75048)