Add first draft of triage-2021-01-26

Author: rylev

triage/2021-01-26.md

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+2021-01-26 Triage Log
+
+TODO: Summary
+
+Triage done by **@rylevick**.
+Revision range: [e05409a02c6e73a3dea6da98798468db2910ca59..1483e67addd37d9bd20ba3b4613b678ee9ad4d68](https://perf.rust-lang.org/?start=e05409a02c6e73a3dea6da98798468db2910ca59&end=1483e67addd37d9bd20ba3b4613b678ee9ad4d68&absolute=false&stat=instructions%3Au)
+
+5 Regressions, 2 Improvements, 2 Mixed
+??? of them in rollups
+
+#### Regressions
+
+Deprecate-in-future the constants superceded by RFC 2700 [#80958](https://github.com/rust-lang/rust/issues/80958)
+- Small regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=57a71ac0e17e4f7070b090ab7bdc5474d8e37ecc&end=339e19697a39a78f4d669c217b7d21109215de41&stat=instructions:u) (up to 2.4% on `incr-patched: Job` builds of `regex-check`)
+- Unsure why this might be causing performance regressions as this change does not touch much code and only produces lint warnings when the user opts into them. 
+- The author of the PR [was asked why this might be the case](https://github.com/rust-lang/rust/pull/80958#issuecomment-767837811).
+
+Rollup of 11 pull requests [#81240](https://github.com/rust-lang/rust/issues/81240)
+- Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=339e19697a39a78f4d669c217b7d21109215de41&end=a243ad280a4ac57c1b8427e30e31c5b505cf10de&stat=instructions:u) (up to 2.0% on `full` builds of `keccak-check`)
+- Revert kicked off [here](https://github.com/rust-lang/rust/pull/81420) to test a possible culprit in [#81178](https://github.com/rust-lang/rust/pull/81178).
+
+Generate metadata by iterating on DefId instead of traversing the HIR tree 1/N [#80919](https://github.com/rust-lang/rust/issues/80919)
+- Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=446cbc9db080c63c8742c418bcaa44c808f7e033&end=85e355ea9bd86ac6580a5d422a65dbf689845808&stat=instructions:u) (up to 1.0% on `incr-patched: println` builds of `style-servo-check`)
+- Contact the PR author to see why the perf regressions were not discussed in the initial PR (even though perf runs were done). Waiting on [their response](https://github.com/rust-lang/rust/pull/80919#issuecomment-767854932).
+
+Rollup of 14 pull requests [#81355](https://github.com/rust-lang/rust/issues/81355)
+- Very large regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=1d0d76f8dd4f5f6ecbeab575b87edaf1c9f56bb8&end=d3163e96553ae8cb1fca0e62084b124e8b98310b&stat=instructions:u) (up to 12.6% on `full` builds of `deeply-nested-async-debug`)
+
+Prevent query cycles in the MIR inliner [#68828](https://github.com/rust-lang/rust/issues/68828)
+- Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=7fba12bb1d3877870758a7a53e2fe766bb19bd60&end=f4eb5d9f719cd3c849befc8914ad8ce0ddcf34ed&stat=instructions:u) (up to 2.1% on `full` builds of `keccak-check`)
+- Moderate improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=7fba12bb1d3877870758a7a53e2fe766bb19bd60&end=f4eb5d9f719cd3c849befc8914ad8ce0ddcf34ed&stat=instructions:u) (up to -1.8% on `full` builds of `ctfe-stress-4-check`)
+
+#### Improvements
+
+Visit only terminators when removing unneeded drops [#81122](https://github.com/rust-lang/rust/issues/81122)
+- Moderate improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=a243ad280a4ac57c1b8427e30e31c5b505cf10de&end=65767e56537e20903b54ecde7c371cbfb1b201b0&stat=instructions:u) (up to -2.0% on `full` builds of `keccak-check`)
+
+Rollup of 15 pull requests [#81304](https://github.com/rust-lang/rust/issues/81304)
+- Moderate improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=4d0dd02ee07bddad9136f95c9f7846ebf3eb3fc5&end=1279b3b9232e4c44112d98f19cfa8846776d1fe8&stat=instructions:u) (up to -1.1% on `incr-unchanged` builds of `helloworld-opt`)
+
+#### Mixed
+
+mark raw_vec::ptr with inline [#79113](https://github.com/rust-lang/rust/issues/79113)
+- Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=f4eb5d9f719cd3c849befc8914ad8ce0ddcf34ed&end=ff6ee2a70218543f410e557f390e246131847572&stat=instructions:u) (up to 4.2% on `full` builds of `cranelift-codegen-opt`)
+- Moderate improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=f4eb5d9f719cd3c849befc8914ad8ce0ddcf34ed&end=ff6ee2a70218543f410e557f390e246131847572&stat=instructions:u) (up to -1.5% on `incr-patched: println` builds of `cargo-opt`)
+
+#### Nags requiring follow up
+
+TODO: Nags
+
+