2020-07-28 Triage Log

See the README for details on how this log was gathered.

Triage done by njn. Revision range: 71384101ea3b030b80f7def80a37f67e148518b0..efc02b03d18b0cbaa55b1e421d792f70a39230b2

2 regressions, 1 improvement, none of them on rollups.

Regressions

• Serialize span hygiene data #72121 (instructions): Up to 3.5% losses on numerous benchmarks. This was expected, and the author and reviewer deemed it reasonable for the improvement in error messages.
• Polymorphization #69749 (instructions): Up to 1.8% losses (ignoring the noisy script-servo-2-opt results), which was expected. (Hopefully extensions to this change will allow for compile time improvements in the future.) This feature was later disabled due to correctness issues, but the disabling was performance neutral, oddly enough.

Improvements

• Revert libbacktrace -> gimli #74613 (instructions): Up to 25.5% wins across many benchmarks. A reversion of one of last week‘s regressions. This PR may re-land because it’s a big functional improvement and the regressions are mostly on very short-running benchmarks. If it does it will be after consideration and with intention.