Performance Testing with rustc-perf

# Performance testing ## rustc-perf A lot of work is put into improving the performance of the compiler and preventing performance regressions. The [rustc-perf](https://github.com/rust-lang/rustc-perf) project provides several services for testing and tracking performance. It provides hosted infrastructure for running benchmarks as a service. At this time, only `x86_64-unknown-linux-gnu` builds are tracked. A "perf run" is used to compare the performance of the compiler in different configurations for a large collection of popular crates. Different configurations include "fresh builds", builds with incremental compilation, etc. The result of a perf run is a comparison between two versions of the compiler (by their commit hashes). You can also use `rustc-perf` to manually benchmark and profile the compiler [locally](../profiling/with_rustc_perf.md). ### Automatic perf runs After every PR is merged, a suite of benchmarks are run against the compiler. The results are tracked over time on the <https://perf.rust-lang.org/> website. Any changes are noted in a comment on the PR. ### Manual perf runs Additionally, performance tests can be ran before a PR is merged on an as-needed basis. You should request a perf run if your PR may affect performance, especially if it can affect performance adversely. To evaluate the performance impact of a PR, write this comment on the PR: `@bors try @rust-timer queue` > **Note**: Only users authorized to do perf runs are allowed to post this > comment. Teams that are allowed to use it are tracked in the [Teams > repository](https://github.com/rust-lang/team) with the `perf = true` value in > the `[permissions]` section (and bors permissions are also required). If you > are not on one of those teams, feel free to ask for someone to post it for you > (either on Zulip or ask the assigned reviewer). This will first tell bors to do a "try" build which do a full release build for `x86_64-unknown-linux-gnu`. After the build finishes, it will place it in the queue to run the performance suite against it. After the performance tests finish, the bot will post a comment on the PR with a summary and a link to a full report. If you want to do a perf run for an already built artifact (e.g. for a previous try build that wasn't benchmarked yet), you can run this instead: `@rust-timer build <commit-sha>` You cannot benchmark the same artifact twice though. More information about the available perf bot commands can be found [here](https://perf.rust-lang.org/help.html). More details about the benchmarking process itself are available in the [perf collector documentation](https://github.com/rust-lang/rustc-perf/blob/master/collector/README.md).

The rustc-perf project is used to track and prevent performance regressions in the Rust compiler. It runs benchmarks on popular crates to compare compiler performance between different configurations. Performance tests are run automatically after every PR merge, and manually via the `@bors try @rust-timer queue` command before merging a PR. The results are tracked on the perf.rust-lang.org website, and summaries are posted as comments on the corresponding PRs.