docs(perf): add performance section to CONTRIBUTING.md (#4846)

* docs(perf): add performance section to CONTRIBUTING.md

As requested here: https://github.com/prettier/prettier/pull/4789#issuecomment-401861558

* feat(scripts): add perf-repeat, perf-repeat-inspect, perf-benchmark

- update CONTRIBUTING.md to reference these scripts

*  diaereses

CONTRIBUTING.md

@@ -22,3 +22,28 @@ Run `yarn lint --fix` to automatically format files.
 If you can, take look at [commands.md](commands.md) and check out [Wadler's paper](http://homepages.inf.ed.ac.uk/wadler/papers/prettier/prettier.pdf) to understand how Prettier works.
 
 If you want to know more about Prettier's GitHub labels, see the [Issue Labels](https://github.com/prettier/prettier/wiki/Issue-Labels) page on the Wiki.
+
+## Performance
+
+If you're contributing a performance improvement, the following Prettier CLI options can help:
+
+- `--debug-repeat N` uses a naïve loop to repeat the formatting `N` times and measures the average run duration. It can be useful to highlight hot functions in the profiler. The measurements are printed at the debug log level, use `--loglevel debug` to see them.
+- `--debug-benchmark` uses [`benchmark`](https://npm.im/benchmark) module to produce statistically significant duration measurements. The measurements are printed at the debug log level, use `--loglevel debug` to see them.
+
+For convenience, the following commands for profiling are available via `package.json` `scripts`.
+
+_Unfortunately, [`yarn` simply appends passed arguments to commands, cannot reference them by name](https://github.com/yarnpkg/yarn/issues/5207), so we have to use inline environment variables to pass them._
+
+- `PERF_FILE=<filename> PERF_REPEAT=[number-of-repetitions:1000] yarn perf-repeat` starts the naïve loop. See the CLI output for when the measurements finish, and stop profiling at that moment.
+- `PERF_FILE=<filename> PERF_REPEAT=[number-of-repetitions:1000] yarn perf-repeat-inspect` starts the naïve loop with `node --inspect-brk` flag that pauses execution and waits for Chromium/Chrome/Node Inspector to attach. Open [`chrome://inspect`](chrome://inspect), select the process to inspect, and activate the CPU Profiler, this will unpause execution. See the CLI output for when the measurements finish, and stop the CPU Profiler at that moment to avoid collecting more data than needed.
+- `PERF_FILE=<filename> yarn perf-benchmark` starts the `benchmark`-powered measurements. See the CLI output for when the measurements finish.
+
+In the above commands:
+
+- `yarn && yarn build` ensures the compiler-optimized version of Prettier is built prior to launching it. Prettier's own environment checks are defaulted to production and removed during the build. The build output is cached, so a rebuild will happen only if the source code changes.
+- `NODE_ENV=production` ensures Prettier and its dependencies run in production mode.
+- `node --inspect-brk` pauses the script execution until Inspector is connected to the Node process.
+- `--loglevel debug` ensures the `--debug-repeat` or `--debug-benchmark` measurements are printed to `stderr`.
+- `> /dev/null` ensures the formatted output is discarded.
+
+In addition to the options above, you can use [`node --prof` and `node --prof-process`](https://nodejs.org/en/docs/guides/simple-profiling/), as well as `node --trace-opt --trace-deopt`, to get more advanced performance insights.

package.json

@@ -111,6 +111,9 @@
     "test": "jest",
     "test:dist": "node ./scripts/test-dist.js",
     "test-integration": "jest tests_integration",
+    "perf-repeat": "yarn && yarn build && cross-env NODE_ENV=production node ./dist/bin-prettier.js --debug-repeat ${PERF_REPEAT:-1000} --loglevel debug ${PERF_FILE:-./index.js} > /dev/null",
+    "perf-repeat-inspect": "yarn && yarn build && cross-env NODE_ENV=production node --inspect-brk ./dist/bin-prettier.js --debug-repeat ${PERF_REPEAT:-1000} --loglevel debug ${PERF_FILE:-./index.js} > /dev/null",
+    "perf-benchmark": "yarn && yarn build && cross-env NODE_ENV=production node ./dist/bin-prettier.js --debug-benchmark --loglevel debug ${PERF_FILE:-./index.js} > /dev/null",
     "lint": "cross-env EFF_NO_LINK_RULES=true eslint . --format node_modules/eslint-friendly-formatter",
     "lint-docs": "prettylint {.,docs,website,website/blog}/*.md",
     "build": "node ./scripts/build/build.js",