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diff --git a/docs/debugging.md b/docs/debugging.md new file mode 100644 index 00000000000..85065fe32db --- /dev/null +++ b/docs/debugging.md @@ -0,0 +1,70 @@ +# Servo debugging guide + +There are a few ways to debug Servo. `mach` supports a `--debug` flag that +searches a suitable debugger for you and runs servo with the appropriate +arguments under it: + +``` +./mach run --debug test.html +``` + +You can also specify an alternative debugger using the `--debugger` flag: + +``` +./mach run --debugger=my-debugger test.html +``` + +You can also, of course, run directly your debugger on the Servo binary: + +``` +$ gdb --args ./target/debug/servo test.html +``` + +## Debugging SpiderMonkey. + +You can build Servo with a debug version of SpiderMonkey passing the +`--debug-mozjs` flag to `./mach build`. + +Note that this sometimes can cause problems when an existing build exists, so +you might have to delete the `mozjs` build directory, or run `./mach clean` +before your first `--debug-mozjs` build. + +## Debugging Servo with [rr][rr]. + +To record a trace under rr you can either use: + +``` +$ ./mach run --debugger=rr testcase.html +``` + +Or: + +``` +$ rr record ./target/debug/servo testcase.html +``` + +### Running WPT tests under rr's chaos mode. + +Matt added a mode to Servo's testing commands to record traces of Servo running +a test or set of tests until the result is unexpected. + +To use this, you can pass the `--chaos` argument to `mach test-wpt`: + +``` +$ ./mach test-wpt --chaos path/to/test +``` + +Note that for this to work you need to have `rr` in your `PATH`. + +Also, note that this might generate a lot of traces, so you might want to delete +them when you're done. They're under `$HOME/.local/share/rr`. + +### Known gotchas + +If you use a Haswell processor that supports Hardware Lock Ellision, rr might +not work for you. There's a `rr` [bug][rr-bug] open about this. Until that gets +fixed, you can ensure that the `parking_lot` dependency isn't built with the +`nightly` feature, which as of this writing is the only dependency that uses it. + +[rr]: http://rr-project.org/ +[rr-bug]: https://github.com/mozilla/rr/issues/1883 |