Build Metrics is a catch-all term for performance measures that are generated by the Firefox build system and tracked by Perfherder.
Number of static constructors found by the compiler in the Firefox C++ codebase. Lower is better. Static constructors are undesirable because their initialization imposes an unavoidable time penalty every time Firefox is started.
Size in bytes of the Firefox installer. Lower is better here, especially on space-restricted platforms like Android.
Amount of time it takes to build Firefox in automation on a specific platform / configuration. Lower is better.
Number of compiler warnings detected during a build. Lower is better.
Due to the way the build system works, compiler warnings are not consistently detected. So the value may fluctuate from build to build even if the number of compiler warnings didn’t actually change. Since Perfherder alerts are calculated based on the mean value of a range, a regression may be reported as a fractional value.