Dead Code Elimination or DCE is a compiler feature which removes unused code from the output. After typing, the compiler evaluates the DCE entry-points (usually the main-method) and recursively determines which fields and types are used. Used fields are marked accordingly and unmarked fields are then removed from their classes.
DCE has three modes which are set when invoking the command line:
The DCE-algorithm works well with typed code, but may fail when dynamic or reflection is involved. This may require explicit marking of fields or classes as being used by attributing the following metadata:
@:keep: If used on a class, the class along with all fields is unaffected by DCE. If used on a field, that field is unaffected by DCE.
@:keepSub: If used on a class, it works like
@:keepon the annotated class as well as all subclasses.
@:keepInit: Usually, a class which had all fields removed by DCE (or is empty to begin with) is removed from the output. By using this metadata, empty classes are kept.
If a class needs to be marked with
@:keep from the command line instead of editing its source code, there is a compiler macro available for doing so:
--macro keep('type dot path') See the haxe.macro.Compiler.keep API for details of this macro. It will mark package, module or sub-type to be kept by DCE and includes them for compilation.
The compiler automatically defines the flag
dce with a value of either
"full" depending on the active mode. This can be used in conditional compilation.
DCE was originally implemented in Haxe 2.07. This implementation considered a function to be used when it was explicitly typed. The problem with that was that several features, most importantly interfaces, would cause all class fields to be typed in order to verify type-safety. This effectively subverted DCE completely, prompting the rewrite for Haxe 2.10.
DCE for the