Tags are identifiers that link compiler flags with specific files. Usually, they are defined in a files group with the tags attribute as a comma-separated list, and with the tag attribute on a compiler flag node. Files are then compiled with all the flags that have matching tags.
By restricting tags to certain files, only a sub-set of files needs to be recompiled when conditions change, and files without the relevant tags can reuse their object files. This can save a lot of compilation time, since some flags only apply to a few files.
Files can override the group tags by specifying their own tags attribute. Groups can add tags with the tag node.
Some tags have standard meanings when compiling Haxe code:
haxe - adds all the required compiler flags to get Haxe-generated code to compile correctly, and should be added to all files that depend directly or indirectly on hxcpp.h.static - will add the STATIC_LINK define when appropriate, which is used for generating CFFI glue code. It should be added to CFFI code that might generate static libraries.gc - These flags only affect the garbage-collection files.hxstring - These flags only affect String.cpp.optimization tags - each file is assumed to have exactly 1 optimization tag. If none is explicitly specified, then the default is used depending on whether it is a debug or release build. They are:
optim-std - alias for debug or release depending on build.debugreleaseoptim-noneoptim-size - optimize for size.Setting one of these tags is useful for compiling your library in release mode, even if haxe has -debug. Some very big files are slow to compile in release mode, so using a less optimized mode can be faster.
The tags attribute can be added to a Haxe-generated file using the @:fileXml meta, e.g.:
@:fileXml("tags='haxe,optim-none'") class MyClass { ... }
Here, the class is compiled with the normal Haxe flags, but has optimizations disabled, which can lead to much faster compilation times in some circumstances.