Things have been coming along very well with my Summer of Code project to add dynamically loaded plugins to the Glasgow Haskell Compiler. In my first week of coding post-finals I’ve got a lot done. I’ll be discussing two of the headline items in this post.
Proof Of Concept Plugin Loading
GHC is capable of dynamically loading plugins specified on the command line from any installed package, and running the compiler phases that they install. To give you an idea of what that looks like, here is the current code for my sample-plugin project:
There’s a lot of work still to do here: the biggie is allowing “annotations” a-la languages like C# that let you mark identifiers or expressions in the language with extra stuff that meta-programs can make use of. For example, you might want to tag which functions you want your compiler plugin to analyse or add instrumenting code to. It’s quite hard to get this feature right, and I’ll probably be posting some more about the issues involved later as I get closer to implementing it.
GHC compiles your programs in a classic pipelined style: the main stages in a typical pipeline would be lexing, parsing, typechecking, desugaring, optimization and finally code generation. Although most of these stages have to run in a particular order, some of the stages can potentially run in multiple orders, most notably those sub-stages that make up the “optimization” stage I mentioned.
This is relevant to plugins because we need to be able to say when any phases you install should run. However, it also turns out that we use this feature to allow compiler users to control when inlining and source code rewrite rules should be applied, as documented in the user guide.
The current system we use is a bit ugly and just establishes a mapping between controlled things and the natural numbers to establish an ordering. This week, I’ve proposed (and partially implemented) a system for phase control that uses phase names that are declared in PHASE pragmas and henceforth exported and imported just like any other Haskell name, so for example:
This establishes a new phase C that must run before GHCs constructor specialization phase. This phase is in turn used to control the activation of the “silly” rule, and the phase exported so it can be referred to by other modules.
If you have any comments about this system, please make yourself heard on glasgow-haskell-users!
I’m fairly pleased with my progress so far and having a great time finally doing some coding again after the long exam period!
Hopefully this coming week will see me complete the implementation of the new phase control system and a refactoring of GHCs existing pipeline construction to take into account that phase information. I should then be able to move on to some issues more directly related to plugins, such as the rather thorny issue of the annotation system.