2010/6/22 José Romildo Malaquias
Hello.
I have been teaching an introductory course on compiler construction to our undergraduates students using Appel's "Modern Compiler Implementation in Java". There are also versions of the book in ML and C. The books explain how to write a compiler for the Tiger programming language.
Now I want to implement a Tiger compiler in Haskell.
The lexer and parser (built with the help of alex and happy) and the type checker are already working.
Next step is implementing a variable escape analysis phase, needed for the intermediate code generator. The objective of this phase is to find out if each variable escapes or not. In general an escaping variable should be allocated in the stack frame (in memory). Non escaping variables may be allocated in the stack frame or in a register.
Generally, a variable escapes if it is passed by reference, its address is taken (using C's & operator, for instance), or it is accessed from a nested function. Only the last is possible with Tiger.
The approach adopted by Appel in his books is easy: a muttable field in the abstract syntax of variable declarations, of for expressions, and of function formal parameters, which introduce new variables, is used to collect the escaping information of the variable. In the ML version of the book this is a reference to boolean (bool ref). Initially, in a conservative approach, the reference is initialized to true.
In the variable escaping analysis phase of the compiler, a function findEscape looks for escaping variables and record this information in the escape fields of the abstract syntax. To do this the entire abstract syntax tree is traversed looking for escaping uses of every variable, and, when found, the field is set to true.
findEscape uses a symbol table to accomplish its work, binding variable names to a reference to boolean (the same reference used in the abstract syntax tree). When processing a variable declaraction, for instance, it inserts the variable name into the symbol table, binding it to its escaping field. When processing an expression which is a simple variable, if the variable occurs in a nested function, its binding in the symbol table is set to true. This reflects directly in the abstract syntax tree of the variable declaration, as the escape field in the variable declaration and the binding in the symbol table are the same reference to bool.
I am look for good ways to implement the variable escaping analysis phase in Haskell, which is a pure language. I see two alternatives:
1) adopt the same approach as Appel used in his ML version of the compiler: use a mutable reference in the IO monad (Data.IORef) to hold the variable escaping information, and write everything inside the IO monad
2) build a new abstract syntax tree with updated information regarding variable escaping
The second option is more elegant in my point of view, but would be much less efficient than the first one.
In Haskell you will get a lot of sharing between the original AST and the updated AST. This greatly reduces the overhead of #2. Have you read Chris Okasaki's Purely Functional Datastructures book? My other bit of advice would be to code it in the purely functional way and then do some benchmarking / profiling to see that it does have sufficiently good performance characteristics. If you find that it's getting awkward to manage the purely functional code then refactor that code into a monad that hides the tedious bits. I had some other comments to make but others here have already covered it :) (Such as using Writer or State or ST instead of IO, etc). Jason