Compilation: Pass#1
Since I decided to add cross-reference support for CoolBasic (see blog entry “Highly Modular”), some major changes to the entire process were needed. Therefore, I’ve rewritten the lexer in preparation for Pass#1. Due to the nature of these changes, the way inner lists work also needed a review. I’m quite happy with the results. Lexing algorithms improved, for example the assignment operator is now better distinguished from the comparison operator. The assignment is now automatically cast for statements like Optional in function declaration, variable declaration, or the For…Next assignment. This will ease further parsing in Pass#1 and Pass#2.
Pass#1 compiles the symbol table i.e list of all identifiers. Basically, the compiler will scan for declaration statements and then create a collection of their names. Some secondary information such as path, relationship, block entry/end and signature are also formulated. Pass#1 can also process the hierarchy integrity as well as perform some initial checks to the syntax. We can’t create exact pointers to parent entities yet because program structures can be declared in random order within the source. Other tasks include generation of enums and trimming the code from entity modifiers. In general, Pass#1 is just a fast sweep and it doesn’t analyse the syntax in great depth. Deep scan belongs to Pass#2 and perhaps Pass#3.
Constants are interesting because they can be used to define each others, for example Const a = 1 : Const b = a + 3. It’s, however, possible to create a circular reference which renders the evaluation of the constant’s value, impossible. You can’t make exceptions to the general reference resolver (requiring Constants to be always declared before using) either. Being unable to specify constants as part of another constant expression, is stupid, too. This means I have to implement a recursive search at later passes when constants’ true values are pre-calulated. Pass#1 doesn’t yet do anything about the constants’ values anyway. In VB.NET constants aren’t really replacing values within the source; they’re constant variables i.e Static ReadOnly. They are, however, pre-evaluated at compiling time to determine constant expressions and thus discarding false condition code blocks. This is where CoolBasic will differ – constants will be transformed into literal values which means speed increase at runtime.
More about Pass#2 when I’m done with this…