[antlr-interest] philosophy about translation

[Paul Johnson]

I'm not knocking the single-tree approach; I'm sure it has its uses. 
However, my own opinion is that multiple in-situ tree transformations 
are extremely useful, if not indispensable. Some quick examples, off the 
top of my head, in addition to Andy's:

1 - constant reduction. You've got a subtree of, say, a dozen or more 
nodes which represent something like ((CONSTA << 16) & CONSTB) | (CONSTC 
 >> 2)). There's no point whatever keeping this in the tree and 
redundantly parsing and re-evaluating it on every AST pass. Why not just 
evaluate it once in a constant reduction pass, snip off the branch, and 
replace it with a single CONST node?

2 - if your language has compound assignments (ie. something like 'a += 
b') then it's pointless and confusing to keep this in the tree, and it 
complicates code generation. Why do not just replace the branch with 
another branch that unrolls the assignment? What if your language is 
even worse than this - 'x = a += b, a + c'? I expect you all know what 
language that is.

3 - Similarly, if your language has incr/decr operators, these are 
easily handled by modifying the AST to unroll them.

4 - What if your operators have side-effects, which you can't handle 
till the next sequence point? Doing this in a single static tree is very 
difficult. It's simple if you construct a new expression and insert it 
into the tree at some later point.

5 - what if your language has multiple ways to do the same thing? 
Indirection and array access, for example? It makes little sense to code 
this redundancy into the AST. You can instead have a pass which, for 
example, eliminates indirections and replaces them with array accesses.

And so on. Bottom line - the AST should represent the programmer's 
intent, which is never obvious from the bare syntax of whatever language 
they attempted to code that intent in. Sometimes, it can take you many 
passes to find out what that intent actually was. Why tie one arm behind 
your back by sticking with your first AST till the bitter end?

Paul