Sohail Aslam Compiler Construction Notes Set:5-90
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Evaluation Methods
A number of ways can be used to evaluate the attributes. When using Dynamic method,
the compiler application builds the parse tree and then builds the dependence graph. A
topological sort of the graph is carried out and attributes are evaluated or defined in
topological order. In rule-based (or treewalk) methodology, the attribute rules are
analyzed at compiler-generation time. A fixed (static) ordering is determined and the
nodes in the dependency graph are evaluated this order. In oblivious (passes, dataflow)
methodology, the attribute rules and parse tree are ignored. A convenient order is picked
at compiler design time and used.
Attribute grammars have not achieved widespread use due to a number of problems. For
example: non-local computation, traversing parse tree, storage management for short-
lived attributes and lack of high-quality inexpensive tools. However, a variation of
attribute grammars and evaluation schemes is used in many compilers. This variation is
called ad-hoc analysis.
In rule-based evaluators, a sequence of actions is associated with grammar productions.
Organizing actions required for context-sensitive analysis around structure of the
grammar leads to powerful, albeit ad-hoc, approach which is used on most parsers. A
snippet of code (action) is associated with each production that executes at parse time
In top-down parsers, the snippet is added to the appropriate parsing routine. In a bottom-
up shift-reduce parsers, the actions are performed each time the parser performs a
reduction. Here the LR(1) skeleton parser indicating the place where the snippet is
executed.
stack.push(dummy); stack.push(0);
done = false; token = scanner.next();
while (!done) {
s = stack.top();
if( Action[s,token] == โreduce Aโฮฒโ) {
invoke the code snippet
stack.pop(2ร|ฮฒ|);
s = stack.top();
stack.push(A);
stack.push(Goto[s,A]);
}
else if( Action[s,token] == โshift iโ){
stack.push(token); stack.push(i);
token = scanner.next();
}
}
