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Attribute Grammar Example
This example illustrates the most common use of attribute grammars —
prototyping compilers. It is adapted from B. Courcelle “Attribute grammars: theory
and applications”,
Formalization of Programming Concepts
, Lect. Notes in Comp.
Sci., V.107.
This example concerns itself with only a small part of a compiler, namely
translating assignment statements. In particular, it focuses on the process of
allocating temporary storage for the evaluation of expressions. For instance, in
the expression a+b*c, the result of b*c must first be computed and stored in
some memory location d, and then the addition a+d performed. In complicated
expressions, numerous intermediate results must be generated and the locations
where they are stored retained for later use as appropriate. This methodology is
described here by means of an attribute grammar.
Some pool of available temporary memory locations must be assumed. In order
to focus on the desired issues, this is taken to be the symbolic locations L0, L1,
L2, … Courcelle expresses the code for an assignment statement in terms of
“three address instructions”. That is, the usual precedence is used to resolve the
order of operations, and suitable temporary locations are determined, but the
code is symbolically indicated to avoid computer architecture details. Also,
temporary locations are reused when their prior use is completed. For example,
the assignment
X := 3.14 * (X+Y)
will be translated into
L0 := 3.14;
L1 := X;
L2 := Y;
L1 := L1+L2;
L0 := L0*L1;
X:= L0.
This attribute grammar uses the attribute ‘free’ to denote the first unused
temporary memory location available, the attribute ‘res’ to denote the memory
location used to hold the result of an expression, term, etc., and ‘code’ to denote
the sequence of instructions for an assignment, expression, etc. The semantic
rules also use the function ‘next’ applied to temporary memory locations with the
obvious result (e.g., next(L0) = L1, etc.).
