Compiler Design and Construction - Old Questions

6.  Describe the L- attributed definitions. How L-attributed definitions are evaluated?

An syntax directed definition that uses both synthesized and inherited attribute with restriction of not taking values from right siblings is called L-attributed definition.

Mathematically,

A syntax-directed definition is L-attributed if each inherited attribute of X_j on the right side of A → X₁ X₂ … X_n depends only on

• The attributes of the symbols X₁, X₂, …, X_j-1
• The inherited attributes of A

Consider a production: S → ABC

Here, S can take values from A, B, and C (synthesized). 'A' can take values from S only. B can take values from S and A. C can get values from S, A, and B.

L – attributed definitions allow for a natural order of evaluating attributes: depth-first and left to right. Every S-attributed syntax-directed definition is also L-attributed and the above rule applied only for inherited attributes.

Procedure for Depth First Evaluation of L – Attributed Definitions:

Procedure dfvisit(n: node);     //depth-first evaluation

    for each child m of n, from left to right do

          evaluate inherited attributes of m;

          dfvisit(m);

    evaluate synthesized attributes of n

Example:

A → XY

X.i = A.i

Y.i = X.s

A.s = Y.s