1. COMPILER DESIGN 11CS30013 & 11CS30014 Group 33 23 October 2013

2. Topics Covered
Evaluation of a parse tree containing both Synthesized and Inherited attributes
L – attributed grammars
23 October 2013
Compiler Design – CS31003

3. Evaluation of parse tree with Inherited attributes
Dependency Graph : A graph with the attributes of the symbols in the parse tree as nodes and directed edges indicating the flow of attributes.
Evaluation of a parse tree of an S-attributed SDD is simple. It needs just a Post Order Traversal of the dependency graph.
However, doing the same with a grammar having inherited attributes as well is incorrect.
23 October 2013
Compiler Design – CS31003

4. Evaluation of parse tree with Inherited attributes7
/14 A F X 10
/11
Consider the dependency graph on the right.
Objective : To find a linear order of traversal using Topological sort on the DAG
implies that FinishTime(u) > FinishTime(v)
and that u must come before v in the linear order.
Implying that the linear order must be descending w.r.t. FinishTime Y Z B E 8
/13 9
/12 u v Q P C D 1 /6 4 /5 R G 2 /3
Black lines denote the productions.
Red arrows are the actual edges of the dependency graph – denoting the movement of attributes
Therefore, the linear order is :
A B E F D C G
Attributes should be calculated in this order so that the parse tree can be calculated with success
29 December 2018
Compiler Design – CS31003

5. L – attributed SDD
An L – attributed SDD can contain both Synthesized and Inherited attributes but with certain constraints.
The constraints are :
Synthesized attribute of a symbol can be calculated using the inherited attribute of the parent only and not the synthesized attribute.
Consider a production of the form : A -> X1 X2 … Xi … Xn
Then, for the calculation of the inherited attribute of Xi, the synthesized and inherited attributes of only X1, X2, … , Xi-1 are allowed to be used
There should be no cycle in the dependency graph denoting the calculation of attributes
Set of grammars supported by S-attributed SDD is a subset of the set of grammars supported by L-attributed SDD
Both LR and LL parsers are able to parse languages having L-attributed grammars
29 December 2018
Compiler Design – CS31003

6. L – attributed SDD A  X Y An example of Invalid L-attributed SDD
Consider the production with the semantic rules
A  X Y
A.i = f(X.j, Y.k)
X.i = f(A.i, Y.k)
The grammar is Invalid because for the calculation of the attribute i of the symbol X, we need an attribute of Y – which comes after X in the production
29 December 2018
Compiler Design – CS31003

7. Applications of L – attributed SDD
Facilitates translation of source code to target code
Parse Tree Intermediate Code
Evaluation of the parse tree. While evaluation of the parse tree, we can :
Print
Update data structures (particularly, Symbol Table)
Construct Data structures (particularly, Syntax Tree)
Execute internal codes
Semantic Analysis
Side Effect : Functions/procedures clubbed with semantic rules.
For example, insertion of datatype into the symbol table entry of a variable when the declaration of that variable is encountered.
23 October 2013
Compiler Design – CS31003

8. Evaluation of L-attributed SDD (example)
Consider the following L-attributed grammar :
Terminals : int, float, id
Non Terminals : D(start symbol), T, L
Attributes : L.inh(INHERITED), T.type
Productions with semantic rules :
D  T L L.inh = T.type
T  int T.type = integer
T  float T.type = float
L  L1 , id L1.inh = L.inh addTable(L.inh, id.lexVal)
L  id addTable(L.inh, id.lexVal)
addTable() function adds the datatype to the identifier
23 October 2013
SIT-IIT KGP

9. Evaluation of L-attributed SDD (example)
Input string : int x , y D T L
inh = “integer”
type = “integer”
inh = “integer” L , id
lexVal = “y”
int
addTable(integer, x) id
lexVal = “x”
23 October 2013
Compiler Design – CS31003