Chapter 5 Syntax Directed Translation

Chapter 5
Syntax Directed Translation

Outline
Syntax Directed Definitions
Evaluation Orders of SDD’s
Applications of Syntax Directed Translation
Syntax Directed Translation Schemes

Introduction
We can associate information with a language
construct by attaching attributes to the grammar
symbols.
A syntax directed definition specifies the values of
attributes by associating semantic rules with the
grammar productions.
Production Semantic Rule
E->E1+T E.code=E1.code||T.code||’+’
• We may alternatively insert the semantic actions inside the grammar
E -> E1+T {print ‘+’}

Syntax Directed Definitions
A SDD is a context free grammar with attributes and
rules
Attributes are associated with grammar symbols and
rules with productions
Attributes may be of many kinds: numbers, types,
table references, strings, etc.
Synthesized attributes
A synthesized attribute at node N is defined only in
terms of attribute values of children of N and at N it
Inherited attributes
An inherited attribute at node N is defined only in
terms of attribute values at N’s parent, N itself and N’s
siblings

Example of S-attributed SDD
1) L -> E n
2) E -> E1 + T
3) E -> T
4) T -> T1 * F
5) T -> F
6) F -> (E)
7) F -> digit
Production Semantic Rules
L.val = E.val
E.val = E1.val + T.val
E.val = T.val
T.val = T1.val * F.val
T.val = F.val
F.val = E.val
F.val = digit.lexval

Example of mixed attributes
1) T -> FT’
2) T’ -> *FT’1
3) T’ -> ε
1) F -> digit
T’.inh = F.val
T.val = T’.syn
T’1.inh = T’.inh*F.val
T’.syn = T’1.syn
T’.syn = T’.inh
F.val = F.val = digit.lexval

Evaluation orders for SDD’s
A dependency graph is used to determine the order of
computation of attributes
Dependency graph
For each parse tree node, the parse tree has a node for
each attribute associated with that node
If a semantic rule defines the value of synthesized
attribute A.b in terms of the value of X.c then the
dependency graph has an edge from X.c to A.b
If a semantic rule defines the value of inherited
attribute B.c in terms of the value of X.a then the
dependency graph has an edge from X.c to B.c
Example!

Ordering the evaluation of
attributes
If dependency graph has an edge from M to N then M
must be evaluated before the attribute of N
Thus the only allowable orders of evaluation are
those sequence of nodes N1,N2,…,Nk such that if
there is an edge from Ni to Nj then i<j
Such an ordering is called a topological sortof a
graph
Example!

S-Attributed definitions
An SDD is S-attributed if every attribute is
synthesized
We can have a post-order traversal of parse-tree to
evaluate attributes in S-attributed definitions
postorder(N) {
for (each child C of N, from the left) postorder(C);
evaluate the attributes associated with node N;
}
S-Attributed definitions can be implemented during
bottom-up parsing without the need to explicitly create
parse trees

L-Attributed definitions
A SDD is L-Attributed if the edges in dependency graph
goes from Left to Right but not from Right to Left.
More precisely, each attribute must be either
Synthesized
Inherited, but if there us a production A->X1X2…Xn and
there is an inherited attribute Xi.a computed by a rule
associated with this production, then the rule may only use:
 Inherited attributes associated with the head A
 Either inherited or synthesized attributes associated with the
occurrences of symbols X1,X2,…,Xi-1 located to the left of Xi
 Inherited or synthesized attributes associated with this occurrence
of Xi itself, but in such a way that there is no cycle in the graph

Application of Syntax Directed
Translation
Type checking and intermediate code generation
(chapter 6)
Construction of syntax trees
Leaf nodes: Leaf(op,val)
Interior node: Node(op,c1,c2,…,ck)
Example:
1) E -> E1 + T
2) E -> E1 - T
3) E -> T
4) T -> (E)
5) T -> id
6) T -> num
E.node=new node(‘+’, E1.node,T.node)
E.node=new node(‘-’, E1.node,T.node)
E.node = T.node
T.node = E.node
T.node = new Leaf(id,id.entry)
T.node = new Leaf(num,num.val)

Syntax tree for L-attributed
definition
+
1) E -> TE’
2) E’ -> + TE1’
3) E’ -> -TE1’
4) E’ -> ∈
5) T -> (E)
6) T -> id
7) T -> num
E.node=E’.syn
E’.inh=T.node
E1’.inh=new node(‘+’, E’.inh,T.node)
E’.syn=E1’.syn
E1’.inh=new node(‘+’, E’.inh,T.node)
E’.syn=E1’.syn
E’.syn = E’.inh
T.node = E.node
T.node=new Leaf(id,id.entry)
T.node = new Leaf(num,num.val)

Syntax directed translation
schemes
An SDT is a Context Free grammar with program
fragments embedded within production bodies
Those program fragments are called semantic actions
They can appear at any position within production body
Any SDT can be implemented by first building a parse tree
and then performing the actions in a left-to-right depth
first order
Typically SDT’s are implemented during parsing without
building a parse tree

Postfix translation schemes
Simplest SDDs are those that we can parse the grammar
bottom-up and the SDD is s-attributed
For such cases we can construct SDT where each action is
placed at the end of the production and is executed along
with the reduction of the body to the head of that
production
SDT’s with all actions at the right ends of the production
bodies are called postfix SDT’s

Example of postfix SDT
1) L -> E n {print(E.val);}
2) E -> E1 + T {E.val=E1.val+T.val;}
3) E -> T {E.val = T.val;}
4) T -> T1 * F {T.val=T1.val*F.val;}
5) T -> F {T.val=F.val;}
6) F -> (E) {F.val=E.val;}
7) F -> digit {F.val=digit.lexval;}

Parse-Stack implementation of
postfix SDT’s
In a shift-reduce parser we can easily implement
semantic action using the parser stack
For each nonterminal (or state) on the stack we can
associate a record holding its attributes
Then in a reduction step we can execute the semantic
action at the end of a production to evaluate the
attribute(s) of the non-terminal at the leftside of the
production
And put the value on the stack in replace of the
rightside of production

Example
L -> E n {print(stack[top-1].val);
top=top-1;}
E -> E1 + T {stack[top-2].val=stack[top-2].val+stack.val;
top=top-2;}
E -> T
T -> T1 * F {stack[top-2].val=stack[top-2].val+stack.val;
top=top-2;}
T -> F
F -> (E) {stack[top-2].val=stack[top-1].val
top=top-2;}
F -> digit

SDT’s with actions inside
productions
For a production B->X {a} Y
If the parse is bottom-up then
we perform action “a” as soon as
this occurrence of X appears on
the top of the parser stack
If the parser is top down we
perform “a” just before we
expand Y
Sometimes we cant do things as
easily as explained above
One example is when we are
parsing this SDT with a bottom-
1) L -> E n
2) E -> {print(‘+’);} E1 + T
3) E -> T
4) T -> {print(‘*’);} T1 * F
5) T -> F
6) F -> (E)
7) F -> digit {print(digit.lexval);}

SDT’s with actions inside
productions (cont)
 Any SDT can be
implemented as follows
1. Ignore the actions and
produce a parse tree
2. Examine each interior
node N and add actions
as new children at the
correct position
3. Perform a postorder
traversal and execute
actions when their nodes
are visited
L
E
+E
{print(‘+’);}
T
F
digit
{print(4);}
T
T F*
digit
{print(5);}
F
digit
{print(3);}
{print(‘*’);}

SDT’s for L-Attributed definitions
We can convert an L-attributed SDD into an SDT
using following two rules:
Embed the action that computes the inherited
attributes for a nonterminal A immediately before that
occurrence of A. if several inherited attributes of A are
dpendent on one another in an acyclic fashion, order
them so that those needed first are computed first
Place the action of a synthesized attribute for the head
of a production at the end of the body of the production

Example
S -> while (C) S1 L1=new();
L2=new();
S1.next=L1;
C.false=S.next;
C.true=L2;
S.code=label||L1||C.code||label||L2||S1.code
S -> while ( {L1=new();L2=new();C.false=S.next;C.true=L2;}
C) {S1.next=L1;}
S1{S.code=label||L1||C.code||label||L2||S1.code;}

Readings
Chapter 5 of the book

Chapter 5 Syntax Directed Translation

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