12-Syntax Directed Definition – Evaluation Order-09-06-2023.ppt

2
Syntax-Directed Translation
Syntax-directed translation refers to a method of compiler implementation where the source
language translation is completely driven by the parser. A common method of syntax-directed
translation is translating a string into a sequence of actions by attaching one such action to each
rule of a grammar.
1. We associate information with the programming language constructs by attaching attributes
to grammar symbols.
2. Values of these attributes are evaluated by the semantic rules associated with the
production rules.
3. Evaluation of these semantic rules:
– may generate intermediate codes
– may put information into the symbol table
– may perform type checking
– may issue error messages
– may perform some other activities
– in fact, they may perform almost any activities.
4. An attribute may hold almost any thing.
– a string, a number, a memory location, a complex record.

3
Syntax-Directed Definitions and Translation Schemes
When we associate semantic rules with productions, we use two notations:
– Syntax-Directed Definitions
– Translation Schemes
A. Syntax-Directed Definitions:
– give high-level specifications for translations
– hide many implementation details such as order of evaluation of semantic actions.
– We associate a production rule with a set of semantic actions, and we do not say when they
will be evaluated.
B. Translation Schemes:
– indicate the order of evaluation of semantic actions associated with a production rule.
– In other words, translation schemes give a little bit information about implementation
details.

4
Syntax-Directed Translation
• Conceptually with both the syntax directed definition and translation
scheme we
– Parse the input token stream
– Build the parse tree
– Traverse the tree to evaluate the semantic rules at the parse tree nodes.
Input string parse tree dependency graph evaluation order for
semantic rules
Conceptual view of syntax directed translation

5
Syntax-Directed Definitions
1. A syntax-directed definition is a generalization of a context-free
grammar in which:
– Each grammar symbol is associated with a set of attributes.
– This set of attributes for a grammar symbol is partitioned into two subsets called
• synthesized and
• inherited attributes of that grammar symbol.
– Each production rule is associated with a set of semantic rules.
2. The value of an attribute at a parse tree node is defined by the semantic rule
associated with a production at that node.
3. The value of a synthesized attribute at a node is computed from the values of
attributes at the children in that node of the parse tree
4. The value of an inherited attribute at a node is computed from the values of
attributes at the siblings and parent of that node of the parse tree

9
Syntax-Directed Definitions
Examples:
Synthesized attribute : E→E1+E2 { E.val =E1.val + E2.val}
Inherited attribute :A→XYZ {Y.val = 2 * A.val}
1. Semantic rules set up dependencies between attributes which can be
represented by a dependency graph.
2. This dependency graph determines the evaluation order of these
semantic rules.
3. Evaluation of a semantic rule defines the value of an attribute. But a
semantic rule may also have some side effects such as printing a value.

10
Annotated Parse Tree
• A parse tree showing the values of attributes at each node is called
an annotated parse tree.
• Values of Attributes in nodes of annotated parse-tree are either,
– initialized to constant values or by the lexical analyzer.
– determined by the semantic-rules.
• The process of computing the attributes values at the nodes is called
annotating (or decorating) of the parse tree.
• Of course, the order of these computations depends on the
dependency graph induced by the semantic rules.

11
Syntax-Directed Definition
In a syntax-directed definition, each production A→α is associated
with a set of semantic rules of the form:
b=f(c1,c2,…,cn)
where f is a function and b can be one of the followings:
 b is a synthesized attribute of A and c1,c2,…,cn are attributes of the
grammar symbols in the production ( A→α ).
OR
 b is an inherited attribute from one of the grammar symbols in α
(on the right side of the production), and c1,c2,…,cn are attributes of the
grammar symbols in the production ( A→α ).

12
Attribute Grammar
• So, a semantic rule b=f(c1,c2,…,cn) indicates that the attribute b
depends on attributes c1,c2,…,cn.
• In a syntax-directed definition, a semantic rule may just evaluate
a value of an attribute or it may have some side effects such as
printing values.
• An attribute grammar is a syntax-directed definition in which the
functions in the semantic rules cannot have side effects (they can only
evaluate values of attributes).

13
Syntax-Directed Definition -- Example
Production Semantic Rules
L → E n print(E.val)
E → E1 + T E.val = E1.val + T.val
E → T E.val = T.val
T → T1 * F T.val = T1.val * F.val
T → F T.val = F.val
F → ( E ) F.val = E.val
F → digit F.val = digit.lexval
1. Symbols E, T, and F are associated with a synthesized attribute val.
2. The token digit has a synthesized attribute lexval (it is assumed that it is evaluated by
the lexical analyzer).
3. Terminals are assumed to have synthesized attributes only. Values for attributes of
terminals are usually supplied by the lexical analyzer.
4. The start symbol does not have any inherited attribute unless otherwise stated.

14
S-attributed definition
• A syntax directed translation that uses synthesized attributes exclusively
is said to be a S-attributed definition.
• A parse tree for a S-attributed definition can be annotated by evaluating
the semantic rules for the attributes at each node, bottom up from leaves
to the root.

15
Annotated Parse Tree -- Example
Input: 5+3*4 L
E.val=17 n
E.val=5 + T.val=12
T.val=5 T.val=3 * F.val=4
F.val=5 F.val=3 digit.lexval=4
digit.lexval=5 digit.lexval=3

16
Dependency Graph
Input: 5+3*4 L
E.val=17 n
E.val=5 + T.val=12
T.val=5 T.val=3 * F.val=4
F.val=5 F.val=3 digit.lexval=4
digit.lexval=5 digit.lexval=3

17
Inherited attributes
• An inherited value at a node in a parse tree is defined in terms of
attributes at the parent and/or siblings of the node.
• Convenient way for expressing the dependency of a programming
language construct on the context in which it appears.
• We can use inherited attributes to keep track of whether an identifier
appears on the left or right side of an assignment to decide whether the
address or value of the assignment is needed.
• Example: The inherited attribute distributes type information to the
various identifiers in a declaration.

18
Syntax-Directed Definition – Inherited Attributes
D → T L L.in = T.type
T → int T.type = integer
T → real T.type = real
L → L1 , id L1.in = L.in, addtype(id.entry,L.in)
L → id addtype(id.entry,L.in)
1. Symbol T is associated with a synthesized attribute type.
2. Symbol L is associated with an inherited attribute in.

19
Annotated parse tree
Input: real p,q,r annotated parse tree
parse tree D
D
T L T.type=real L1.in=real
real L , id3 real L1.in=real , id3
L , id2 L1.in=real , id2
id1 id1

Dependency Graph
• Directed Graph
• Shows interdependencies between attributes.
• If an attribute b at a node depends on an attribute c, then the semantic rule for b at that
node must be evaluated after the semantic rule that defines c.
• Construction:
– Put each semantic rule into the form b=f(c1,…,ck) by introducing dummy
synthesized attribute b for every semantic rule that consists of a procedure call.
– E.g.,
• L  E n print(E.val)
• Becomes: dummy = print(E.val)
– The graph has a node for each attribute and an edge to the node for b from the
node for c if attribute b depends on attribute c.
20

Dependency Graph Construction
for each node n in the parse tree do
for each attribute a of the grammar symbol at n do
construct a node in the dependency graph for a
for each node n in the parse tree do
for each semantic rule b = f(c1,…,cn)
associated with the production used at n do
for i= 1 to n do
construct an edge from
the node for ci to the node for b
21

Dependency Graph Construction
• Example
• Production Semantic Rule
E→E1 + E2 E.val = E1.val + E2.val
E . val
E1. val + E2 . Val
• E.val is synthesized from E1.val and E2.val
• The dotted lines represent the parse tree that is not part of the
dependency graph.
22

Dependency Graph
23
D → T L L.in = T.type
T → int T.type = integer
T → real T.type = real
L → L1 id L1.in = L.in,
addtype(id.entry,L.in)
L → id addtype(id.entry,L.in)

Evaluation Order
• A topological sort of a directed acyclic graph is any ordering
m1,m2…mk of the nodes of the graph such that edges go from nodes
earlier in the ordering to later nodes.
. i.e if there is an edge from mi to mj them mi appears before mj in the ordering
• Any topological sort of dependency graph gives a valid order for
evaluation of semantic rules associated with the nodes of the parse tree.
• The dependent attributes c1,c2….ck in b=f(c1,c2….ck ) must be available before f
is evaluated.
• Translation specified by Syntax Directed Definition
• Input string parse tree dependency graph evaluation order for
semantic rules
24

Evaluation Order
• a4=real;
• a5=a4;
• addtype(id3.entry,a5);
• a7=a5;
• a9=a7;
25

Evaluating Semantic Rules
• Parse Tree methods
– At compile time evaluation order obtained from the topological sort of dependency
graph.
– Fails if dependency graph has a cycle
• Rule Based Methods
– Semantic rules analyzed by hand or specialized tools at compiler construction
time
– Order of evaluation of attributes associated with a production is pre-determined at
compiler construction time
• Oblivious Methods
– Evaluation order is chosen without considering the semantic rules.
– Restricts the class of syntax directed definitions that can be implemented.
– If translation takes place during parsing order of evaluation is forced by parsing
method.
26

27
Syntax Trees
Syntax-Tree
– an intermediate representation of the compiler’s input.
– A condensed form of the parse tree.
– Syntax tree shows the syntactic structure of the program while
omitting irrelevant details.
– Operators and keywords are associated with the interior nodes.
– Chains of simple productions are collapsed.
Syntax directed translation can be based on syntax tree as well as
parse tree.

Syntax Tree-Examples
Expression:
+
5 *
3 4
• Leaves: identifiers or constants
• Internal nodes: labelled with
operations
• Children: of a node are its
operands
if B then S1 else S2
if - then - else
Statement:
• Node’s label indicates what kind
of a statement it is
• Children of a node correspond to
the components of the statement
28
B S1 S2

Constructing Syntax Tree for Expressions
• Each node can be implemented as a record with several fields.
• Operator node: one field identifies the operator (called label of the node) and
remaining fields contain pointers to operands.
• The nodes may also contain fields to hold the values (pointers to values) of
attributes attached to the nodes.
• Functions used to create nodes of syntax tree for expressions with binary
operator are given below.
– mknode(op,left,right)
– mkleaf(id,entry)
– mkleaf(num,val)
Each function returns a pointer to a newly created node.
29

Constructing Syntax Tree for Expressions-
Example: a-4+c
1. p1:=mkleaf(id,entrya);
2. p2:=mkleaf(num,4);
3. p3:=mknode(-,p1,p2)
4. p4:=mkleaf(id,entryc);
5. p5:= mknode(+,p3,p4);
• The tree is constructed bottom
up.
30
+
- id
id num 4
to entry for c
to entry for a

A syntax Directed Definition for Constructing
Syntax Tree
1. It uses underlying productions of the grammar to schedule the calls of
the functions mkleaf and mknode to construct the syntax tree
2. Employment of the synthesized attribute nptr (pointer) for E and T to
keep track of the pointers returned by the function calls.
PRODUCTION SEMANTIC RULE
E  E1 + T E.nptr = mknode(“+”,E1.nptr ,T.nptr)
E  E1 - T E.nptr = mknode(“-”,E1.nptr ,T.nptr)
E  T E.nptr = T.nptr
T  (E) T.nptr = E.nptr
T  id T.nptr = mkleaf(id, id.lexval)
T  num T.nptr = mkleaf(num, num.val)
31

Annotated parse tree depicting construction of
syntax tree for the expression a-4+c
32
E.nptr + T.nptr
E.nptr - T.nptr
T.nptr num
id
id
+
-
nu
m
id
id
E.nptr
Entry for a
Entry for c

33
S-Attributed Definitions
1. Syntax-directed definitions are used to specify syntax-directed translations.
2. To create a translator for an arbitrary syntax-directed definition can be difficult.
3. We would like to evaluate the semantic rules during parsing (i.e. in a single pass, we will parse
and we will also evaluate semantic rules during the parsing).
4. We will look at two sub-classes of the syntax-directed definitions:
– S-Attributed Definitions: only synthesized attributes used in the syntax-directed
definitions.
– All actions occur on the right hand side of the production.
– L-Attributed Definitions: in addition to synthesized attributes, we may also use inherited
attributes in a restricted fashion.
5. To implement S-Attributed Definitions and L-Attributed Definitions we can evaluate semantic
rules in a single pass during the parsing.
6. Implementations of S-attributed Definitions are a little bit easier than implementations of L-
Attributed Definitions

Bottom-Up Evaluation of S-Attributed Definitions
• A translator for an S-attributed definition can often be implemented with the
help of an LR parser.
• From an S-attributed definition the parser generator can construct a translator
that evaluates attributes as it parses the input.
• We put the values of the synthesized attributes of the grammar symbols a stack
that has extra fields to hold the values of attributes.
– The stack is implemented by a pair of arrays val & state
– If the ith state symbol is A the val[i] will hold the value of the attribute
associated with the parse tree node corresponding to this A.
34

• We evaluate the values of the attributes during reductions.
A  XYZ A.a=f(X.x,Y.y,Z.z) where all attributes are synthesized.
state val state val
top 
 top
• Synthesized attributes are evaluated before each reduction.
• Before XYZ is reduced to A, the value of Z.z is in val[top], that of Y.y in val[top-1]
and that of X.x in val[top-2].
• After reduction top is decremented by 2.
• If a symbol has no attribute the corresponding entry in the array is undefined.
35
Z
Y
X
.
Z.z
Y.y
X.x
.
A
.
A.a
.

36
L → E n print(val[top-1])
E → E1 + T val[ntop] = val[top-2] + val[top]
E → T
T → T1 * F val[ntop] = val[top-2] * val[top]
T → F
F → ( E ) val[ntop] = val[top-1]
F → digit
1. At each shift of digit, we also push digit.lexval into val-stack.
2. At all other shifts, we do not put anything into val-stack because other terminals do
not have attributes (but we increment the stack pointer for val-stack).

37
Bottom-Up Evaluation -- Example
• At each shift of digit, we also push digit.lexval into val-stack.
Input state val semantic rule
5+3*4n - -
+3*4n 5 5
+3*4n F 5 F → digit
+3*4n T 5 T → F
+3*4 n E 5 E → T
3*4n E+ 5-
*4 n E+3 5-3
*4n E+F 5-3 F → digit
*4n E+T 5-3 T → F
4n E+T* 5-3-
n E+T*4 5-3-4
n E+T*F 5-3-4 F → digit
n E+T 5-12 T → T1 * F
n E 17 E → E1 + T
En 17- L → E n
L 17

L-Attributed Definitions
• When translation takes place during parsing, order of evaluation is linked to the order in which
the nodes of a parse tree are created by parsing method.
• A natural order can be obtained by applying the procedure dfvisit to the root of a parse tree.
• We call this evaluation order depth first order.
• L-attributed definition is a class of syntax directed definition whose attributes can always be
evaluated in depth first order( L stands for left since attribute information flows from left to
right).
dfvisit(node n)
{
for each child m of n, from left to right
{
evaluate inherited attributes of m
dfvisit(m)
}
evaluate synthesized attributes of n
}

L-Attributed Definitions
A syntax-directed definition is L-attributed if each inherited attribute of Xj,
where 1≤j≤n, on the right side of A → X1X2...Xn depends only on
1. The attributes of the symbols X1,...,Xj-1 to the left of Xj in the
production
2. The inherited attribute of A
Every S-attributed definition is L-attributed, since the restrictions apply only to
the inherited attributes (not to synthesized attributes).

A Definition which is not L-Attributed
Productions Semantic Rules
A → L M L.in=l(A.i)
M.in=m(L.s)
A.s=f(M.s)
A → Q R R.in=r(A.in)
Q.in=q(R.s)
A.s=f(Q.s)
This syntax-directed definition is not L-attributed because the semantic rule Q.in=q(R.s)
violates the restrictions of L-attributed definitions.
• When Q.in must be evaluated before we enter to Q because it is an inherited attribute.
• But the value of Q.in depends on R.s which will be available after we return from R. So,
we are not be able to evaluate the value of Q.in before we enter to Q.

Translation Schemes
• In a syntax-directed definition, we do not say anything about the evaluation times of the
semantic rules (when the semantic rules associated with a production should be
evaluated).
• Translation schemes describe the order and timing of attribute computation.
• A translation scheme is a context-free grammar in which:
– attributes are associated with the grammar symbols and
– semantic actions enclosed between braces {} are inserted within the right sides of
productions.
Each semantic rule can only use the information compute by already executed semantic
rules.
• Ex: A → { ... } X { ... } Y { ... }
Semantic Actions

Translation Schemes for S-attributed Definitions
• useful notation for specifying translation during parsing.
• Can have both synthesized and inherited attributes.
• If our syntax-directed definition is S-attributed, the construction of the corresponding
translation scheme will be simple.
• Each associated semantic rule in a S-attributed syntax-directed definition will be inserted
as a semantic action into the end of the right side of the associated production.
Production Semantic Rule
E → E1 + T E.val = E1.val + T.val a production of a syntax directed
definition
⇓
E → E1 + T { E.val = E1.val + T.val } the production of the
corresponding translation scheme

A Translation Scheme Example
• A simple translation scheme that converts infix expressions to the
corresponding postfix expressions.
E → T R
R → + T { print(“+”) } R1
R → ε
T → id { print(id.name) }
a+b+c ab+c+
infix expression postfix expression

A Translation Scheme Example (cont.)
E
T R
id {print(“a”)} + T {print(“+”)} R
id {print(“b”)} + T {print(“+”)} R
id {print(“c”)} ε
The depth first traversal of the parse tree (executing the semantic actions in that order)
will produce the postfix representation of the infix expression.

Inherited Attributes in Translation Schemes
• If a translation scheme has to contain both synthesized and inherited attributes, we have
to observe the following rules to ensure that the attribute value is available when an
action refers to it.
1. An inherited attribute of a symbol on the right side of a production must be
computed in a semantic action before that symbol.
2. A semantic action must not refer to a synthesized attribute of a symbol to the right
of that semantic action.
3. A synthesized attribute for the non-terminal on the left can only be computed after
all attributes it references have been computed (we normally put this semantic action at
the end of the right side of the production).
• With a L-attributed syntax-directed definition, it is always possible to construct a
corresponding translation scheme which satisfies these three conditions (This may not
be possible for a general syntax-directed translation).

Inherited Attributes in Translation Schemes: Example
S →A1A2 {A1.in=1; A2.in=2}
A →a { print (A.in)}
S
A1 A2 {A1.in=1; A2.in=2}
a {print (A.in)} a {print (A.in)}

A Translation Scheme with Inherited Attributes
D → T {L.in = T.type } L
T → int { T.type = integer }
T → real { T.type = real }
L → {L1.in = L.in } L1, id {addtype(id.entry,L.in)}
L → id {addtype(id.entry,L.in)}
• This is a translation scheme for an L-attributed definitions

Bottom Up evaluation of Inherited Attributes
• Removing Embedding Semantic Actions
In bottom-up evaluation scheme, the semantic actions are evaluated during reductions.
• During the bottom-up evaluation of S-attributed definitions, we have a parallel stack to
hold synthesized attributes.
• Problem: where are we going to hold inherited attributes?
• A Solution:
– We will convert our grammar to an equivalent grammar to guarantee to the followings.
– All embedding semantic actions in our translation scheme will be moved into the
end of the production rules.
– All inherited attributes will be copied into the synthesized attributes (most of the
time synthesized attributes of new non-terminals).
– Thus we will be evaluate all semantic actions during reductions, and we find a
place to store an inherited attribute.

Removing Embedding Semantic Actions
• To transform our translation scheme into an equivalent translation
scheme:
1. Remove an embedding semantic action Si, put new a non-terminal Mi
instead of that semantic action.
2. Put that semantic action Si into the end of a new production rule Mi→ε
for that non-terminal Mi.
3. That semantic action Si will be evaluated when this new production
rule is reduced.

A→ {S1} X1 {S2} X2 ... {Sn} Xn
⇓
remove embedding semantic actions
A→ M1 X1 M2 X2 ... Mn Xn
M1→ε {S1}
M2→ε {S2}
.
.
Mn→ε {Sn}

E → T R
R → + T { print(“+”) } R
R → ε
⇓
remove embedding semantic actions
E → T R
R → + T M R
R → ε
M → ε { print(“+”)
print( + ) }

Inheriting attributes on parser stack
• A bottom up parser reduces the RHS of a production A→XY by removing X and Y
from the top of the stack and replacing them by A.
• Suppose X has a synthesized attribute X.s which is already in the stack.
• If the inherited attrtibute Y.i is defined by the copy rule X.s=Y.i, then the value of X.s
can where Y.i is called for.
• Copy rule plays an important role in the evaluation of inherited attributes during
bottom up parsing.
Productions Semantic Rules
D → T L
T → int val[ntop]=integer
T → real val[ntop]=real
L → L1, id addtype(val[top],val[top-3])
L → id addtype(val[top],val[top-1])

12-Syntax Directed Definition – Evaluation Order-09-06-2023.ppt

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