Syntax directed translation

Syntax-directed translation
Syntax-directed translation (SDT) refers to a method of
compiler implementation where the source language
translation is completely driven by the parser.
The parsing process and parse trees are used to direct
semantic analysis and the translation of the source
program.
We can augment grammar with information to control
the semantic analysis and translation. Such grammars are
called attribute grammars.
2

Associate attributes with each grammar symbol that
describes its properties.
An attribute has a name and an associated value.
With each production in a grammar, give semantic rules
or actions.
The general approach to syntax-directed translation is to
construct a parse tree or syntax tree and compute the
values of attributes at the nodes of the tree by visiting them
in some order.
3

There are two ways to represent the semantic rules
associated with grammar symbols.
Syntax-Directed Definitions (SDD)
Syntax-Directed Translation Schemes (SDT)
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Syntax-Directed Definitions
A syntax-directed definition (SDD) is a context-free
grammar together with attributes and rules.
Attributes are associated with grammar symbols and rules
are associated with productions.
PRODUCTION SEMANTIC RULE
E → E1 + T E.code = E1.code || T.code || ‘+’
5

Syntax-Directed Definitions
SDDs are highly readable and give high-level
specifications for translations.
But they hide many implementation details.
For example, they do not specify order of evaluation of
semantic actions.
Syntax-Directed Translation Schemes (SDT) embeds
program fragments called semantic actions within
production bodies
SDTs are more efficient than SDDs as they indicate the
order of evaluation of semantic actions associated with a
production rule.
6

Inherited Attributes
An INHERITED ATTRIBUTE for a non-terminal B
at a parse-tree node N is defined by a semantic rule
associated with the production at the parent of N. The
production must have B as a symbol in its body.
An inherited attribute at node N is defined only in
terms of attribute values at N's parent, N itself, and N's
siblings.
7

Synthesized Attributes
Each of the non-terminals has a single synthesized attribute, called val.
An SDD that involves only synthesized attributes is called S-attributed
Each rule computes an attribute for the non-terminal at the head of a
production from attributes taken from the body of the production.
8

Evaluating an SDD at the Nodes of a Parse Tree
A parse tree, showing the value(s) of its attribute(s)
is called an annotated parse tree.
With synthesized attributes, evaluate attributes in
bottom-up order.
9

Annotated Parse Tree for 3*5+4n10

give annotated parse trees for the following
expressions:
11

SDD for expression grammar with
inherited attributes
14

Annotated parse-tree for the input
real id1, id2, id3
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SDD for expression grammar with
inherited attributes
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Annotated Parse Tree for 3*5 18

1) L -> En L.val = E.val
2) E -> TE' E'.inh = T.val
E.val = E'.syn
3) E' -> +TE_1' E_1'.inh = E'.inh + T.val
E'.syn = E_1'.syn
4) E' -> ε E'.syn = E'.inh
5) T -> FT' T'.inh = F.val
T.val = T'.syn
6) T' -> *FT_1' T_1'.inh = T'.inh * F.val
T'.syn = T_1'.syn
7) T' -> ε T'.syn = T'.inh
8) F -> (E) F.val = E.val
9) F -> digit F.val = digit.lexval
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SDD with circular dependencies no guarantee in the
order of evaluation.
22

Evaluation Orders for SDD's
"Dependency graphs“ tool for determining an
evaluation order for the attribute instances in a given
parse tree.
annotated parse tree shows the values of attributes,
a dependency graph helps to determine how those
values can be computed.
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Dependency graphs
Edges express constraints implied by the semantic
rules.
Each attribute is associated to a node
If a semantic rule associated with a production p
defines the value of synthesized attribute A.b in terms
of the value of X.c, then graph has an edge from X.c to
A.b
If a semantic rule associated with a production p
defines the value of inherited attribute B.c in terms of
value of X.a, then graph has an edge from X.a to B.c
24

Topological Sort
 A dependency graph characterizes the possible order in
which we can evaluate the attributes at various nodes of a
parse tree.
 If there is an edge from node M to N, then attribute
corresponding to M first be evaluated before evaluating
N.
 Thus the allowable orders of evaluation are N1, N2, ...,
Nk such that if there is an edge from Ni to Nj then i < j.
 Such an ordering embeds a directed graph into a linear
order, and is called a topological sort of the graph.
 If there is any cycle in the graph, then there are no
topological sorts.
27

S-Attributed
If every attribute is synthesized.
S-attributed SDD can be evaluated in bottom up order of
the nodes of the parse tree.
28

L-attributed definitions
 Synthesized, or
 Inherited, but with the rules limited as follows .
Suppose that there is a production A -> XIX2 · ..
Xn , and that there is an inherited attribute Xi·a
computed by a rule associated with this
production. Then the rule may use only:
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L-attributed definitions
Inherited attributes associated with the head A.
Either inherited or synthesized attributes
associated with the occurrences of symbols Xl ,
X2 , ••• ,Xi-l located to the left of Xi.
Inherited or synthesized attributes associated
with this occurrence of Xi itself, but only in
such a way that there are no cycles in a
dependency graph formed by the attributes of
this Xi .
30

Semantic Rules with Controlled
Side Effects
Permit incidental side effects that do not disturb
attribute evaluation.
Impose restriction on allowable evaluation orders,
so that the same translation is produced for any
allowable order.
31

Applications of Syntax-Directed
Translations
 Construction of Syntax Trees
 Syntax trees are useful for representing programming language constructs like
expressions and statements.
 Each node of a syntax tree represents a construct; the children of the node
represent the meaningful components of the construct.
 e.g. a syntax-tree node representing an expression E1 + E2 has label + and two
children representing the sub expressions E1 and E2
32

Applications of Syntax-
Directed Translations
 Each node is implemented by objects with suitable number of fields; each
object will have an op field that is the label of the node with additional fields
as follows:
 If the node is a leaf, an additional field holds the lexical value for the leaf . This is
created by function Leaf(op, val)
 If the node is an interior node, there are as many fields as the node has children in
the syntax tree. This is created by function Node(op, c1, 2,...,ck) .
33

Syntax Directed Translation
Schemes
 SDT can be implemented by first building a parse
tree and then performing the actions in a left-to-
right depth-first order
 Postfix Translation Schemes:each semantic action
can be placed at the end of production and
executed along with the reduction of body to the
head of the production.
36

Parser-Stack Implementation
of Postfix SDTs
38

Semantic Actions during
Parsing
 when shifting
 push the value of the terminal on the semantic
stack
 when reducing
 pop k values from the semantic stack, where k is
the number of symbols on production’s RHS
 push the production’s value on the semantic stack
39

SDTs with Actions inside
Productions
 Action can be placed at any position in the production body.
 Action is performed immediately after all symbols left to it are
processed.
 Given B —> X { a } Y , an action a is done after
 we have recognized X (if X is a terminal), or
 all terminals derived from X (if X is a nonterminal).
40

SDTs with Actions inside
Productions
 If bottom-up parser is used, then action a is performed as soon as X appears on
top of the stack.
 If top-down parser is used, then action a is performed
 just before Y is expanded (if Y is nonterminal), or
 check Y on input (if Y is a terminal).
 Any SDT can be implemented as follows:
 Ignoring actions, parse input and produce parse tree.
 Add additional children to node N for action in α, where A → α.
 Perform preorder traversal of the tree, and as soon as a node labeled by an action is
visited, perform that action.
41

Semantic Actions during
Parsing
42

Type Checking?
 Type checking is the process of verifying that each operation
executed in a program respects the type system of the
language.
 This generally means that all operands in any
expression are of appropriate types and number.
 Much of what we do in the semantic analysis phase is type
checking.
44

type system
 The compiler must then determine that these type
expressions conform to a collection of logic al
rules that is called the type system for the source
language
47

Static and Dynamic
checking
52

Syntax directed translation

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