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Syntax-Directed Translation

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Source Program

Target Lang. Prog.

Semantic Analyzer

Syntax Analyzer

Lexical Analyzer

Front

End

Back End

Int. Code Generator

Intermediate Code

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Semantic Analyzer (Static Checker)

Catches all remaining errors

Semantic Analyzer checks the sourceprogram for semantic errors and gathers

type information for the subsequent codegeneration phase.

Compiler must do more than recognize

whether a sentence belongs to thelanguage

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Semantic Analyzer

Find remaining errors that would make program

invalid

undefined variables, types

type errors that can be caught statically

Figure out useful information for later phases types of all expressions

data layout

Static checks done by the compiler

Dynamic checks done at run time

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Semantic Analysis

Source code

Lexical Analysis

Syntactic Analysis

Semantic Analysis

lexical

errors

syntaxerrors

semantic

errors

tokens

Syntax / Parse Tree

Annotated Parse Tree

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Semantic Analyzer

The semantic analyzer completes the symbol table with

information on the characteristics of each identifier.

In symbol table one entry is created for each identifier and

constant.

Scope is taken into account. Two different variables with the same name will have

different entries in the symbol table.

Nodes in the Syntax Tree containing identifiers or constants

really contain pointers into the symbol table.

The semantic analyzer completes the table using

information from declarations and perhaps the context

where the identifiers occur.

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Kinds of Checks Uniqueness checks

Certain names must be unique ( a var. is defined exactly once)

Many languages require variable declarations

Flow-of-control checks Match control-flow operators with structures

Example: break applies to innermost loop/switch or break is used

within a scope whose closing statement is missing.

Type checks Check compatibility of operators and operands

Name-related checks (var. that is defined previously isused)

Logical checks (Not done by Semantic Analyzer) Program is syntactically and semantically correct, but does not do the

correct thing

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Semantic Analyzer (Cont.)

Example of semantic errors that are tackled

statically: Undeclared identifiers Unreachable statements Identifiers used in the wrong context Multiply declared identifier Index out of bounds Wrong number or types of args to call Incompatible types for operation Break statement outside switch/loop Goto with no label Methods called with the wrong number of

parameters or with parameters of the wrong type. ...

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Semantic Analyzer v/s Syntax Analyzer

There can be overlapping of Syntax and SemanticAnalysis phase, by evaluating semantic rules duringparsing, without explicitly constructing a parse treeor a graph showing dependencies betweenattributes.

This is done generally in single passimplementation of a compiler and this is importantcompile time efficiency.

There are two notations for associating semanticrules with productions of CFG: Syntax Directed Definitions

Translation Schemes

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Semantic Analyzer

Semantic Analysis is performed in a syntax-directedfashion by augmenting the Context Free Grammar with

with each grammar symbol it associates a set ofattributes, e.g) type of construct or value ofconstruct

With each production, a set of semantic rules andthe parsing process by evaluation of attributesusing semantic rules and the parsing process byevaluation of attributes.

There are two schemes for this:

Sattributed scheme

Lattributed scheme

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Introduction

Here we will learn: translation of languages guided by

context-free grammar.

A Syntax Directed Definition uses a context free grammar tospecify the syntactic structure of input:

With each grammar symbol, it associates a set of attributes;e.g.) Type of construct, value of the construct, Numbers,types, table references, or strings etc.

And with each production, a set of semantic rules forcomputing values of the attributes associated with the

symbols appearing in that production.

The grammar and the set of semantic rules constitute theSyntax Directed Definition .

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Syntax Directed Definitions The output for each input x is specified in the following manner:

Construct a Parse tree for x. Suppose a node n in the parse tree is labeled by the

grammar symbol X.

We write X.a to denote the value of attribute a of X at that

node. The value of X.a at n is computed using the semantic rule for

attribute a associated with the production X used at thatparticular node n in the parse tree.

A parse tree showing the attribute values at each node iscalled anAnnotated 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.

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Syntax Directed Definitions

A SDD is a generalization of a CFG in whicheach grammar symbol has an associated set ofattributes partitioned into two sets:

Synthesized Attributes

Inherited Attributes

If we think of a node for the grammar symbol in aparse tree as a record with fields for holding

information, then an attribute corresponds to thename of a field.

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Form of a Syntax Directed Definition

In a SDD, each grammar production A is associated with it

a set of semantic rules of the form:

b := f ( c1, c2, .ck)

where f is a function and either:

b is a Synthesized Attribute of A, and c1, c2, .ck are

attributes belonging to the grammar symbols of theproductions, or

b is an inherited attribute of one of the grammar symbolson the right side of production A , c1, c2, .ck are

attributes belonging to grammar symbols of production A .

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Inherited & Synthesized AttributeLet X0 X1 ... Xn be a production

If the computing rule of X0s attribute is of theform A(X0) = f ( A(X1), ... , A(Xn) )

Synthesized attribute

If the computing rule of Xjs attribute is of theform A(Xj) = f ( A(X0), ..., A(Xi), )

Inherited attribute

Terminals have intrinsic attributes Lexical values supplied by the lexical

analyzer

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Synthesized Attributes

Attributes are values attached to the nodes of aparse tree.

The identifiers $$, $1, $2, etc., in Yacc actions aresynthesized attributes.

The value of a synthesized attribute at a node iscomputed from the values of attributes at thechildren of that node in parse tree.

Synthesized attributes are values that can be

computed bottom-up in the parse tree. Synthesized attributes can be easily computed by

a shift-reduce parser that keeps the values of theattributes on the parsing stack.

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Attribute Grammars Example Each grammar symbols has

a set ofattributes E.g. the value of E1 is the

attribute E1.val

Each grammar rule has a set

of rules over the symbolattributes

Copy rules

Semantic Function rules

E.g. sum, quotient

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Evaluating an SDD at the Nodes of a Parse Tree Example :

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S-Attributed Definitions

An SDD is S-attributedif every attribute is synthesized.

Example shown in the previous slide is an example of S-attributed definition.

When an SDD is S-attributed, we can evaluate its attributes inany bottom-up order of the nodes of the parse tree.

postorder(N){

for ( each child CofN, from the left )postorder(C);

evaluate the attributes associated with nodeN;

}

S-attributed definitions can be implemented during bottom-upparsing, since a bottom-up parse corresponds to a postordertraversal.

An S-attributed SDD can be implemented naturally in

conjunction with an LR parser.

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Inherited Attributes and Bottom Up TranslationSome inherited attributes might not be available when we are

reducing by a certain production.Consider the translation scheme:

PRODUCTION SEMANTIC RULE

D p T L L.in = T.typeT p int T.type = integer

T p real T.type = real

Lp L1 , id L1.in = L.in

addtype(id.entry, L.in)

Lp id addtype(id.entry, L.in)

Attempt top down parsing overreal id1, id2 , id3

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Example with Inherited Attributes Even though inherited attrib can be simulated by synthesized it is

more natural to write Syntax-Directed Definitions using inherited.

below in is an inherited attribute ofL

PRODUCTION SEMANTIC RULE

D p T L L.in=

T.typeT p int T.type =integer

T p real T.type =real

L p L1 , id L1.in = L.in

addtype(id.entry, L.in)L p id addtype(id.entry, L.in)

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Example

Example real id1, id2 , id3

identry=id2

id

entry=id3

,

L

in=real

T

type=real

D

real

id

entry=id1

,

L

in=real

L

in=real

addtype(id1,real)

addtype(id2,real)

addtype(id3,real)

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L-Attributed Definitions A syntax-directed definition is called L-Attributed

if In the semantic rules of a production

Ap X1Xnthe inherited attributes of Xj depend only on

The attributes (synthesized or inherited) of the symbolsX1Xj-1 The inherited attributes of A

All S-attributed Definitions are also L-attributed.

S-attributed schemes do not permit inherited attributes andare too restrictive.

It is often more natural to use syntax directed definitions withinherited attributes.

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L-Attributed Definitions An SDD is L-attributed Between the attributes associated with a production body,

SDG edges can go from left to right, but not from right to left

(hence L-attributed). Each attribute must be either

Synthesized, or

Inherited, but with the rules limited as follows. Suppose that

there is a productionA pX1X2 Xn, and that there is aninherited attributeXi.a computed by a rule associated withthis production. Then the rule may use only:

Inherited attributes associated with the headA.

Either inherited or synthesized attributes associated withthe occurrence of symbolsX1X2 Xi-1 located to the leftofXi.

Inherited or synthesized attributes associated with thisoccurrence ofXi itself, but only in such a way that there

are no cycles in a DG formed by the attributes of thisXi.

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L-Attributed

Definitions

Example:

The SDD in Fig. is L-attributed.

Example:Any SDD containing the following production and rules cannotbe L-attributed.

A p B C A.s =B.b;

B.i = f(C.c,A.s)

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Evaluating an SDD at the Nodes of a Parse Tree

Imagine that the rules of an SDD are applied by first

constructing a parse tree and then using the rules to evaluateall of the attributes at each of the nodes of the parse tree(annotated parse tree).

How do we construct an annotated parse tree?

In what order do we evaluate attributes? Before we can evaluate an attribute at a node of a parsetree, we must evaluate all the attributes upon which itsvalue depends.

With synthesized attributes, we can evaluate attributes in any

bottom-up order, such as that of a postorder traversal of theparse tree.

For SDD with both inherited and synthesized attributes, thereis no guarantee that there is even one order in which toevaluate attributes at nodes.

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Evaluating an SDD at the Nodes of a Parse Tree Example:

Fig. 5.4: An SDD based on a grammar

suitable for top-down parsing.

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Conceptual View of SDT

Conceptually with both syntax directed definitions and

translation schemes, input token stream is parsed, theparse tree is build, and then the tree is traversed asneeded to evaluate the semantic rules at the parse treenodes.

Inputstring Parsetree

Dependencygraph

Evaluationorder forsemantic rules

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Dependency Graphs

Dependency graphs are a useful tool fordetermining an evaluation order for the attributeinstances in a given parse tree.

A dependency graph (DG) depicts the flow of

information among the attribute instances in aparticular parse tree; an edge from one attributeinstance to another means that the value of thefirst is needed to compute the second.

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Dependency Graphs

Edges express constraints implied by the semantic

rules. For each parse tree node, say a node labeled by grammar

symbolX, the DG has a node for each attribute associatedwithX.

Suppose that a semantic rule associated with a productionp defines the value of synthesized attributeA.b in terms ofthe value ofX.c. Then, the DG has an edge fromX.ctoA.b.

Suppose that a semantic rule associated with a productionp defines the value of inherited attribute B.cin terms of thevalue ofX.a. Then, the DG has an edge fromX.a to B.c.

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Example:

Ep E1 + T E.val = E1.val+ T.val

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Dependency Graphs Example 5.5:

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Applications of Syntax-Directed Translation

Construction of syntax trees

Two SDDs for constructing syntax trees forexpressions

1.An S-attribute definition, suitable for use

during bottom-up parsing2.L-attributed, suitable for use during top-down

parsing

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Translation Scheme A translation scheme is a Context Free Grammar in which

program fragments called semantic actions are embeddedwithin the right sides of productions

A Translation scheme is like a SDD, except that the order of

evaluation of semantic rules is explicitly shown.

The position at which an action is to be executed is shown byenclosing it between braces and writing it within the right side of

a production:

e.g.) rest + term { print(+) } rest_1

rest

+ term { print ( + ) } rest_1

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Translation Scheme A Translation Scheme generates an output for each

sentence x generated by the underlying grammar byexecuting the actions in the order they appear during adepthfirst traversal of a parse tree of x.

In figure below the action { print (+) } will be performed

after the subtree for term is traversed but before child forrest_1 is visited.

rest

+ term { print ( + ) } rest_1

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Syntax-Directed Translation Schemes

Syntax-directed translation schemes are a complementary

notation to SDDs. All of the applications of SDDs discussed in previous slides

can be implemented using SDT schemes.

Typically, SDTs are implemented during parsing, withoutbuilding a parse tree.

We focus on the use of SDTs to implement two important

classes of SDDs: The underlying grammar is LR-parsable, and the SDD is

S-attributed.

The underlying grammar is LL-parsable, and the SDD is L-attributed.

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Comparison A syntax-directed translation scheme embeds program

fragments called semantic actions within productionbodies, as in

Ep E1 + T {print +}

SDDs can be more readable, and hence more useful forspecifications.

SD translation scheme can be more efficient, thus moreuseful for implementations.

L-attributed translations

S-attributed translations

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Postfix Translation Schemes SDTs with all actions at the right ends of the production

bodies are calledpostfix

SD

Ts. Example :

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SDTs With Actions Inside Productions

An action may be placed at any position within the body of

a production. It is performed immediately after all symbols to its left are

processed.

Example: desk calculator running example into an SDTthat prints the prefix form of an expression.

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SDTs With Actions Inside Productions

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Eliminating Left Recursion from SDTs When transforming the grammar, treat the actions as if they

were terminal symbols.

Example 5.17:

A pAE | FA p FR

R p ER | I

EpE1 + T{print(+);}

Ep T

Ep TR

R p + T{print(+);}R

Rp I

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Example

exprp expr+ term {print(+) }exprp expr- term {print(-) }exprp termterm p 0 {print(0) }term p 9 {print(9) }

Show SDT for the string w=

5 + 4 + 6

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Translation eg 2

Side-effects and Translation Schemes.

Do the construction as before but:

Side-effect in front of a symbol will be executed in astate when we make the move following that symbol

to another state. Side-effects on the rightmost end are executed during

reduce actions.

E p EE p E + E {print(+)}

| E * E {print(*)}

| {parenthesis++} ( E ) {parenthesis--}

| id { print(id); print(parenthesis); }

Do for example id*(id+id)$

side-effects

attachedtothesymbols

totherightofthem.