Context Sensitive Analysis and Attribute Grammar - Compiler Design - Dr. D. P. Sharma - NIT Surathkal by wahid311

8/7/2019 Context Sensitive Analysis and Attribute Grammar - Compiler Design - Dr. D. P. Sharma - NIT Surathkal by wahid311

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ContextContext

SensitiveSensitiveAnalysisAnalysis

ContextContext

SensitiveSensitiveAnalysisAnalysis


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Beyond SyntaxBeyond SyntaxBeyond SyntaxBeyond Syntaxvoid fie(int a, int b)

{ .... }

void fee() {

int f[3],g[1],h,i,j,k;char* p;

fie(h, i, ab);

k = f*i+j;

h = g[17];

p = 10;

}

what is wrong withthis program?


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{ .... }

void fee() {


fie(h, i, ab);

k = f*i+j;

h = g[17];

p = 10;

}

dimension ofg is 1,

index is 17


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{ .... }

void fee() {


fie(h, i, ab);

k = f*i+j;

h = g[17];

p = 10;

}

wrong numberofargs to function fie


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{ .... }

void fee() {


fie(h, i, ab);

k = f*i+j;

h = g[17];

p = 10;

}

f is an array;used withoutindex


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{ .... }

void fee() {


fie(h, i, ab);

k = f*i+j;

h = g[17];

p = 10;

}

10 is not a

characterstring


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Beyond SyntaxBeyond SyntaxBeyond SyntaxBeyond Syntax

To generate code, the

compiler

needs to answ

er

many questions


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Is x a scaler, an array or afunction?

Is x declared before it isused?

Is the expression x*y+ztype-consistent?


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Beyond SyntaxBeyond SyntaxBeyond SyntaxBeyond Syntax In a[i,j,k] does a have

three dimensions?

Does *preference theresult of new?

Do p and qrefer to thesame memory location?


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three dimensions?




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three dimensions?




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These questions are part ofcontext-sensitive analysis

Answers depend on values,not parts of speech

Answers may involvecomputation


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How can we answer thesequestions?

Use formal methods Context-sensitive

grammars Attribute grammars


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Use ad-hoc techniques

Symbol tables

ad-hoc code


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Attribute GrammarsAttribute GrammarsAttribute GrammarsAttribute Grammars A CFG is augmented with a

set of rules

Each symbol in the derivationhas a set of values orattributes

Rules specify how to computea value for each attribute


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set of rules




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set of rules




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Attribute GrammarsAttribute GrammarsAttribute GrammarsAttribute Grammars

grammar forsigned binary

numbers (SBN)

Number Sign List

Sign +

List List Bit | BitBit 01


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derivation for 1

Number Sign List List Bit

1


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For -101Number Sign List

Sign List Bit

Sign List1 Sign List Bit1 Sign List0 1

Sign Bit0 1 Sign 1 0 1 1 0 1


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Foran attributed version ofSBN,the following attributes are needed

Symbol AttributesNumber val

Sign neg

List pos, val

Bit pos, val


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We will add rules tocompute decimal value of a

signed binary number


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Productions Attribution Rules

NumberSign List List.posn 0

ifSign.neg thenNumber.valn List.valelse Number.valn List.val

Sign + Sign.negn false

Sign Sign.negn true


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Productions Attribution Rules

NumberSign List List.posn 0

ifSign.neg thenNumber.valn List.valelse Number.valn List.val

Sign + Sign.negn false

Sign Sign.negn true


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Attribute GrammarsAttribute GrammarsAttribute GrammarsAttribute GrammarsProductions Attribution Rules

List0 List1 Bit List1.posn List0.pos + 1Bit.posn List0.posList0.valn List1.val + Bit.val

ListBit Bit.posn List.pos

List.valn Bit.val

Bit 0 Bit.valn 0

Bit 1 Bit.valn 2Bit.pos


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Attributes are associated withnodes in parse tree

Rules are value assignmentsassociated with productions


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Attributes are associated withnodes in parse tree

Rules are value assignmentsassociated with productions


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Rules and parse tree definean attribute dependence

gr

aph Graph must be acyclic


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Number

Sign List

Bit

1

List.pos n 0List.valn Bit.val =1

Number.valn List.val =1

Bit.pos n 0

Bit.valn 2Bit.pos =1

Sign.neg

n true


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AttributesAttributesAttributesAttributes

Attributes are distinguishedbased on the direction ofvalueflow

1. Synthesized attributes

2. Inherited attributes


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Synthesized AttributesSynthesized AttributesSynthesized AttributesSynthesized Attributes

Attributes of a node whose

values are defined wholly in

terms of attributes ofnodeschildren and from constants

are called synthesized

attributes


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Synthesized AttributesSynthesized AttributesSynthesized AttributesSynthesized Attributes

Values used to compute

synthesized attributes flow

bottom-up in the parse tree

Good match to LR parsing


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Inherited AttributesInherited AttributesInherited AttributesInherited Attributes

Attributes whose values aredefined in terms of

nodes own attributes,

nodes siblings

nodes parent

Values flow top-down andlaterally in the parse tree


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Inherited AttributesInherited AttributesInherited AttributesInherited Attributes

Attributes whose values aredefined in terms of

nodes own attributes,

nodes siblings

nodes parent

Values flowtop-down andlaterally in the parse tree


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Number

SignList

Bit

1

List

List

Bit

1Bit

0

Parse tree for101


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Number

SignList

Bit

1

val: -5

List

List

Bit

1Bit

0

attributed tree

pos: 0val: 5

pos: 0val: 1

pos: 1

val: 4

pos: 1val: 0

pos: 2val: 4

pos: 2val: 4

neg: true


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Number

SignList

Bit

1

val: -5

List

List

Bit

1Bit

0

Inherited Attributes

pos: 0val: 5

pos: 0val: 1

pos: 1

val: 4

pos: 1val: 0

pos: 2val: 4

pos: 2val: 4

neg: true


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Number

SignList

Bit

1

val: -5

List

List

Bit

1Bit

0

Synthesized Attributes

pos: 0

val: 5

pos: 0

val: 1

pos: 1

val: 4

pos: 1

val: 0

pos: 2

val: 4

pos: 2

val: 4

neg: true


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Number

SignList

Bit

1

val: -5

List

List

Bit

1Bit

0

Together

pos: 0

val: 5

pos: 0

val: 1

pos: 1

val: 4

pos: 1

val: 0

pos: 2

val: 4

pos: 2

val: 4

neg: true


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Number

SignList

Bit

1

val: -5

List

List

Bit

1Bit

0

peel away parse tree

pos: 0

val: 5

pos: 0

val: 1

pos: 1

val: 4

pos: 1

val: 0

pos: 2

val: 4

pos: 2

val: 4

neg: true


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1

val: -5

1

0

dependence graph

pos: 0

val: 5

pos: 0

val: 1

pos: 1

val: 4

pos: 1

val: 0

pos: 2

val: 4

pos: 2

val: 4

neg: true


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1

val: -5

1

0

dependence graph

pos: 0

val: 5

pos: 0

val: 1

pos: 1

val: 4

pos: 1

val: 0

pos: 2

val: 4

pos: 2

val: 4

neg: true

must be acyclic!


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Evaluation MethodsEvaluation MethodsEvaluation MethodsEvaluation Methods

Dynamic methods

Build the parse tree

Build the dependence graph

Topological sort the graph

Define attributes in topologicalorder


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Dynamic methods






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Dynamic methods






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Dynamic methods






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Dynamic methods






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Rule-based (treewalk)

Analyze attribute rules at

compiler-generation time Determine a fixed (static)

ordering

Evaluate nodes in that order


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Oblivious (passes, dataflow)

Ignore rules and parse tree

Pick a convenient order (atdesign time) and use it


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ProblemsProblemsProblemsProblems

Attribute grammars havenot achieved widespread

use due to a myriad ofproblems


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non-local computation

traversing parse tree

storage management forshort-lived attributes

lack of high-qualityinexpensive tools


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AdAd--Hoc AnalysisHoc AnalysisAdAd--Hoc AnalysisHoc Analysis

In rule-based evaluators, asequence ofactions are

associatedw

ith gr

ammar

productions


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Organizing actions requiredfor context-sensitive analysis

ar

ound str

uctur

e of thegrammar leads to powerful,

albeit ad-hoc, approach which

is used on most parsers


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A snippet of code (action) isassociated with eachproduction that executes atparse time

In top-down parsers, the

snippet is added to theappropriate parsing routine


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A snippet of code (action) isassociated with eachproduction that executes atparse time

In top-down parsers, the

snippet is added to theappropriate parsing routine


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In a bottom-up shift-reduceparsers, the actions areperformed each time theparserperforms a reduction.

LR(1) Sk l t PLR(1) Sk l t PLR(1) Sk l t PLR(1) Sk l t P


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LR(1) Skeleton ParserLR(1) Skeleton ParserLR(1) Skeleton ParserLR(1) Skeleton Parserstack.push(dummy); stack.push(0);

done = false; token = scanner.next();while (!done) {

s = stack.top();

if( Action[s,token] == reduce AF) {

stack.pop(2v|F|);

s = stack.top();

stack.push(A);

stack.push(Goto[s,A]);

}

else if( Action[s,token] == shifti){stack.push(token); stack.push(i);

token = scanner.next();

}



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LR(1) Skeleton ParserLR(1) Skeleton ParserLR(1) Skeleton ParserLR(1) Skeleton Parserstack.push(dummy); stack.push(0);

done = false; token = scanner.next();while (!done) {

s = stack.top();


stack.pop(2v|F|);

s = stack.top();

stack.push(A);

stack.push(Goto[s,A]);

}

else if( Action[s,token] == shifti){stack.push(token); stack.push(i);

token = scanner.next();

}



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LR(1) Skeleton ParserLR(1) Skeleton ParserLR(1) Skeleton ParserLR(1) Skeleton Parser


stack.pop(2v|F|);

s = stack.top();

stack.push(A);

stack.push(Goto[s,A]);}

invoke the code snippet


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Productions Code snippet

NumberSign List Number.val Sign.valv List.val

Sign + Sign.val 1

Sign Sign.val 1

List Bit List.val Bit.val

List0 List1 Bit List0.val 2vList1.val + Bit.val

Bit 0 Bit.val 0

Bit 1 Bit.val 1

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Context Sensitive Analysis and Attribute Grammar - Compiler Design - Dr. D. P. Sharma - NIT Surathkal by wahid311