Compiler Construction

Recap

Ran Shaham and Ohad ShachamSchool of Computer Science

Tel-Aviv University

22

Test

25/02/2009 at 9:00 חומר פתוח Look at Ran’s webpage for previous tests

Possible questions Extend IC Parsing …

33

Scanning

CLASS,CLASS_ID(Hello),LB,BOOLEAN,ID(state),SEMI …

Issues in lexical analysis:

Language changes:

New keywords,

New operators,

New meta-language features, e.g., annotations

// An example programclass Hello { boolean state; static void main(string[] args) { Hello h = new Hello(); boolean s = h.rise(); Library.printb(s); h.setState(false); } boolean rise() { boolean oldState = state; state = true; return oldState; } void setState(boolean newState) { state = newState; }}

44

Parsing and ASTCLASS,CLASS_ID(Hello),LB,BOOLEAN,ID(state),SEMI …

prog

class_list

class

field_method_list

field field_method_list

type ID(state)

BOOLEAN

method

field_method_list

…

…

parser uses stream of tokensand generate derivation tree

Issues in syntax analysis:

Grammars: LL(1), LR(0)

Building LR(0) parsers

Transition diagram

Parse table

Running automaton

Conflict resolution

Factoring

Ambiguity

TA1

55

Parsing and AST

prog

class_list

class

field_method_list

field field_method_list

type ID(state)

BOOLEAN

method

field_method_list

…

…

Syntax tree builtduring parsing

parser uses stream of tokenand generate derivation

tree

CLASS,CLASS_ID(Hello),LB,BOOLEAN,ID(state),SEMI …

ProgAST

ClassAST

classList

FieldAST[0]type:BoolTypename:state

MethodAST[0]

MethodAST[1]

MethodAST[2]

…

…

methodListfieldList

Should know difference between derivation tree and AST

Know how to build AST from input

66

FieldsOrMethods ::=

Field:field FieldsOrMethods:next{: RESULT = next;

RESULT. addField(field); :} |

Method:method FieldsOrMethods:next {: RESULT = next;

RESULT.addMethod(method); :} |

/* empty */{: RESULT = new FieldsMethods(); :};

77

Questions

Is the following grammar in LR(0) ?Build a parser for the grammarRun an input string using your parser

88

Question

Is the following grammar is LR(0)?

S -> B $

B -> id P | id ( E ]

P -> epsilon | ( E )

E -> B | B,E

A grammar with epsilon production is not LR(0)

99

Semantic analysis

ProgAST

ClassAST

classList

FieldAST[0]type:BoolType

MethodAST[0]

MethodAST[1]

MethodAST[2]

…

…

methodListfieldList

Representing scopes Type-checking Semantic checks

SymbolKindType

HelloclassHello

SymbolKindTypeProperties

statefieldbooleaninstance

mainmethodstring[]->voidstatic

risemethodvoid->booleaninstance

setStatemethodboolean->voidinstance

SymbolKindType

newStateparamint

(Program)

(Hello)

(setState)

…

1010

Semantic conditions

What is checked in compile-time and what is checked in runtime?

EventC/R

Program execution halts

Break/continue inside a while statement

Array index within bound

In Java the cast statement(Integer)f is legal

In Java method o.m(…) is illegal since m is private

1111

Semantic conditions

What is checked in compile-time and what is checked in runtime?

EventC/R

Program execution haltsR (undecidable in general)

Break/continue inside a while statement

C

Array index within boundR (undecidable in general)

In Java the cast statement(A)f is legal

Depends: if A is sub-type of f then checked during runtime (raising exception), otherwise flagged as an error during compilation

In Java method o.m(…) is illegal since m is private

C

1212

Question – extend IC

Support Java override annotation inside comments // @Override Annotation is written above method to indicate it overrides a

method in superclass Describe the phases in the compiler affected by the

change and the changes themselves

class A { void rise() {…}}class B extends A { // @Override void rise() {…}}

class A { void rise() {…}}class B extends A { // @Override void ris() {…}}

Legal program Illegal program

1313

Answer

The change affects the lexical analysis, syntax analysis and semantic analysis

Does not affect later phasesUser semantic condition

1414

Changes to scanner

Add pattern for @Override inside comment state patterns

Add Java code to action to comments – instead of not returning any token, we now return a token for the annotation

What if we want to support multiple annotations in comments?

boolean override=false;%%<INITIAL> // { override=false; yybegin(comment); }<comment> @Override { override=true; }<comment> \n { if (override) return new Token(…,override,…) }

1515

Changes to parser and AST

method static type name params ‘{‘ mbody ‘}’

| type name params ‘{‘ mbody ‘}’

| OVERRIDE type name params ‘{‘ mbody ‘}’

Add a Boolean flag to the method AST node to indicate that the method is annotated

1616

Changes to semantic analysis

Suppose we have an override annotation above a method m in class A

We check the following semantic conditions1. class A extends a superclass (otherwise it does not make

sense to override a method)2. Traverse the superclasses of A by going up the class

hierarchy until we find the first method m and check that it has the same signature as A.mIf we fail to find such a method we report an error

1717

3מועד ב, שאלה 2006

class ByRefExample { static void main(string [] args) { int x = 5; increment(x); Library.println(x); }

static void increment(int & y) { y = y + 1; }}

1818

3מועד ב, שאלה 2006

Treat by reference as address Lexer

Add token for &

Parser Add reference type

Semantic analysis Compare the actual parameter type and the reference type Treat the scope checking

1919

3מועד ב, שאלה 2006

IR Add support for by reference

Code generation Caller

Add the pointer of the actual parameter to the stuck instead of the parameter

Callee Treat the parameter as a pointer

2020

Question

Add constructors to IC

Solution: treat the constructor as a function and call when object allocatedMark the constructorCheck that every object has a constructorCall the constructor on allocation

IRCode generation

2121

Question

Add break(exp) exp is from type int break should exit exp loop Cannot perform break(n) can be perform only inside n nested

loops!!!

int x=2;while(true) {

while(true) {break(x);

}}

Can we generally check this in compile time?

2222

Translation to IR Accept annotated AST and translate functions into lists of

instructions Compute offsets for fields and virtual functions

Issues: dispatch tables, weighted register allocation Question: give the method tables for Rectangle and Square

class Shape { boolean isShape() {return true;} boolean isRectangle() {return false;} boolean isSquare() {return false;} double surfaceArea() {…}}class Rectangle extends Shape { double surfaceArea() {…} boolean isRectangle() {return true;}}class Square extends Rectangle { boolean isSquare() {return true;}}

2323

Answer

Shape_isShape

Rectangle_isRectangle

Shape_isSqaure

Rectangle_surfaceArea

Shape_isShape

Rectangle_isRectangle

Sqaure_isSqaure

Rectangle_surfaceArea

Method table for rectangle Method table for square

2424

Possible question

Suppose we wish to provide type information during runtime like instanceof in Java

x instanceof A returns true iff x is exactly of type A (in Java it can also be subtype of A)

Describe the changes in runtime organization needed to support this operator and the translation to IR

2525

Answer Use the pointers to the dispatch table as the type

indicators

Translate x instanceof A asMove x,R0MoveField R0.0,R0Compare R0,_DV_A

If we want to support the Java operator represent the type hierarchy during runtime and generate code to

search up the hierarchy Keep info for each type for its ancestors and check in constant

time

2626

Question – Activation record

Run the program on the input and show the stack at after …

Add a “new” function that allocates on the stack How to do it? Disadvantages

Stack size is limited Live until the method returns

2727

“To Callee-save or toCaller-save?”

Callee-saved registers need only be saved when callee modifies their value

Some conventions exist (cdecl)%eax, %ecx, %edx – caller save%ebx, %esi, %edi – callee save%esp – stack pointer%ebp – frame pointerUse %eax for return value

2828

Register allocation

Sethi Ullman can only handle expressions without side effect Can change the execution order without hurt the program

semantics Possible question

Extend Sethi Ullman to ternary trees

Global register allocation Is it optimal?

IR registers are treated as local variables

When we have an actual spill we use the stuck

2929

Translating call/return to IR

TR[e1.foo(e2)] R1 := TR[e1]

R2 := TR[e2]

VirtualCall R1.cfoo(x=R2),R

Constant representing offset

of method f in dispatch table

of class type of e1