Compiler Construction

Recap

2

Announcements

• PA4: extension until end of exam period– Not a single day more, for any reason!

• PA5: bonus exercise– Will be posted immediately after semester ends

3

Exam

• 18/02/2013 at 9:00• Past exams on my website• פתוח חומר• Possible questions

– Extend IC– Parsing– Register allocation– …

4

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; }}

5

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

…

…

Issues in syntax analysis: Grammars: LL(1), LR(0) Ambiguity

Parser uses token stream, and generates derivation tree

6

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 token stream, and generates 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

7

Question: Parsing

• 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 productions is not LR(0)

8

Other possible questions

• Is the following grammar in LR(k)?• Build a parser for given grammar• Run an input string using your parser• …

9

Semantic analysis

ProgAST

ClassAST

classList

FieldAST[0]type:BoolType

MethodAST[0]

MethodAST[1]

MethodAST[2]

…

…

methodListfieldList

• Representing scopes• Type checking• Semantic checks

Symbol Kind Type

Hello class Hello

Symbol Kind Type Properties

state field boolean instance

main method string[]->void static

rise method void->boolean instance

setState method boolean->void instance

Symbol Kind Type

newState param int

(Program)

(Hello)

(setState)

…

10

Semantic conditions

• What is checked at compile time, and what is checked at runtime?

Event C/R

Program execution halts R (undecidable in general)

break/continue inside a while statement

C

Array index within bound R (undecidable in general)

In Java, the cast statement(A)f is legal

Depends: if A is subtype of f then checked at runtime (potentially raising exception); otherwise flagged as an error during compilation

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

C

11

Question: IC language• 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

12

Answer

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

• Does not affect later phases– User-level semantic condition

13

Changes to scanner

• Add pattern for @Override inside comment state patterns• Add Java action code to comments: Instead of not returning

any token, we now return a token for the annotation

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

14

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

15

Changes to semantic analysis• Suppose we have an override annotation above a method

m in class A

• We check the following semantic conditions:1. 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, then we report an error

16

Question: IC language

Add constructors to IC (must be called)

Answer

• Treat the constructor as a function, and call when object allocated– Lexical analysis: nothing– Parsing: AST node for constructor– Semantic analysis:

• Check that every class has a constructor• Actual/formal compatibility

– IR/code generation:• Call the constructor on allocation

18

Translation to IR• Accept annotated AST and translate functions

into lists of instructions– Compute offsets for fields and virtual methods– Issues: dispatch tables, weighted register allocation

Question: IR

• 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;}}

20

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

21

Question: IR

• Suppose we wish to provide type information at runtime – Similar to 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

22

Answer• Use the pointer to the dispatch table as the type indicator

• 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 at runtime and generate code to search

up the hierarchy– Keep ancestor info for each type to enable constant-time checking

23

Register allocation

• Sethi Ullman– can only handle expressions without side effect

• Global register allocation

• IR registers are treated as local variables

• When we have an actual spill we use the stack

24

Weighted register allocation

• Can save registers by reordering subtree computations• Label each node with its weight

– Weight = number of registers needed– Leaf weight known– Internal node weight

• w(left) > w(right) then w = left• w(right) > w(left) then w = right• w(right) = w(left) then w = left + 1

• Choose heavier child as first to be translated

• Have to check that there are no side effects

25

Weighted reg. alloc. example

b

5 c

*

array access

+

a

base index

W=1

W=0 W=1

W=1W=1

W=2

W=2

Phase 1: - check absence of side-effects in expression tree - assign weight to each AST node

R0 := TR[a+b[5*c]]

26

ReminderR0 := TR[a+(b+(c*d))]

b

c d

*

+

+

aR0

R1

R2

Translation using all optimizationsshown until now uses 3 registers

R2

R1

left child first

b

c d

*

+

+

a

R0

Managed to save two registers

R0

R0

right child firstR0R0

27

Sethi Ullman• What type of tree is worst case for SU with respect to the tree’s height?

28

Sethi Ullman• What type of tree is best case for SU with respect to the tree’s height?

.

.

.

29

Sethi Ullman• What type of tree maximizes the ratio between registers allocated by

traversing the tree left-to-right and right-to-left?

.

.

.