imple oo lang 2

7/28/2019 Imple OO Lang 2

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Implementing

Object-Oriented Languages - Part 2Y.N. Srikant

Computer Science and Automation

Indian Institute of Science

Bangalore 560 012

NPTEL Course on Compiler Design


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Y.N. Srikant 2

Outline of the Lecture

Language requirements

Mapping names to methods Variable name visibility

Code generation for methods

Simple optimizations Parts of this lecture are based on the book,

Engineering a Compiler, by Keith Cooper andLinda Torczon, Morgan Kaufmann, 2004, sections6.3.3 and 7.10.

Topics 1,2,3, and, 4 were covered in Part 1 of the lecture.


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Y.N. Srikant 3

Example of Class Hierarchy with

Complete Method Tablesn: 0

fee

fum

n: 1

fee

fum

n: 2

fee

fum

x: 5

x: 5

y: 3

z:

foe foe

fie

x: 2

y: 0

z:

y: 3

fum

...

fee...

foe...

fee

...

fee...

fie...

one

two

threec

a

b

object

class

method


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Y.N. Srikant 4

Implementing Multiple Inheritance

%new_

pointer

%new_

pointer

%new_

pointer

fum...

foe...

fee

...

fie...

class a class b

class c


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Y.N. Srikant 5


class

pointer

a data

members

b data

members

c data

members

object layout

for objects

of class c

class

pointer

a data

members

object layout

for objects

of class a

classpointer

b datamembers

object layout

for objects

of class b


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Y.N. Srikant 6


- Fixed Offset Method Record the constant offset in the method

table along with the methods

Offsets for this example are as follows: (c) fee : 0, (a) fie: 0, (b) foe : 8, (b) fum : 8,

assuming that instance variables of class a take 8bytes

Generated code adds this offset to thereceivers pointer address before invoking the

method

class

pointer

a data

members

b data

members

c data

members

object layout

for objects

of class c


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Y.N. Srikant 7


%new_

pointer

%new_

pointer

%new_

pointer

fum...

foe...

fee

...

fie...

class a class b

class c

class

pointer

a data

members

b data

members

c data

members

object layout

for objects

of class c

data pointer for c.foe()


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Y.N. Srikant 8


- Trampoline Functions Create trampoline functions for each method

of class b A function that increments this (pointer to

receiver) by the required offset and then invokes

the actual method fromb. On return, it decrements the receiver pointer, if it

was passed by reference


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Y.N. Srikant 9


Trampolines appear to be more expensive

than the fixed offset method, but not really so They are used only for calls to methods inherited

fromb

In the other method, offset (possibly 0) was added for allcalls

Method inlining will make it better than option 1,since the offset is a constant

Finally, a duplicate class pointer (pointing toclass c) may need to be inserted just before

instance variables ofb (for convenience)


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Y.N. Srikant 10

Fast Type Inclusion Tests

The need

If classY is a subclass of class X X a = new Y(); //a is of type base class of Y, okay

// other code omittedY b = a; // a holds a value of type Y

The above assignment is valid, but the following is not

X a = new X();// other code omittedY b = a; // a holds a value of type X

Runtime type checking to verify the above is

needed J ava has an explicit instanceoftest that requires a

runtime type checking


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Y.N. Srikant 11


Searching the

Class Hierarchy Graph Store the class hierarchy graph in memory

Search and check if one node is an ancestorof another

Traversal is straight forward to implement

only for single inheritance Cumbersome and slow for multiple

inheritance

Execution time increases with depth of classhierarchy


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Y.N. Srikant 12

Class Hierarchy Graph -

Example

A

B C

D E

G

F

Single

inheritance A

B C

D E

G

F

HMultiple

inheritance


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Y.N. Srikant 13


Binary Matrix

0 1 0 0 1

0 0 1 0 1

1 0 0 1 0

1 0 0 0 1

0 0 1 0 0

C1 C2 C3 C4 C5

C1

C2

C3

C4

C5

Class types

Class types

BM [Ci, Cj] = 1, iff Ci is a subclass ofCj

Tests areefficient, butMatrix will belarge in practice.

The matrix canbe compacted,but thisincreases

access time.This can handlemultipleinheritance also.


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Y.N. Srikant 14

Relative (Schuberts) Numbering

A

B C

D E

G

F

{3,3}

{1,1}

{5,5}{3,4}

{3,6}{1,2}

{1,7}

{ la

, ra

} for a node a

:

ra

is the ordinal number of the

node a

in a postorder

traversal

of the tree. Let denotesubtype of

relation. All

descendants of a node are

subtypes of that node.

is reflexive and transitive.

la

= min { rp

| p is a descendant

of

a }.

Now,

a

b, iff

lb

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