spof03-2-2

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Languages and Compilers

(SProg og Oversttere)

Bent Thomsen

Department of Computer ScienceAalborg University

With acknowledgement to Norm Hutchinson whos slides this lecture is based on.

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Compilation

So far we have treated language processors (including

compilers) as black boxes

Now we take a first look "inside the box": how are

compilers built.

And we take a look at the different phases and their

relationships

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The Phases of a Compiler

Syntax Analysis

Contextual Analysis

Code Generation

Source Program

Abstract Syntax Tree

Decorated Abstract Syntax Tree

Object Code

Error Reports

Error Reports

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Example Program

We now look at each of the three different phases in a little

more detail. We look at each of the steps in transforming

an example Triangle program into TAM code.

! This program is useless except for

! illustration

let var n: integer;

var c: char

in begin

c := &;n := n+1

end

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1) Syntax Analysis

Syntax Analysis

Source Program


Error Reports

Note: Not all compilers construct an

explicit representation of an AST. (e.g. on

a single pass compiler generally no need

to construct an AST)

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1) Syntax Analysis -> AST

Program

LetCommand

SequentialDeclaration

n Integer c Char c & n n + 1

Ident Ident Ident Ident Ident Ident Ident OpChar.Lit Int.Lit

SimpleT

VarDecl

SimpleT

VarDecl

SimpleV

Char.Expr

SimpleV

VNameExp Int.Expr

AssignCommand BinaryExpr

SequentialCommand

AssignCommand

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2) Contextual Analysis -> Decorated AST

Contextual Analysis


Error Reports


Contextual analysis:

Scope checking: verify that all applied occurrences ofidentifiers are declared

Type checking: verify that all operations in the program areused according to their type rules.

Annotate AST:

Applied identifier occurrences => declaration

Expressions => Type

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2) Contextual Analysis -> Decorated AST

Program

LetCommand

SequentialDeclaration

n

Ident Ident Ident Ident

SimpleT

VarDecl

SimpleT

VarDecl

Integer c Char c & n n + 1

Ident Ident Ident OpChar.Lit Int.Lit

SimpleV

Char.Expr

SimpleV

VNameExp Int.Expr

AssignCommand BinaryExpr

SequentialCommand

AssignCommand

:char:char

:int

:int:int :int

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Contextual Analysis

Finds scope and type errors.

AssignCommand

:char

Example 1:

:int

***TYPEERROR (incompatible types in

assigncommand)

Example 2:

foo

Ident

SimpleV

foo not found

***SCOPEERROR: undeclared variable foo

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3) Code Generation

Assumes that program has been thoroughly

checked and is well formed (scope & type rules) Takes into account semantics of the source

language as well as the target language.

Transforms source program into target code.

Code Generation


Object Code

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3) Code Generation

let var n: integer;

var c: char

in begin

c := &;

n := n+1end

PUSH 2

LOADL 38

STORE 1[SB]

LOAD 0

LOADL 1CALL add

STORE 0[SB]

POP 2

HALT

n

Ident Ident

SimpleT

VarDecl

Integer

address = 0[SB]

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Compiler Passes

A pass is a complete traversal of the source program, or

a complete traversal of some internal representation of

the source program.

A pass can correspond to a phase but it does not have

to! Sometimes a single pass corresponds to several phases

that are interleaved in time.

What and how many passes a compiler does over the

source program is an important design decision.

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Single Pass Compiler

Compiler Driver

Syntactic Analyzer

calls

calls

Contextual Analyzer Code Generator

calls

Dependency diagram of a typical Single Pass Compiler:

A single pass compiler makes a single pass over the source text,parsing, analyzing and generating code all at once.

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Multi Pass Compiler

Compiler Driver

Syntactic Analyzer

callscalls

Contextual Analyzer Code Generator

calls

Dependency diagram of a typical Multi Pass Compiler:

A multi pass compiler makes several passes over the program. Theoutput of a preceding phase is stored in a data structure and used by

subsequent phases.

input

Source Text

output

AST

input output

Decorated AST

input output

Object Code

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Example:The Triangle Compiler Driver

public class Compiler {

public static void compileProgram(...) {Parser parser = new Parser(...);Checker checker = new Checker(...);Encoder generator = new Encoder(...);

Program theAST = parser.parse();

checker.check(theAST);generator.encode(theAST);

}public void main(String[] args) {

... compileProgram(...) ...}

}

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Compiler Design Issues

Single Pass Multi Pass

Speed

Memory

Modularity

Flexibility

Global optimization

Source Language

better worse

better for

large programs

(potentially) better

for small programsworse better

betterworse

impossible possible

single pass compilers are not possible

for many programming languages

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Language Issues

Example Pascal:

Pascal was explicitly designed to be easy to implement

with a single pass compiler:

Every identifier must be declared before it is first use.

var n:integer;

procedure inc;

begin

n:=n+1

end

Undeclared Variable!

procedure inc;

begin

n:=n+1

end;

var n:integer;

?

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Language Issues

Example Pascal:

Every identifier must be declared before it is used.

How to handle mutual recursion then?

procedure ping(x:integer)

begin

... pong(x-1); ...

end;

procedure pong(x:integer)

begin

... ping(x); ...

end;

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Language Issues

Example Pascal:

Every identifier must be declared before it is used.

How to handle mutual recursion then?

forward procedure pong(x:integer)

procedure ping(x:integer)

begin

... pong(x-1); ...

end;

procedure pong(x:integer)

begin

... ping(x); ...

end;

OK!

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Language Issues

Example Java:

identifiers can be declared before they are used.

thus a Java compiler need at least two passes

Class Example {

void inc() { n = n + 1; }

int n;

void use() { n = 0 ; inc(); }

}

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Keep in mind

There are many issues influencing the design of a new

programming language: Choice of paradigm

Syntactic preferences

Even the compiler implementation

e.g no of passes available tools

There are many issues influencing the design of new

compiler: No of passes

The source, target and implementation language

Available tools

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