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Chapter V: Compiler

Overview: To study the design and operation of compiler for

high-level programming languages. Contents

Basic compiler (one-pass compiler) functions Machine-dependent extension:

(object-code generation & code optimization) Compiler design alternative:

multi-pass compiler, interpreters, p-code compilers & compiler-compilers.

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Example

Basic compiler functions

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Basic compiler functions (cont.)

Source program Regard each statement as a sequence of token.

The task of scanning the source statement, recognizing and classifying the various tokens, is known as lexical analysis. (scanner)

Recognized all tokens as some language construct by the grammar. This process is called syntactic analysis or parsing. (parser)

Generation of object code.

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Compilation process

Scanning (lexical analysis) Parsing (syntactic analysis)

Code generation

Ps. It can achieve in a single pass !

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Grammars

A grammar for a programming language is a formal description of the syntax, of programs and individual statements written in the language.

The difference between syntax and semantics, E.g.,

I := J + K X := Y + I where X,Y : Real I,J,K : IntegerThey are identical syntax.However, the semantic are quite different.

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Grammars (cont.)

BNF (Backus-Naur Form) A kind of syntax description. Simple. Widely used. It provide capabilities that are sufficient for most purposes.

BNF consists of a set of rules, each of which defines the syntax of some construct in the programming language. E.g., <read> ::= READ ( <id-list>)

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Grammars (cont.)

<read> ::= READ ( <id-list>) <id-list> ::= id | <id-list>, id

Character strings enclosed between < and > are called nonterminal symbol.

Character strings not enclosed between < and > are called terminal symbol (I.e, tokens).

E.g., READ(value, sum, x, y)

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Simplified Pascal grammar

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Simplified Pascal grammar (cont.)

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Simplified Pascal grammar (cont.)

To display the analysis of a source statement in terms of a grammar a a tree (parse tree or syntax tree).

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The parse tree for VARIANCE := SUMSQ DIV 100 – MEAN * MEAN

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Grammars (cont.)

Draw parse tree for ALPHA – BETA * GAMMA

If there is more than one possible parse tree for a given statement, the grammar is said to be ambiguous.

The ambiguous grammar would leave doubt about what object code should be generated.

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Lexical analysis (scanning)

Scanning the program to be compiled and recognizing the tokens that make up the source statements.

Scanner are usually designed to recognize keywords, operators, and identifiers, integer, floating-point numbers, character strings, …,etc.

The identifier might be defined by the rules: <ident> ::= <letter> | <ident> <letter> | <ident> <digit> <letter> ::= A | B | C | D | … | Z <digit> ::= 0 | 1 | 2 | 3 | … | 9

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Token coding scheme

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Lexical scan

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The lexical scanning

It must deal with the following cases: For example,

DO 10 I = 1, 100 DO 10 I =1 (FORTRAN ignores blank in the statement)

IF (THEN .EQ. ELSE) THEN IF = THENELSE THEN = IFENDIF

A number of tools have been developed for automatically constructing lexical scanners from specifications stated in a special-purpose language.

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Modeling Scanners as Finite Automata

The tokens of most programming languages can be recognized by a finite automation.

Starting state vs. final state. If the automation stops in a final state, we say that

it recognizes (or accept) the string being scanned, otherwise, it fails to recognize the string.

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Modeling Scanners as Finite Automata (cont.)

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Modeling Scanners as Finite Automata (cont.)

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Modeling Scanners as Finite Automata (cont.)

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Modeling Scanners as Finite Automata (cont.)

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The implementation of finite automata

Using algorithm code (for Fig. 5.8 (b))

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Using tabular representation

The implementation of finite automata (cont.)