UMBC

Introduction to Compilers

CMSC 431Shon Vick

01/28/02

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UMBCWhat is a compiler?

• Translates source code to target code– Source code is typically a high level

programming language (Java, C++, etc) but does not have to be

– Target code is often a low level language like assembly or machine code but does not have to be

• Can you think of other compilers that you have used – according to this definition?

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UMBCOther Compilers

• Javadoc -> HTML• SQL Query output -> Table• Poscript -> PDF• High level description of a circuit -

> machine instructions to fabricate circuit

The C

om

pila

tion P

roce

ss

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UMBCThe analysis Stage

• Broken up into four phases– Lexical Analysis (also called scanning

or tokenization)– Parsing– Semantic Analysis– Intermediate Code Generation

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UMBCLexing Example

double d1;double d2;d2 = d1 * 2.0;

double TOK_DOUBLE reserved wordd1 TOK_ID variable name; TOK_PUNCT has value of “;”double TOK_DOUBLE reserved wordd2 TOK_ID variable name ; TOK_PUNCT has value of “;”d2 TOK_ID variable name = TOK_OPER has value of “=”d1 TOK_ID variable name* TOK_OPER has value of “*”2.0 TOK_FLOAT_CONST has value of 2.0; TOK_PUNCT has value of “;”

lexemes

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UMBCSyntax and Semantics

• Syntax - the form or structure of the expressions – whether an expression is well formed

• Semantics – the meaning of an expression

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UMBCSyntactic Structure

• Syntax almost always expressed using some variant of a notation called a context-free grammar (CFG) or simply grammar– BNF– EBNF

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UMBCA CFG has 4 parts

• A set of tokens (lexemes), known as terminal symbols

• A set of non-terminals• A set of rules (productions) where each

production consists of a left-hand side (LHS) and a right-hand side (RHS) The LHS is a non-terminal and the RHS is a sequence of terminals and/or non-terminal symbols.

• A special non-terminal symbol designated as the start symbol

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UMBCAn example of BNF

syntax for real numbers

<r> ::= <ds> . <ds><ds> ::= <d> | <d> <ds><d> ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7| 8 | 9

< > encloses non-terminal symbols::= 'is' or 'is made up of ' or 'derives' (sometimes denoted with an arrow ->) | or

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UMBCExample

• On the example from the previous slide:– What are the tokens?– What are the lexemes?– What are the non terminals?– What are the productions?

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UMBCBNF Points

• A non terminal can have more than RHS or an OR can be used

• Lists or sequences are expressed via recursion

• A derivation is just a repeated set of production (rule) applications

• Examples

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UMBCExample Grammar

<program> -> <stmts><stmts> -> <stmt> | <stmt> ; <stmts><stmt> -> <var> = <expr><var> -> a | b | c | d<expr> -> <term> + <term> | <term> - <term><term> -> <var> | const

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UMBCExample Derivation

<program> => <stmts> => <stmt> => <var> = <expr> => a = <expr> => a = <term> + <term> => a = <var> + <term> => a = b + <term> => a = b + const

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UMBCParse Trees

• Alternative representation for a derivation

• Example parse tree for the previous example

var expr=

term+

var

b

const

stmts

stmt

terma

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UMBCAnother Example

Expression -> Expression + Expression | Expression - Expression | ... Variable | Constant |...Variable -> T_IDENTIFIERConstant -> T_INTCONSTANT | T_DOUBLECONSTANT

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UMBCThe Parse

Expression -> Expression + Expression -> Variable + Expression

-> T_IDENTIFIER + Expression -> T_IDENTIFIER + Constant -> T_IDENTIFIER + T_INTCONSTANT

a + 2

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UMBCParse Trees

PS -> P | P PS

P -> | '(' PS ')' | '<' PS '>' | '[' PS ']'

What’s the parsetree for this statement ? < [ ] [ < > ] >

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UMBCEBNF - Extended BNF

• Like BNF except that• Non-terminals start w/ uppercase • Parens are used for grouping terminals • Braces {} represent zero or more

occurrences (iteration ) • Brackets [] represent an optional construct ,

that is a construct that appears either once or not at all.

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UMBCEBNF example

Exp -> Term { ('+' | '-') Term }Term -> Factor { ('*' | '/') Factor }Factor -> '(' Exp ')' | variable | constant

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UMBCEBNF/BNF

• EBNF and BNF are equivalent

• How can {} be expressed in BNF?

• How can ( ) be expressed?

• How can [ ] be expressed?

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UMBCSemantic Analysis

• The syntactically correct parse tree (or derivation) is checked for semantic errors

• Check for constructs that while valid syntax do not obey the semantic rules of the source language.

• Examples:– Use of an undeclared/un-initialized variable– Function called with improper arguments– Incompatible operands and type mismatches,

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UMBCExamples

int i;int j;i = i + 2;

int arr[2], c;c = arr * 10;

Most semantic analysis pertains to the checking of

types.

void fun1(int i);double d;d = fun1(2.1);

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UMBC Intermediate Code Generation

• Where the intermediate representation of the source program is created.

• The representation can have a variety of forms, but a common one is called three-address code (TAC)

• Like assembly – the TAC is a sequence of simple instructions, each of which can have at most three operands.

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UMBCExample

_t1 = b * c_t2 = b * d_t3 = _t1 + _t2a = _t3

a = b * c + b * d

Note temps

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UMBCAnother Example

_t1 = a > b if _t1 goto L0 _t2 = a - c a = _t2L0: t3 = b * c c = _t3

if (a <= b) a = a - c;c = b * c;

Note TempsSymbolic addresses

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UMBCNext Time

• Finish introduction to compilation stages

• Read Aho/Sethi/Ullman Chapter 1

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UMBC

Selected References

• Compilers Principles, Techniques and Tools, Aho, Sethi, and Ullman

• http://www.stanford.edu/class/cs143/