编译器设计04

1 .CS 153: Concepts of Compiler Design August 30 Class Meeting Department of Computer Science San Jose State University Fall 2018 Instructor: Ron Mak www.cs.sjsu.edu/~mak 1

2 .2 Syntax Diagrams Chapter 3

3 .3 How to Scan for Tokens Suppose the source line contains IF (index >= 10) THEN The scanner skips over the leading blanks. The current character is I , so the next token must be a word . The scanner extracts a word token by copying characters up to but not including the first character that is not valid for a word, which in this case is a blank. The blank becomes the current character. The scanner determines that the word is a reserved word .

4 .4 How to Scan for Tokens, cont ’ d The scanner skips over any blanks between tokens. The current character is ( . The next token must be a special symbol . After extracting the special symbol token, the current character is i . The next token must be a word . After extracting the word token, the current character is a blank.

5 .5 How to Scan for Tokens, cont ’ d Skip the blank. The current character is > . Extract the special symbol token. The current character is a blank. Skip the blank. The current character is 1 , so the next token must be a number . After extracting the number token, the current character is ) .

6 .6 How to Scan for Tokens, cont ’ d Extract the special symbol token. The current character is a blank. Skip the blank. The current character is T , so the next token must be a word . Extract the word token. Determine that it ’ s a reserved word . The current character is

7 .7 Basic Scanning Algorithm Skip any blanks until the current character is nonblank. In Pascal, a comment and the end-of-line character each should be treated as a blank. The current (nonblank) character determines what the next token is and becomes that token ’ s first character. Extract the rest of the next token by copying successive characters up to but not including the first character that does not belong to that token. Extracting a token consumes all the source characters that constitute the token. After extracting a token, the current character is the first character after the last character of that token .

8 .8 Pascal-Specific Subclasses

9 .9 Class PascalScanner protected Token extractToken () throws Exception { skipWhiteSpace (); Token token; char currentChar = currentChar (); // Construct the next token. The current character determines the // token type. if ( currentChar == EOF) { token = new EofToken (source); } else if ( Character.isLetter ( currentChar )) { token = new PascalWordToken (source); } else if ( Character.isDigit ( currentChar )) { token = new PascalNumberToken (source); } ... return token; } The first character determines the type of the next token. wci.frontend.pascal.PascalScanner

10 .10 Pascal-Specific Token Classes Each class PascalWordToken , PascalNumberToken , PascalStringToken , PascalSpecial-SymbolToken , and PascalErrorToken is is a subclass of class PascalToken . PascalToken is a subclass of class Token . Each Pascal token subclass overrides the default extract() method of class Token . The default method could only create single-character tokens. Loosely coupled. Highly cohesive.

11 .11 Class PascalWordToken protected void extract () throws Exception { StringBuilder textBuffer = new StringBuilder (); char currentChar = currentChar (); // Get the word characters (letter or digit). The scanner has // already determined that the first character is a letter. while ( Character.isLetterOrDigit ( currentChar )) { textBuffer.append ( currentChar ); currentChar = nextChar (); // consume character } text = textBuffer.toString (); // Is it a reserved word or an identifier? type = ( RESERVED_WORDS.contains ( text.toLowerCase ())) ? PascalTokenType.valueOf ( text.toUpperCase ()) // reserved word : IDENTIFIER ; // identifier } wci.frontend.pascal.tokens.PascalWordToken

12 .11 Class PascalWordToken protected void extract () throws Exception { StringBuilder textBuffer = new StringBuilder (); char currentChar = currentChar (); // Get the word characters (letter or digit). The scanner has // already determined that the first character is a letter. while ( Character.isLetterOrDigit ( currentChar )) { textBuffer.append ( currentChar ); currentChar = nextChar (); // consume character } text = textBuffer.toString (); // Is it a reserved word or an identifier? type = ( RESERVED_WORDS.contains ( text.toLowerCase ())) ? PascalTokenType.valueOf ( text.toUpperCase ()) // reserved word : IDENTIFIER ; // identifier } wci.frontend.pascal.tokens.PascalWordToken

13 .13 Pascal Number Tokens A Pascal integer literal constant is an unsigned integer. A Pascal real literal constant starts with an unsigned integer (the whole part ) followed by either A decimal point followed by another unsigned integer (the fraction part ), or An E or e , optionally followed by + or - , followed by an unsigned integer (the exponent part ), or A whole part followed by an exponent part. Any leading + or – sign before the literal constant is a separate token.

14 .14 Class PascalNumberToken For the token string "31415.926e-4" , method extractNumber () passes the following parameter values to method computeFloatValue () : wholeDigits "31415" fractionDigits "926" exponentDigits "4" exponentSign - Compute variable exponentValue : 4 as computed by computeIntegerValue() -4 after negation since exponentSign is - -7 after subtracting fractionDigits.length() Compute 31415926 x 10 -7 = 3.1415926 A bit of a hack!

15 .15 Syntax Error Handling Error handling is a three-step process: Detect the presence of a syntax error. Flag the error by pointing it out or highlighting it, and display a descriptive error message. Recover by moving past the error and resume parsing. For now, we ’ ll just move on, starting with the current character, and attempt to extract the next token. SYNTAX_ERROR message source line number beginning source position token text syntax error message

16 .16 Class PascalParserTD public void parse () throws Exception { ... // Loop over each token until the end of file. while (!((token = nextToken ()) instanceof EofToken )) { TokenType tokenType = token.getType (); if ( tokenType != ERROR) { // Format each token. sendMessage (new Message(TOKEN, new Object[] { token.getLineNumber (), token.getPosition (), tokenType , token.getText (), token.getValue ()})); } else { errorHandler.flag (token, ( PascalErrorCode ) token.getValue (), this); } } ... } wci.frontend.pascal.PascalParserTD

17 .17 Program: Pascal Tokenizer Verify the correctness of the Pascal token subclasses . Verify the correctness of the Pascal scanner . Demo (Chapter 3)

18 .Assignment #2 Write a scanner for the Java language. Add a new Java front end. 18

19 .19 Quick Review of the Framework TO: FROM: Our next topic: The symbol table Chapter 4

20 .20 The Symbol Table: Basic Concepts Purpose To store information about certain tokens during the translation process (i.e., parsing and scanning) What information to store? Anything that ’ s useful! For an identifier: name data type how it ’ s defined (as a variable, type, function name, etc.)

21 .21 The Symbol Table: Basic Operations Enter new information. Look up existing information. Update existing information.

22 .22 The Symbol Table: Conceptual Design Each entry in the symbol table has a name attributes At the conceptual level, we don ’ t worry about implementation. Goal: The symbol table should be source language independent.

23 .23 What Needs a Symbol Table? A Pascal program Identifiers for constant, type, variable, procedure, and function names. A Pascal procedure or function Identifiers for constant, type, variable, procedure, and function names. Identifiers for formal parameter (argument) names. A Pascal record type Identifiers for field names.

24 .24 The Symbol Table Stack Language constructs can be nested . Procedures and functions are nested inside a program. Procedures and functions can be nested inside of each other. Record types are defined within programs, procedures, and functions. Record types can be nested inside of each other. Therefore, symbol tables need to be kept on a symbol table stack .

25 .25 The Symbol Table Stack, cont ’d Whichever symbol table is on top of the stack is the local symbol table . The first symbol table created (the one at the bottom of the stack) is the global symbol table . It stores the predefined information, such as entries for the names of the standard types integer , real , char , and boolean . During the translation process, symbol tables are pushed onto and popped off the stack … … as the parser enters and exits nested procedures, functions, record types, etc. Global symbol table

26 .26 The Symbol Table Stack, cont ’ d For now, we ’ ll have only have a single symbol table. Therefore, the local symbol table is the global symbol table. We won’t need multiple symbol tables until we start to parse declarations. Implementing the symbol table stack now will make things easier for us later. Global symbol table

27 .27 Symbol Table Interfaces Key operations Enter into the local symbol table, the table currently at the top of the stack. Look up (search for) an entry only in the local symbol table. Look up an entry in all the symbol tables in the stack. Search from the top (the local) table down to the bottom (global) table. Each symbol table has a nesting level . 0: global 1: program 2: top-level procedure 3: nested procedure, etc.

28 .28 Why All the Interfaces? We ’ ve defined the symbol table components entirely with interfaces. Other components that use the symbol table will code to the interfaces , not to specific implementations. Loose coupling provides maximum support for flexibility. symTabStack = SymTabFactory.createSymTabStack (); SymTabEntry entry = symTabStack.lookup (name);

29 .29 Why All the Interfaces? cont’d We ’ ll be able to implement the symbol table however we like. We can change the implementation in the future without affecting the users. But not change the interfaces. The interfaces provide an API for the symbol table. Callers of the symbol table API only need to understand the symbol table at the conceptual level .