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Lesson 6. CDT301 – Compiler Theory , Spring 2011 Teacher : Linus Källberg. Outline. Code generation using syntax-directed translation Lexical analysis. Code generation using syntax-directed translation. Syntax-directed translation. Add attributes to the grammar symbols

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## Lesson 6

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**Lesson 6**CDT301 – CompilerTheory, Spring 2011 Teacher: Linus Källberg**Outline**• Code generation usingsyntax-directedtranslation • Lexicalanalysis**Syntax-directedtranslation**• Add attributes to the grammar symbols • Add semantic actions to the grammar • Syntax-directed translation scheme • “Inject” code into the parser**SDT example(Section2.3 in the book)**• Expression grammar: expr → expr + num | expr – num | num • Infix to postfixnotation**SDT example(Section2.3 in the book)**• Formal definition: • postfix(num) = num • postfix( (E) ) = postfix(E) • postfix(E1opE2) = postfix(E1) postfix(E2) op**Exercise (1)**Translate the following infix expressions intopostfix notation: • 78 • 3 – 2 – 1 • (8 + 19 * 3) • 3 * (17) / (92 + 8) Assumeconventional operator precedence and associativity.**SDT example(Section2.3 in the book)**• Translationscheme: expr → expr + num { print(num.value);print('+') } | expr – num { print(num.value);print('–') } | num { print(num.value) }**SDT example(Section2.3 in the book)**• Extendedparsetree for 1 + 2 – 3: expr expr – num (3) { print(num.value); print('-') } expr + num (2) { print(num.value); print('+') } num (1) { print(num.value) }**Exercise (2)**Traverse the followingextendedparsetree in a depth-first, left-right order and execute the semanticactions: expr expr – num (3) { print(num.value); print('-') } expr + num (2) { print(num.value); print('+') } num (1) { print(num.value) }**Leftrecursion elimination**expr → num { print(num.value) } rest rest → + num { print(num.value);print('+') } rest rest → - num { print(num.value);print('-') } rest rest → ε**Exercise (3)**Draw the parsetree for 1 + 2 – 3(i.e. num + num – num) with the new grammar. Include the semanticactions as leafnodes. Thentraverse it and execute the semanticactions.**Syntax-directed definitions**• Similar to translation schemes • More “abstract” or “declarative”**Lexicalanalysis**• “Lexical analyzer”/“scanner”/“tokenizer” • Simplifies the parser: • Removes white spaces • Removes comments • Identifies lexemes and returns tokens**Tokens**• Name + attribute • Attributes: • Line and columnnumber • Identifiername/symbol table index • Numericalvalue • … • Lexemes**Differingrequirements**• Allowspaces in identifiers? • Example: Fortran 90 • Allowkeywords as identifiers? • Example: PL/1 • Language support for configuringthe lexicalanalysis? • Example: TeX**Implementinglexicalanalysis**• Finite statemachine? • Hard-coded? • Use a generator tool?**intlineno = 1, attribute = NONE;**intGetNextToken(void) { char t; for (t = ReadChar(); t != 0; t = ReadChar()) { if (t == ' ' || t == '\t') /* Skipwhitespaces */ elseif (t == '\n') lineno++; elseif ('0' <= t && t <= '9') { attribute= GetNum(t); return NUM; } else { /* Error handling */ attribute= NONE; return UNKNOWN_TOKEN; } } return EOF; /* End of file token */ }**intGetNum(char t) {**intnum= 0; for(; '0' <= t && t <= '9'; t = ReadChar()) { num*= 10; num+= t – '0'; } // Put back the char that caused the loop to exit PutBack(t); returnnum; }**Differentiatingbetweenkeywords and identifiers**• Twostrategies: • Keyword table • Test for keywords before identifiers**Errorrecovery**• Often hard to detect • Misspelled keywords = valid identifiers • Misspelled identifiers hard to detect • Recovery strategies: • Panic mode • Try to “fix” the input**Conclusion**• Code generation usingsyntax-directedtranslation • Lexicalanalysis**Next time**• Stack machinecode • Generating stack machinecodeusing SDT

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