In this section, we will explore the process of building a simple compiler using ALGOL. This will involve understanding the basic components of a compiler, writing code to handle these components, and testing our compiler with simple programs.
Key Concepts
-
Compiler Basics
- Lexical Analysis: Tokenizing the input source code.
- Syntax Analysis: Parsing tokens to ensure they follow the grammar rules.
- Semantic Analysis: Checking for semantic errors.
- Intermediate Code Generation: Creating an intermediate representation of the source code.
- Optimization: Improving the intermediate code.
- Code Generation: Producing the target code.
- Code Linking and Loading: Preparing the code for execution.
-
Tools and Techniques
- Lexers and Parsers: Tools for lexical and syntax analysis.
- Abstract Syntax Trees (ASTs): Data structures representing the syntax of the source code.
- Symbol Tables: Data structures for storing information about variables and functions.
- Intermediate Representations (IRs): Simplified versions of the source code for optimization and code generation.
Step-by-Step Guide
Step 1: Lexical Analysis
Lexical analysis involves breaking down the source code into tokens. Tokens are the smallest units of meaning, such as keywords, identifiers, operators, and literals.
Example Code: Lexical Analyzer in ALGOL
BEGIN
STRING sourceCode;
ARRAY tokens[1:100] OF STRING;
INTEGER i, tokenIndex;
PROCEDURE isLetter(c: CHAR): BOOLEAN;
BEGIN
RETURN (c >= 'A' AND c <= 'Z') OR (c >= 'a' AND c <= 'z');
END;
PROCEDURE isDigit(c: CHAR): BOOLEAN;
BEGIN
RETURN c >= '0' AND c <= '9';
END;
PROCEDURE tokenize;
BEGIN
tokenIndex := 1;
FOR i := 1 STEP 1 UNTIL LENGTH(sourceCode) DO
IF isLetter(sourceCode[i]) THEN
tokens[tokenIndex] := "IDENTIFIER";
tokenIndex := tokenIndex + 1;
ELSE IF isDigit(sourceCode[i]) THEN
tokens[tokenIndex] := "NUMBER";
tokenIndex := tokenIndex + 1;
ELSE
tokens[tokenIndex] := "SYMBOL";
tokenIndex := tokenIndex + 1;
END;
END;
END;
sourceCode := "int x = 10;";
tokenize;
FOR i := 1 STEP 1 UNTIL tokenIndex - 1 DO
PRINT(tokens[i]);
END;
END;Step 2: Syntax Analysis
Syntax analysis involves parsing the tokens to ensure they follow the grammar rules of the language. This is typically done using a parser.
Example Code: Simple Parser in ALGOL
BEGIN
ARRAY tokens[1:100] OF STRING;
INTEGER tokenIndex;
PROCEDURE parseExpression;
BEGIN
IF tokens[tokenIndex] = "IDENTIFIER" THEN
tokenIndex := tokenIndex + 1;
IF tokens[tokenIndex] = "SYMBOL" THEN
tokenIndex := tokenIndex + 1;
IF tokens[tokenIndex] = "NUMBER" THEN
PRINT("Valid Expression");
ELSE
PRINT("Syntax Error: Expected NUMBER");
END;
ELSE
PRINT("Syntax Error: Expected SYMBOL");
END;
ELSE
PRINT("Syntax Error: Expected IDENTIFIER");
END;
END;
tokens[1] := "IDENTIFIER";
tokens[2] := "SYMBOL";
tokens[3] := "NUMBER";
tokenIndex := 1;
parseExpression;
END;Step 3: Semantic Analysis
Semantic analysis involves checking for semantic errors, such as type mismatches and undeclared variables.
Example Code: Semantic Analyzer in ALGOL
BEGIN
ARRAY tokens[1:100] OF STRING;
ARRAY symbolTable[1:100] OF STRING;
INTEGER tokenIndex, symbolIndex;
PROCEDURE checkSemantics;
BEGIN
symbolIndex := 1;
FOR tokenIndex := 1 STEP 1 UNTIL LENGTH(tokens) DO
IF tokens[tokenIndex] = "IDENTIFIER" THEN
symbolTable[symbolIndex] := tokens[tokenIndex];
symbolIndex := symbolIndex + 1;
ELSE IF tokens[tokenIndex] = "NUMBER" THEN
IF symbolTable[symbolIndex - 1] = "IDENTIFIER" THEN
PRINT("Valid Semantic");
ELSE
PRINT("Semantic Error: Undeclared Variable");
END;
END;
END;
END;
tokens[1] := "IDENTIFIER";
tokens[2] := "SYMBOL";
tokens[3] := "NUMBER";
checkSemantics;
END;Step 4: Intermediate Code Generation
Intermediate code generation involves creating an intermediate representation of the source code.
Example Code: Intermediate Code Generator in ALGOL
BEGIN
ARRAY tokens[1:100] OF STRING;
ARRAY intermediateCode[1:100] OF STRING;
INTEGER tokenIndex, codeIndex;
PROCEDURE generateIntermediateCode;
BEGIN
codeIndex := 1;
FOR tokenIndex := 1 STEP 1 UNTIL LENGTH(tokens) DO
IF tokens[tokenIndex] = "IDENTIFIER" THEN
intermediateCode[codeIndex] := "LOAD " & tokens[tokenIndex];
codeIndex := codeIndex + 1;
ELSE IF tokens[tokenIndex] = "NUMBER" THEN
intermediateCode[codeIndex] := "PUSH " & tokens[tokenIndex];
codeIndex := codeIndex + 1;
END;
END;
END;
tokens[1] := "IDENTIFIER";
tokens[2] := "SYMBOL";
tokens[3] := "NUMBER";
generateIntermediateCode;
FOR codeIndex := 1 STEP 1 UNTIL LENGTH(intermediateCode) DO
PRINT(intermediateCode[codeIndex]);
END;
END;Step 5: Code Generation
Code generation involves producing the target code from the intermediate representation.
Example Code: Code Generator in ALGOL
BEGIN
ARRAY intermediateCode[1:100] OF STRING;
ARRAY targetCode[1:100] OF STRING;
INTEGER codeIndex, targetIndex;
PROCEDURE generateTargetCode;
BEGIN
targetIndex := 1;
FOR codeIndex := 1 STEP 1 UNTIL LENGTH(intermediateCode) DO
IF intermediateCode[codeIndex] = "LOAD IDENTIFIER" THEN
targetCode[targetIndex] := "MOV R1, IDENTIFIER";
targetIndex := targetIndex + 1;
ELSE IF intermediateCode[codeIndex] = "PUSH NUMBER" THEN
targetCode[targetIndex] := "MOV R2, NUMBER";
targetIndex := targetIndex + 1;
END;
END;
END;
intermediateCode[1] := "LOAD IDENTIFIER";
intermediateCode[2] := "PUSH NUMBER";
generateTargetCode;
FOR targetIndex := 1 STEP 1 UNTIL LENGTH(targetCode) DO
PRINT(targetCode[targetIndex]);
END;
END;Step 6: Testing the Compiler
Testing involves running the compiler with simple programs to ensure it works correctly.
Example Code: Testing the Compiler
BEGIN
STRING sourceCode;
ARRAY tokens[1:100] OF STRING;
ARRAY intermediateCode[1:100] OF STRING;
ARRAY targetCode[1:100] OF STRING;
PROCEDURE compile(source: STRING);
BEGIN
sourceCode := source;
tokenize;
parseExpression;
checkSemantics;
generateIntermediateCode;
generateTargetCode;
END;
sourceCode := "int x = 10;";
compile(sourceCode);
END;Summary
In this section, we have covered the basic steps involved in building a simple compiler using ALGOL. We started with lexical analysis, moved on to syntax and semantic analysis, and then generated intermediate and target code. Finally, we tested our compiler with a simple program. This process provides a foundational understanding of how compilers work and prepares you for more advanced compiler construction techniques.
In the next section, we will explore case studies and projects to apply the concepts learned throughout this course.