Today Winter Compiler Construction T5 AST Mooly Sagiv and Roman Manevich School of Computer Science Tel-Aviv University

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1 Winter Compiler Construction T5 AST Mooly Sagiv and Roman Manevich School of Computer Science Tel-Aviv University Today ic IC Language Today: EBNF Lexical Analysis Syntax Analysis Parsing AST construction AST traversal Visitor pattern AST Symbol Inter. Code Table Rep. Generation etc. (IR) Next week: Annotating ASTs Symbol tables Type-checking exe Executable code EBNF Extended Backus Naur form Extends BNF with regular expressions R* R+ (ABC) R? [R] [R] stands for optional part (same as R?) Not supported by CUP Translate EBNF rules to BNF list ::= x* list ::= x list x ::= y [z] x ::= y y z Abstract Syntax Trees Intermediate program representation Defines a tree What is the root of the AST? Preserves program hierarchy Node types defined by class hierarchy Generated by parser Keywords and punctuation symbols not stored (not relevant once parse tree exists) Provides clear interface to other compiler phases 4 Partial AST hierarchy for IC ASTNode Eclipse type hierarchy view Program ICClass Statement Method Field VirtualMethod StaticMethod LibraryMethod Expression Literal New Call Statement StatementBlock LocalVariable If While declaration 5 6 1

2 AST node contents abstract class ASTNode { int getline() used for error reporting Actions part of IC.cup non terminal Program program non terminal ICClass class non terminal List<ICClass> program ::= :cl {: RESULT = new Program(getLine(), cl) : class Program extends ASTNode { List<ICClass> classes class ICClass extends ASTNode { String name List<Field> fields List<Method> methods // ICClass superclass Generics 7 ::= class:c {: RESULT = new LinkedList<ICClass>() RESULT.add(c) : :cl class:c {: cl.add(c) RESULT = cl : Symbol stack class object stack c cl Reduce ::= :cl class:c {: RESULT = cl.add(c) : pop class pop push In reality, use value field of Symbol object stack Symbol stack RESULT 8 Actions part of IC.cup AST traversal non terminal Program program non terminal ICClass class non terminal List<ICClass> program ::= :cl {: RESULT = new Program(getLine(), cl) : ::= class:c {: RESULT = new LinkedList<ICClass>() RESULT.add(c) : :cl class:c {: cl.add(c) RESULT = cl : Once AST stable want to operate on tree AST traversal for type-checking AST traversal for transformation (IR) AST traversal for pretty-printing (-dump-ast) Each operation in separate pass Reduce ::= :cl class:c {: RESULT = cl.add(c) : object stack Symbol stack RESULT 9 10 Non-Object Oriented approach prettyprint(astnode node) { if (node instanceof Program) { Program prog = (Program) node for (ICClass icc : prog.classes) { prettyprint(icc) else if (node instanceof ICClass) { ICClass icc = (ICClass) node printclass(icc) else if (node instanceof BinaryExpression) { BinaryExpression be = (BinaryExpression) node prettyprint(be.lhs) System.out.println(be.operator) prettyprint(be.rhs) Messy code instanceof + down-casting error-prone Not extensible 11 Visitor Pattern Separate operations on objects of a data structure from object representation Each operation (pass) may be implemented as separate or Use double-dispatch to find right method for object Instance of a design pattern Recommended book Design Patterns / Gang of Four 1

3 Single dispatch - polymorphism What if we need more operations? annotation void op() { void op() { void op() { conceptually, one-dimensional table op index A B C 1 void op1() { void op() { void op() { void op1() { void op() { void op() { void op1() { void op() { void op() { Want to separate complicated operations from data structures 14 What if we need more operations? Visitor pattern in Java class op1 { class op { class op { Problem: OO languages support only single-polymorphism. We seem to need double-polymorphism A x v.(this) v.(this) v.(this) interface Visitor { (A a) (B c) (C c) (A a) { (B c) { (C c) { class op implements Visitor { (A a) { (B c) { (C c) { class op implements Visitor { (A a) { (B c) { (C c) { Double dispatch example Double dispatch example class B { // always calls (B b) v.(this) nd dispatch Visitor v = new op1() // op1// A x = new B() // x can be A/B/C x.accept(v) (A a) { (B b) { class B { // always calls (B b) v.(this) nd dispatch Visitor v = new op1() // op1// A x = new B() // x can be A/B/C x.accept(v) (A a) { (B b) { x.accept(v) 17 v.(this) nd dispatch A B C op1 op1.(b b) op op Visitor pattern conceptually implements two-dimensional table 18

4 Straight Line Program example Printing or example enhanced for loop public class PrettyPrinter implements Visitor { public void print(astnode root) { root.accept(this) prog stmt_list stmt_list stmt stmt_list stmt_list stmt stmt var = expr stmt print(expr) expr expr + expr expr expr-expr expr expr * expr expr expr / expr expr -expr expr ( expr ) expr number expr readi() expr var PrintStmt AssignStmt StmtList Stmt ASTNode Expr BinaryOpExpr UnaryOpExpr NumberExpr ReadIExpr VarExpr (Code available on web site. Demonstrates scanning, parsing, AST + ors) 19 interface Visitor { void (StmtList stmts) void (Stmt stmt) void (PrintStmt stmt) void (AssignStmt stmt) void (Expr expr) void (ReadIExpr expr) void (VarExpr expr) void (NumberExpr expr) void (UnaryOpExpr expr) void (BinaryOpExpr expr) public void (StmtList stmts) { for (Stmt s : stmts.statements) { s.accept(this) System.out.println() // x = *7 public void (AssignStmt stmt) { stmt.varexpr.accept(this) System.out.print("=") stmt.rhs.accept(this) System.out.print("") // x public void (VarExpr expr) { System.out.print(expr.name) // *7 public void (BinaryOpExpr expr) { expr.lhs.accept(this) System.out.print(expr.op) expr.rhs.accept(this) 0 Visitor variations Evaluating or example interface PropagatingVisitor { /** Visits a statement node with a given * context object (book-keeping) * and returns the result * of the computation on this node. Object (Stmt st, Object context) Object (Expr e, Object context) Object (BinaryOpExpr e, Object context) Propagate values down the AST (and back) 1 public class SLPEvaluator implements PropagatingVisitor { public void evaluate(astnode root) { root.accept(this) class Environment { Integer get(varexpr ve) { void update(varexpr ve, int value) { /** x = *7 public Object (AssignStmt stmt, Object env) { Expr rhs = stmt.rhs Integer expressionvalue = (Integer) rhs.accept(this, env) VarExpr var = stmt.varexpr ((Environment)env).update(var, expressionvalue) return null /** expressions like *7 and *y public Object (BinaryOpExpr expr, Object env) { Integer lhsvalue = (Integer) expr.lhs.accept(this, env) Integer rhsvalue = (Integer) expr.rhs.accept(this, env) int result switch (expr.op) { case PLUS: result = lhsvalue.intvalue() + rhsvalue.intvalue() return new Integer(result) AST traversal 1++x 6 root BinaryOpExpr + left right (lhs) class BinaryOpExpr extends Expression { Object return v.(this) Expression lhs, rhs class NumberExpr extends Expression { Object return v.(this) int val Visitor + Generics interface PropagatingVisitor<DownType,UpType> { UpType (Stmt st, DownType d) UpType (Expr e, DownType d) UpType (VarExpr ve, DownType d) public class SLPEvaluator { BinaryOpExpr + (rhs) public Object (BinaryOpExpr e, Object env) { left right Integer lhsvalue=(integer)e.lhs.accept(this,env) 1 Integer rhsvalue=(integer)e.rhs.accept(this,env) int result (lhs) (rhs) switch (expr.op) { case PLUS: result=lhsvalue.intvalue()+rhsvalue.intvalue() NumberExpr NumberExpr VarExpr value = 1 value= name = x return new Integer(result) public Object (NumberExpr e,object env) { return e.value SLPEvaluator ev = new SLPEvaluator() Integer result = (Integer)root.accept(ev) public Object (VarExpr e, Object env) { return ((Environment)env).get(e) (alternative: let accept do tree walking) public class SLPEvaluator implements PropagatingVisitor<Environment,Integer> { public Integer (VarExpr expr, Environment env) { return env.get(expr) 4 4

5 See you next week 5 5