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Abstract Syntax Tree AST Processing Syntax Tree Abstract Syntax Tree AST Processing Compiling Techniques Lecture 7: Abstract Syntax Christophe Dubach 3 October 2017 Christophe Dubach Compiling Techniques Syntax Tree Abstract Syntax Tree AST Processing Table of contents 1 Syntax Tree Semantic Actions Examples Abstract Grammar 2 Abstract Syntax Tree Internal Representation AST Builder 3 AST Processing Object-Oriented Processing Visitor Processing AST Visualisation Christophe Dubach Compiling Techniques Syntax Tree Semantic Actions Abstract Syntax Tree Examples AST Processing Abstract Grammar A parser does more than simply recognising syntax. It can: evaluate code (interpreter) emit code (simple compiler) build an internal representation of the program (multi-pass compiler) In general, a parser performs semantic actions: recursive descent parser: integrate the actions with the parsing functions bottom-up parser (automatically generated): add actions to the grammar Christophe Dubach Compiling Techniques Syntax Tree Semantic Actions Abstract Syntax Tree Examples AST Processing Abstract Grammar Syntax Tree In a multi-pass compiler, the parser builds a syntax tree which is used by the subsequent passes A syntax tree can be: a concrete syntax tree (or parse tree) if it directly corresponds to the context-free grammar an abstract syntax tree if it corresponds to a simplified (or abstract) grammar The abstract syntax tree (AST) is usually used in compilers. Christophe Dubach Compiling Techniques Syntax Tree Semantic Actions Abstract Syntax Tree Examples AST Processing Abstract Grammar Example: Concrete Syntax Tree (Parse Tree) Example: CFG for arithmetic expressions (EBNF form) Expr ::= Term ( ('+'j ' − ') Term ) ∗ Term ::= Factor ( (' ∗ ' j '/') Factor) ∗ Factor ::= number j ' ( ' Expr ' ) ' After removal of EBNF syntax Expr ::= Term Terms Terms ::= ('+' j ' − ') Term Terms j Term ::= Factor Factors Factors ::= (' ∗ ' j '/') Factor Factors j Factor ::= number j ' ( ' Expr ' ) ' After further simplification Expr ::= Term (('+' j ' − ') Expr j ) Term ::= Factor ((' ∗ ' j ' / ' ) Term j ) Factor ::= number j ' ( ' Expr ' ) ' Christophe Dubach Compiling Techniques Syntax Tree Semantic Actions Abstract Syntax Tree Examples AST Processing Abstract Grammar Example: Concrete Syntax Tree (Parse Tree) CFG for arithmetic expressions Expr ::= Term (('+' j ' − ') Expr j ) Term ::= Factor ((' ∗ ' j ' / ' ) Term j ) Factor ::= number j ' ( ' Expr ' ) ' Concrete Syntax Tree for 5 ∗ 3 Term Factor Term '∗' The concrete syntax tree contains number Factor a lot of unnecessary information. '5' number '3' Christophe Dubach Compiling Techniques Syntax Tree Semantic Actions Abstract Syntax Tree Examples AST Processing Abstract Grammar It is possible to simplify the concrete syntax tree to remove the redundant information. For instance parenthesis are not necessary. Exercise 1 Write the concrete syntax tree for 3 ∗ (4 + 5) 2 Simplify the tree. Christophe Dubach Compiling Techniques Syntax Tree Semantic Actions Abstract Syntax Tree Examples AST Processing Abstract Grammar Abstract Grammar These simplifications leads to a new simpler context-free grammar caller Abstract Grammar. Example: abstract grammar for arithmetic expressions Expr ::= BinOp j i n t L i t e r a l BinOp ::= Expr Op Expr Op ::= add j sub j mul j d i v 5 ∗ 3 BinOp intLiteral(5) intLiteral(3) mul This is called an Abstract Syntax Tree Christophe Dubach Compiling Techniques Syntax Tree Semantic Actions Abstract Syntax Tree Examples AST Processing Abstract Grammar Example: abstract grammar for arithmetic expressions Expr ::= BinOp j i n t L i t e r a l BinOp ::= Expr Op Expr Op ::= add j sub j mul j d i v Note that for given concrete grammar, there exist numerous abstract grammar: Expr ::= AddOp j SubOp j MulOp j DivOp j i n t L i t e r a l AddOp ::= Expr add Expr SubOp ::= Expr sub Expr MulOp ::= Expr mul Expr DivOp ::= Expr div Expr We pick the most suitable grammar for the compiler. Christophe Dubach Compiling Techniques Syntax Tree Internal Representation Abstract Syntax Tree AST Builder AST Processing Abstract Syntax Tree The Abstract Syntax Tree (AST) forms the main intermediate representation of the compiler's front-end. For each non-terminal or terminal in the abstract grammar, we define a class. If a non-terminal has any alternative on the rhs (right hand side), then the class is abstract (cannot instantiate it). The terminal or non-terminal appearing on the rhs are subclasses of the non-terminal on the lhs. The sub-trees are represented as instance variable in the class. Each non-abstract class has a unique constructor. If a terminal does not store any information, then we can use an Enum type in Java instead of a class. Christophe Dubach Compiling Techniques Syntax Tree Internal Representation Abstract Syntax Tree AST Builder AST Processing Example: abstract grammar for arithmetic expressions Expr ::= BinOp j i n t L i t e r a l BinOp ::= Expr Op Expr Op ::= add j sub j mul j d i v Corresponding Java Classes abstract class Expr f g c l a s s I n t L i t e r a l extends Expr f i n t i ; IntLiteral( i n t i ) f ... g g c l a s s BinOp extends Expr f Op op ; Expr l h s ; Expr r h s ; BinOp(Op op, Expr lhs , Expr rhs) f ... g g enum Op fADD, SUB, MUL, DIVg Christophe Dubach Compiling Techniques Syntax Tree Internal Representation Abstract Syntax Tree AST Builder AST Processing CFG for arithmetic expressions Expr ::= Term (('+' j ' − ') Expr j ) Term ::= Factor ((' ∗ ' j ' / ' ) Term j ) Factor ::= number j ' ( ' Expr ' ) ' Current Parser (class) Expr parseExpr() f parseTerm (); i f (accept(PLUS j MINUS) ) nextToken (); Expr parseFactor() f parseExpr (); i f (accept(LPAR)) g parseExpr (); expect (RPAR); Expr parseTerm() f e l s e parseFactor (); e x p e c t (NUMBER) ; i f (accept(TIMES j DIV ) ) g nextToken (); parseTerm (); g Christophe Dubach Compiling Techniques Syntax Tree Internal Representation Abstract Syntax Tree AST Builder AST Processing AST building (modified Parser) Expr parseExpr() f Current Parser Expr lhs = parseTerm(); i f (accept(PLUS j MINUS ) ) v o i d parseExpr() f Op op ; parseTerm (); i f ( token == PLUS) i f (accept(PLUS j MINUS ) ) op = ADD; nextToken (); e l s e // token == MINUS parseExpr (); op = SUB ; g nextToken (); Expr rhs = parseExpr(); r e t u r n new BinOp(op, lhs , rhs); r e t u r n l h s ; g Christophe Dubach Compiling Techniques Syntax Tree Internal Representation Abstract Syntax Tree AST Builder AST Processing AST building (modified Parser) Expr parseTerm() f Current Parser Expr lhs = parseFactor(); i f (accept(TIMES j DIV ) ) v o i d parseTerm() f Op op ; parseFactor (); i f (token == TIMES) i f (accept(TIMES j DIV ) ) op = MUL; nextToken (); e l s e // token == DIV parseTerm (); op = DIV ; g nextToken (); Expr rhs = parseTerm(); r e t u r n new BinOp(op, lhs , rhs); r e t u r n l h s ; g Christophe Dubach Compiling Techniques Syntax Tree Internal Representation Abstract Syntax Tree AST Builder AST Processing AST building (modified Parser) Expr parseFactor() f i f (accept(LPAR)) Current Parser Expr e = parseExpr(); expect (RPAR); v o i d parseFactor() f r e t u r n e ; i f (accept(LPAR)) e l s e parseExpr (); IntLiteral il = parseNumber(); expect (RPAR); r e t u r n i l ; e l s e g e x p e c t (NUMBER) ; g IntLiteral parseNumber() f Token n = expect(NUMBER); i n t i = Integer.parseInt(n.data); r e t u r n new IntLiteral(i); g Christophe Dubach Compiling Techniques Syntax Tree Object-Oriented Processing Abstract Syntax Tree Visitor Processing AST Processing AST Visualisation Compiler Pass AST pass An AST pass is an action that process the AST in a single traversal. A pass can for instance: assign a type to each node of the AST perform an optimisation generate code It is important to ensure that the different passes can access the AST in a flexible way. An inefficient solution would be to use instanceof to find the type of syntax node Example i f ( t r e e i n s t a n c e o f IntLiteral) ((IntLiteral)tree).i; Christophe Dubach Compiling Techniques Syntax Tree Object-Oriented Processing Abstract Syntax Tree Visitor Processing AST Processing AST Visualisation Two Ways to Process an AST Object-Oriented Processing Visitor Processing Christophe Dubach Compiling Techniques Syntax Tree Object-Oriented Processing Abstract Syntax Tree Visitor Processing AST Processing AST Visualisation Object-Oriented Processing Using this technique, a compiler pass is represented by a function f () in each of the AST classes. The method is abstract if the class is abstract To process an instance of an AST class e, we simply call e. f (). The exact behaviour will depends on the concrete class implementations Example for the arithmetic expression A pass to print the AST: String toStr () A pass to evaluate the AST: int eval () Christophe Dubach Compiling Techniques Syntax Tree Object-Oriented Processing Abstract Syntax Tree Visitor Processing AST Processing AST Visualisation abstract class Expr f a b s t r a c t String toStr(); abstract int e v a l ( ) ; g c l a s s I n t L i t e r a l extends Expr f i n t i ; String toStr() f r e t u r n ""+i ; g i n t e v a l ( ) f r e t u r n i ; g g c l a s s BinOp extends Expr f Op op ; Expr l h s ; Expr r h s ; String toStr() f r e t u r n lhs.toStr() + op.name() + rhs.toStr(); g i n t e v a l ( ) f s w i t c h ( op ) f case ADD: lhs.eval() + rhs.eval(); break ; case SUB: lhs.eval() − rhs.eval(); break ; case MUL: lhs.eval() ∗ rhs.eval(); break ; case DIV: lhs.eval() / rhs.eval(); break ; g g g Christophe Dubach Compiling Techniques Syntax Tree Object-Oriented Processing Abstract Syntax Tree Visitor Processing AST Processing AST Visualisation Main class c l a s s Main f v o i d main(String[] args) f Expr expr = ExprParser.parse(some i n p u t f i l e ) ; String str = expr.toStr(); i n t result = expr.eval(); g g Christophe Dubach Compiling Techniques Syntax Tree Object-Oriented Processing Abstract Syntax Tree Visitor Processing AST Processing AST Visualisation Visitor Processing With this technique, all the methods from a pass are grouped in a visitor.
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