Syntax Directed Analysis Chapter 5 1 Compiler Construction Syntax Directed Analysis
Syntax-Directed Definitions • Generalizes a context-free grammar • Each grammar symbol has an associated set of attributes – Synthesized attributes – Inherited attributes • The node for a grammar symbol in a parse tree holds information (attributes) 2 Compiler Construction Syntax Directed Analysis
Syntax-directed Translation • Syntax-directed: Augment the productions of a CFG with semantic rules or actions These rules and/or actions define the values of attributes and how to compute them • The augmented grammar is called an attribute grammar • Often a language specification provides the CFG for a language and an English language description of the semantics A compiler writer must deduce an attribute grammar from the English description 3 Compiler Construction Syntax Directed Analysis
Attributes • Value can be – string – number – type – etc. • Values in parse tree node depend on the grammar production responsible for that node 4 Compiler Construction Syntax Directed Analysis
Example: Attributes of an Identifier • Name (lexeme from scanner) • Type – Basic type: int , float , etc. – Array ∗ Dimension ∗ Type – Structured type ( struct , class ): ∗ Field name ∗ Type of fi eld – Procedure (function, method, etc.): ∗ Number and types of parameters ∗ Return type ∗ Size of stack frame • Scope 5 Compiler Construction Syntax Directed Analysis
Synthesized vs. Inherited Attributes • Synthesized – Computed from attributes of child nodes • Inherited – Computed from attributes of sibling nodes and parent node = position + initial * rate 60 6 Compiler Construction Syntax Directed Analysis
Semantic Rules A semantic rule can be associated with a production: Production Semantic Rule E → E 1 + T E . val ← E 1 . val + T . val E → T E . val ← T . val T → T 1 ∗ F T . val ← T 1 . val × F . val T → F T . val ← F . val F → ( E ) F . val ← E . val F → number F . val ← number . val 7 Compiler Construction Syntax Directed Analysis
Example: Declarations Productions: → decl decl type id list ; → id , id list | id id list type id_list ; τ 1 | ... | τ n → type τ id id_list id id_list Semantic Rules: ... id → decl type id list ; { id list.tval ← type.tval } id_list → id , id list 1 | id id list id { id . tval ← id list.tval ; id list 1 . tval ← id list.tval } 8 Compiler Construction Syntax Directed Analysis
Dependencies • Semantic rules set up dependencies among attributes • These dependencies can be represented by a directed graph = position + initial * rate 60 9 Compiler Construction Syntax Directed Analysis
Annotated Parse Tree • Each node of the parse tree has attributes • Such a parse tree is said to be annotated or decorated 10 Compiler Construction Syntax Directed Analysis
Syntax-Directed Definition Each production A → α has an associated a set of semantic rules of the form b = f ( c 1 , c 2 , c 3 ,..., c k ) 11 Compiler Construction Syntax Directed Analysis
Synthesized Attribute In a production of the form A → α an associated semantic rule has the form b = f ( c 1 , c 2 ,..., c k ) • b is a synthesized attribute of A • c 1 , c 2 ,..., c k are attributes belonging to the grammar symbols of the production 12 Compiler Construction Syntax Directed Analysis
Inherited Attribute In a production of the form A → α an associated semantic rule has the form b = f ( c 1 , c 2 ,..., c k ) • b is an inherited attribute of one of the grammar symbols on the right side of the production • c 1 , c 2 ,..., c k are attributes belonging to the grammar symbols of the production 13 Compiler Construction Syntax Directed Analysis
Attributed Evaluation • Unrestricted definition – No restrictions on attribute dependency – Perform a topological sort on the attribute dependency graph and evaluate in topological order – Expensive to evaluate • S-attributed definition – Synthesized attributes only – Attributes may be evaluated bottom-up – Evaluated very efficiently • L-attributed definition – Permits both synthesized and some inherited attributes Inherited attributes restricted to left siblings – Attributes may be evaluated depth-first, left to right Any L-attributed definition may be converted to an S-attributed definition 14 Compiler Construction Syntax Directed Analysis
S-attributed Definition • A syntax-directed definition that uses synthesized attributes exclusively • S stands for synthesized attributes • Attributes are computed from the bottom up • An LR parser can be adapted to implement an S-attributed definition • See example on Pages 281-282 • Do a postorder traversal of the syntax tree to evaluate the attributes 15 Compiler Construction Syntax Directed Analysis
S-attributed Evaluation • All semantic actions follow the right-hand side of a grammar rule • Yacc uses a value stack that parallel’s the LALR parser’s state stack – Values of synthesized attributes are stored on the stack – For a reduction: ∗ Pop attributes of children off the stack ∗ Compute synthesized attributes for this rule ∗ Push computed synthesized attributes on to the stack 16 Compiler Construction Syntax Directed Analysis
S-attributed Evaluation: Yacc Example Expression: NUMBER { $$ = new NumberExpressionNode( yylval.token_info.lexeme, yylval.token_info.line_number); } | Name LPAREN ArgList RPAREN { $$ = new CallExpressionNode( (NameNode *) $1, (ArgListNode *) $3); } | Expression SumOp Expression %prec PLUS { $$ = new SumOpExpressionNode( (ExpressionNode *) $1, (SumOpNode *) $2, (ExpressionNode *) $3); } | Expression ProductOp Expression %prec TIMES { $$ = new ProductOpExpressionNode( (ExpressionNode *) $1, (ProductOpNode *) $2, (ExpressionNode *) $3); } ; 17 Compiler Construction Syntax Directed Analysis
Inherited Attributes • Convenient for expressing the dependence of a programming language construct on the context in which it appears • It is always possible to rewrite a syntax-directed definition to use only synthesized attributes, but it is often more convenient to use inherited attributes • See example on Page 283 18 Compiler Construction Syntax Directed Analysis
L-attributed Grammars Given a production of the form A → X 1 X 2 ... X n an inherited attribute of X j for 1 ≤ j ≤ n is a function of 1. the attributes of the grammar symbols X 1 , X 2 ,..., X j − 1 to the left of X j and 2. the inherited attributes of left side non-terminal A • S-attributed grammars are a subset of these • Semantic actions amy be placed anywhere on the right-hand side of a production • Evaluate with a depth-first traversal 19 Compiler Construction Syntax Directed Analysis
Code Generation from L-attributes Grammar rule: while stmt → while ( expr ) stmt L-attributed grammar rule: while stmt → while ( { fprintf(fout, "__while_top%u:\n", label++); } ) expr { fprintf(fout, "lw $a1 %s\n", temp); fprintf(fout, "beqz $a1 __end_while%u\n", label); } stmt { fprintf(fout, "j __while_top%u\n", label); fprintf(fout, "__end_while%u:\n", label); } 20 Compiler Construction Syntax Directed Analysis
Converting from L-attributed to S-attributed Definition Recall that • In an S-attributed definition the semantic action(s) must appear after all the grammar symbols on the right-hand side of a production • In an L-attributed definition the semantic action(s) may appear anywhere on the right-hand side of a production • Replace a rule such as B { print("Make A") } C → A with two rules → A B D C ǫ { print("Make A") } → D 21 Compiler Construction Syntax Directed Analysis
Evaluation of Attributes When you visit a node evaluate its inherited attributes preorder and its synthesized attributes postorder AttributeEvaluate( Node v ) { for each child w of v from left to right do { Evaluate the inherited attributes of w ; AttributeEvaluate( w ); } Evaluate the synthesized attributes of v ; } In general, inherited attributes are difficult to deal with, especially for bottom-up parsers 22 Compiler Construction Syntax Directed Analysis
Intermediate Representations • Useful for syntax-directed translation • Decouples translation from parsing • Grammars often do not accurately reflect natural program structure • An abstract syntax tree (AST) is an example 23 Compiler Construction Syntax Directed Analysis
Abstract Syntax Tree A parse tree with non-essential information removed (a + b) * c Parse Tree AST E * + id E E * id id ( ) E id E E + id id 24 Compiler Construction Syntax Directed Analysis
AST vs. Parse Tree • Operators are promoted from leaves to internal nodes • Chains of single productions are collapsed • Syntactic details like parentheses, semi-colons, and commas are omitted • Subtree lists are flattened 25 Compiler Construction Syntax Directed Analysis
AST Construction: Operator Promotion Eliminate operator leaf nodes and promote them to interior nodes E * E E E E * 26 Compiler Construction Syntax Directed Analysis
AST Construction: Collapse Child Chains Collapse chains of single children, replacing the chains with their leaf nodes E E E E + id + id id id 27 Compiler Construction Syntax Directed Analysis
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