《东南大学编译原理课件Cp5.pptVIP

Chapter 5 Syntax-Directed Translation Zhai Yuqing Zhou Xiaoyu Approaches to Implement Syntax-Directed Translation Semantic Static Semantic Dynamic Semantic E.g. Annotated parse tree for 3*5+4 Basic Idea(1) Guided by context-free grammar (Translating while or after parsing) Attaching attributes to the grammar symbols representing the program construction. The attributes represent anything we choose: a string, a number, a type, a memory location, etc. Basic Idea(2) The dependency between the attributes of relevant nodes in syntax tree. The value of an attribute at a parse tree node is computed by a “semantic rule” associated with the production used at that node. Basic Idea(3) Evaluation of the semantic rules may generate code, save information in a symbol table, issue error messages, or perform any other activities. Associating Semantic Rules with Productions Syntax-directed Definition. Translation Schemes. Syntax-directed definitions High-level specifications for translations. Hide implementation details. No need to specify explicitly the order in which translation takes place E?E1+E2 {E.val=E1.val+E2.val} Translation Scheme Indicate the order in which semantic rules are to be evaluated Allow some implementation details to be shown. Conceptual View of Syntax-Directed Translation (1) Conceptual View of Syntax-Directed Translation (2) Special cases of syntax-directed definitions can be implemented in a single pass by evaluating semantic rules during parsing, without explicitly constructing a parse tree or a graph showing dependencies between attributes. Syntax-directed Definition A generalization of a context-free grammar in which each grammar symbol has an associated set of attributes. Types of Attribute Synthesized Attribute The value of a synthesized attribute at a node is computed from the values of attributes at the children of that node in the parse tree. Inherited Attribute The value of an inherited attribute is computed from the values of attribut