A programming language requires two major definitions A simple one pass compiler
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1 A programming language requires two major definitions A simple one pass compiler [Syntax: what the language looks like A context-free grammar written in BNF (Backus-Naur Form) usually suffices. [Semantics: what the language means More difficult to describe and usually informal descriptions and examples are used. Infix, prefix and postfix notations. Context-free grammars [Infix: (a*b+c/(d-e))/f [Prefix: /+*ab/c-def [Postfix: ab*cde-/+f/ [Can specify the language syntax in an elegant way [Can help to guide the translation of programs, a syntax-directed translation. [Can usually be directly mapped to a parser generation tool, such as YACC (Yet Another Compiler-Compiler).
2 Context-free grammars (cont.) Language [A production rule stmt if ( expr ) stmt else stmt [Four components Terminal tokens: if, else, (, ). Nonterminals: stmt, expr. Production rules, like above. A start nonterminal, listed first. [The language defined by a grammar is the collection of all possible strings resulted from the application of the production rules of the grammar in any order and possibly infinitely, starting from the start symbol. Parse tree Parse tree (cont.) [A parse tree shows how a string in the language is derived from the grammar. [Properties The root of the tree is labeled by the start symbol. Each leaf is a terminal or ε. All interior nodes are nonterminals. Each nonterminal node reflects a production. [The process of finding a parse tree for a given string is called parsing. [Ambiguous grammar: the same string of tokens can be generated by more than one parse tree. [Examples list list + digit list - digit 0 9. list digit + list digit - list 0 9. String string + string string string 0 9.
3 Associativity of operators Precedence of operators [Left association: operator to the left of an operand takes it first. Examples: expressions, statements. [Right association: operator to the right of an operand takes it first. Examples: most unary operators and assignments in C. [Requires a grammar with nonterminals associated with each level of precedence. term applies * and / on factors expr applies + and on terms expr expr + term expr term term term term * factor term / factor factor factor digit ( expr ) Precedence of operators (cont.) Syntax-directed definitions [Summary: Left or right recursion determines associativity of operators Layer of grammar nonterminal hierarchy determines precedence. [Use a context-free grammar to specify the syntactic structure of the input. [Associate necessary attributes with each grammar symbol. [Use a set of semantic rules to define how attributes are computed with each production rule.
4 Attributes [Synthesized: they can be computed by the attributes of the children of a node. [Inherited: they are determined by the attributes of the parent node. [Hybrid: they are determined by both. [A parse tree showing attributes at each node is called an annotated parse tree. Depth-first traversals [A traversal of a tree is an organized manner to visit each node of the tree once. [Depth-first traversals visit children first before the parent attributes are computed. [Another kind of traversal are breadth-first traversals that visit all children nodes at once, then all grandchildren nodes, and so forth in a layer by layer basis. Translation schemes Simple syntax-directed definitions [Syntax-directed definitions have the grammar production rules and their associated semantic rules listed separately. Therefore the semantic evaluation order is undefined. [Translation schemes allow specific semantic actions to be mixed with production rules and become part of the production rules. Therefore, the order of evaluation of semantic rules is explicitly shown. [The order of translation in the output for each nonterminal corresponds to the order of these nonterminals in the production rules. [Simple translation schemes can be used to implement simple syntaxdirected translations, where output can be generated directly during traversal. [Therefore, no attributes need to be passed from nodes to nodes. Output are produced incrementally without intermediate storages.
5 Parsing Predictive parsing [Most programming language parsers require just a single linear left-toright scan over the input, looking ahead one token at a time. [Top-Down parsing starts with the start symbol, and build the parse tree by expanding matching productions determined by the input. [Bottom-up parsing starts by collecting input tokens until a suitable production can be used to reduce the input into an internal nonterminal node. [Recursive-descent parsing is a topdown method of syntax analysis in which a set of matching recursive procedures are written for the set of nonterminals in the grammar. [A recursive-descent parsing that never needs to backtrack the production rules selected during parsing is called predictive parsing. [For every A α and A β, First(α) and First(β) must be disjoint for predictive parsing to work. Left recursion Tail recursion [Left recursion can make a recursive-descent parser run into an infinite loop. [To fix this, we need to replace it with a right recursion counterpart. A Aα β can be modified as A βr, R αr ε. [The the last statement in a procedure is a recursive call of the same procedure. [Tail recursion can be replaced by iteration to speed up the execution.
6 Lexical analysis Symbol table [Convert input into a stream of syntactic tokens. [Lexeme: a sequence of input characters that comprises a token. [Removal of white space and comments. [Grouping literal constants. [Recognizing identifiers and keywords. [Usually called on demand by the parser when new tokens are needed. [To keep track of information about various terminal and nonterminals. [Examples: reserved word tokens, identifier names and their addresses, symbolic constant values, etc. [Reserved words should be inserted into a symbol table before parsing begins. [Two basic functions: lookup() and insert(). L-values and R-values Homework [The same identifier on the left and right sides of an assignment has different meanings. x=x+1; [The left side specifies where the value is to be stored, while the right side specifies the value itself. [That is, l-values are location addresses, while r-values are simple values. [2.3, 2.4, 2.7, 2.12, 2.14, 2.16(a), 2.16(b).
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