Production rules for parsing the sample token stream in Java

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Example 42 Production rules for parsing the sample token stream
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stmt := if_stmt | assign_stmt if_stmt := IF LPAREN expr RPAREN stmt expr := lval assign_stmt := lval EQUAL expr SEMI lval = ID | arr_access arr_access := ID arr_index+ arr_idx := LBRACKET expr RBRACKET
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The parser performs a derivation by matching the token stream against the production rules If each symbol is connected to the symbol from which it was derived, a parse tree is formed Figure 42 shows a parse tree created using the production rules from Example 42 We have omitted terminal symbols that do not carry names (IF, LPAREN, RPAREN, etc), to make the salient features of the parse tree more obvious
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stmt if_stmt
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expr lval ID(ret) lval arr_access ID(mat) arr_idx expr lval ID(x)
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expr lval arr_idx expr lval ID(y) ID(END_VAL)
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Figure 42 A parse tree derived from the sequence of tokens
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If you would like to build your own parser, the venerable UNIX programs Lex and Yacc have been the traditional way to start in C; if you can choose any language you like, we prefer JavaCC (https://javaccdevjavanet) because it s all-around easier to use and comes complete with a grammar for parsing Java For C and C++, the Edison Design Group (EDG) (http://wwwedgcom) sells an excellent front end EDG sometimes makes its toolkit available for free for academic use The open source Elsa C and C++ parser from UC Berkeley is another good option (http:// wwwcsberkeleyedu/~smcpeak/elkhound/sources/elsa/) Abstract Syntax It is feasible to do signi cant analysis on a parse tree, and certain types of stylistic checks are best performed on a parse tree because it contains the most direct representation of the code just as the programmer wrote it However, performing complex analysis on a parse tree can be inconvenient for a number of reasons The nodes in the tree are derived directly from the grammar s production rules, and those rules can introduce nonterminal
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symbols that exist purely for the purpose of making parsing easy and nonambiguous, rather than for the purpose of producing an easily understood tree; it is generally better to abstract away both the details of the grammar and the syntactic sugar present in the program text A data structure that does these things is called an abstract syntax tree (AST) The purpose of the AST is to provide a standardized version of the program suitable for later analysis The AST is usually built by associating tree construction code with the grammar s production rules Figure 43 shows an AST for our example Notice that the if statement now has an empty else branch, the predicate tested by the if is now an explicit comparison to zero (the behavior called for by C), and array access is uniformly represented as a binary operation
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COMPARE_EQ arr_idx ID(ret) 0 arr_idx ID(mat) ID(x) ID(y) ID(END_VAL)
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Figure 43 An abstract syntax tree
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Depending on the needs of the system, the AST can contain a more limited number of constructs than the source language For example, method calls might be converted to function calls, or for and do loops might be converted to while loops Signi cant simpli cation of the program in this fashion is called lowering Languages that are closely related, such as C and C++, can be lowered into the same AST format, although such lowering runs the risk of distorting the programmer s intent Languages that are syntactically similar, such as C++ and Java, might share many of the same AST node types but will almost undoubtedly have special kinds of nodes for features found in only one language
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Semantic Analysis As the AST is being built, the tool builds a symbol table alongside it For each identi er in the program, the symbol table associates the identi er with its type and a pointer to its declaration or de nition With the AST and the symbol table, the tool is now equipped to perform type checking A static analysis tool might not be required to report typechecking errors the way a compiler does, but type information is critically important for the analysis of an object-oriented language because the type of an object determines the set of methods that the object can invoke Furthermore, it is usually desirable to at least convert implicit type conversions in the source text into explicit type conversions in the AST For these reasons, an advanced static analysis tool has to do just as much work related to type checking as a compiler does In the compiler world, symbol resolution and type checking are referred to as semantic analysis because the compiler is attributing meaning to the symbols found in the program Static analysis tools that use these data structures have a distinct advantage over tools that do not For example, they can correctly interpret the meaning of overloaded operators in C++ or determine that a Java method named doPost() is, in fact, part of an implementation of HttpServlet These capabilities enable a tool to perform useful checks on the structure of the program We use the term structural analysis for these kinds of checks For more, see the sidebar Checking Structural Rules After semantic analysis, compilers and more advanced static analysis tools part ways A modern compiler uses the AST and the symbol and type information to generate an intermediate representation, a generic version of machine code that is suitable for optimization and then conversion into platform-speci c object code The path for static analysis tools is less clear cut Depending on the type of analysis to be performed, a static analysis tool might perform additional transformations on the AST or might generate its own variety of intermediate representation suitable to its needs If a static analysis tool uses its own intermediate representation, it generally allows for at least assignment, branching, looping, and function calls (although it is possible to handle function calls in a variety of ways we discuss function calls in the context of interprocedural analysis in Section 44) The intermediate representation that a static analysis tool uses is usually a higher-level view of the program than the intermediate representation that a compiler uses For example, a C language compiler likely will convert all references to structure elds into byte offsets into the structure for its intermediate representation, while a static analysis tool more likely will continue to refer to structure elds by their names
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