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These optim izations typically are applied either to source code or to a high-level intermediate code that preserves loop structure and the sequence in which operations are performed and that has array accesses in essentially their source-code form. The correspondence between letters and code levels is as follows: The letters at the left in the diagram correspond to the levels o f code appropriate for the corresponding optim izations. Other orders are possible, and the exam ples o f real-world compilers in Chapter 21 present several alternatives, though none o f them includes all o f the optim iza tions in this diagram. For example, you could have a syntax tree such as for (represented inline), where for is an operator, and the elements inside the square brackets are its children (representing C's for syntax) - also composed out of operators etc.ĪSTs are usually generated by compilers in the syntax analysis (parsing) phase as well, and are used later for semantic analysis, intermediate representation, code generation, etc.Order o f Optimizations This flowchart represents a recommended order for performing optim izations in an aggres sive optimizing compiler. Note that the operators themselves are programming constructs in a given language, and don't have to be actual computational operators (like + is): for loops would also be treated in this way. The difference here is that nonterminals and terminals don't play a role, as ASTs don't deal with grammars and string generation, but programming constructs, and thus they represent relationships between such constructs, and not the ways they are generated by a grammar. For example, the syntax tree for the expression i + 9 would have the operator + as root, the variable i as the operator's left child, and the number 9 as the right child. The trees of programming constructs such as expressions, flow control statements, etc - grouped into operators (interior nodes) and operands (leaves). Here's a graphical representation of a parse tree for the expression 9 - 5 + 2 (note the placement of the terminals in the tree and the actual symbols from the expression string):ĪSTs represent the syntactic structure of the some code. They can be used for syntax-directed translation via syntax-directed definitions or translation schemes, for simple tasks such as converting an infix expression to a postfix one. While parsing the compiler also performs various checks to ensure the correctness of syntax - and and syntax error reports can be imbedded into parser code. The leaves are the terminals of the grammar, the actual symbols which appear as identifiers, keywords, and constants in the language / input string, e.g. The root of the parse tree is the most general symbol of the grammar - the start symbol (for example, statement), and the interior nodes represent nonterminal symbols that the start symbol expands to (can include the start symbol itself), such as expression, statement, term, function call. They are tree-like data structures that shows how an input string of terminals (source code tokens) has been generated by the grammar of the language in question.
#DAG EXAMPLES IN COMPILER DESIGN SERIES#
Parse trees are usually generated as the next step after lexical analysis (which turns the source code into a series of tokens that can be viewed as meaningful units, as opposed to just a sequence of characters). They're similar data structures, but they're constructed differently and used for different tasks. Here's an explanation of parse trees (concrete syntax trees, CSTs) and abstract syntax trees (ASTs), in the context of compiler construction.
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21 in Syntax and Semantics of Programming Languages Notice that parens are not present in the AST because the associations are derivable from the tree structure.įor a more through explanation see Compilers and Compiler Generators pg. The AST is an abstract representation of the input. The empty boxes represent whitespace, i.e. The parse tree retains all of the information of the input. The parse tree is a concrete representation of the input.
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The grammar for this example: grammar Expr002 ĪSTLabelType=CommonTree // type of $ee ref etc.Įxpr : multExpr (( '+'^ | '-'^ ) multExpr)* This is based on the Expression Evaluator grammar by Terrence Parr.