A Simple SyntaxDirected Translator

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A Simple Syntax-Directed Translator

• From Chapter 2, The Dragon Book, 2nd ed.

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Overview

• This chapter is an introduction to the compiling
  techniques in Chapters 3 through 6 of the book.

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2.1 Introduction
Abstract syntax tree
Three address code
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2.2 Syntax Definition

• Definition of grammars (2.2.1)
• A context-free grammar has four components
• a set of terminal symbol,
• a set of nonterminal symbols
• a set of productions
• a designated nonterminal as the start symbol
• Example 2.1

list ? list digit list ? list - digit list ?
digit digit ? 0 1 2 3 4 5 6 7 8
9
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2.2 Syntax Definition

• Derivations(2.2.2)
• A grammar derives strings by beginning with the
  start symbol and repeatedly replacing a
  nonterminal by the body of a production for that
  nonterminal.
• Example 2.2
• Derivation of 9-52 using the grammar of Example
  2.1.
• Example 2.3

call ? id ( optparams ) optparams ? params
e params ? params , params param max(x, y)
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2.2 Syntax Definition

• Parse tree (2.2.3)
• A parse tree pictorially shows how the start
  symbol of a grammar derives a string in the
  language.
• Example 2.4

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2.2 Syntax Definition

• Ambiguity (2.2.4)
• A grammar can have more than one parse tree
  generating a given string of terminals. Such a
  grammar is said to be ambiguous.
• Example 2.5

string ? string string string string
0123456789
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2.2 Syntax Definition

• Associativity of operators (2.2.5)
• left-associative
• , -, , /
• Right-associative
• (in C), e.g., abc

right ? letter right letter letter ? a b
... z
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2.2 Syntax Definition

• Precedence of operators (2.2.6)
• has higher precedence than
• Two nontermainals expr and term for the two
  levels of precedence, and an extra one factor for
  generating basic units in expressions

expr ? expr term expr term term term ?
term factor term / factor factor factor ?
digit ( expr )
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2.3 Syntax-Directed Translation

• Syntax-directed translation is done by attaching
  rules or program fragments to productions in a
  grammar.
• The example here
• Translation of infix expressions into postfix
  expression
• Introducing two concepts related to
  syntax-directed translation
• Attributes
• Any quantity associated with a program entity,
• Examples data types of expression, the number of
  instructions in the generated code, or the
  location of the first instruction in the
  generated code for a construct, ...
• Translation schemes
• A notation for attaching program fragment to the
  productions of a grammar

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2.3 Syntax-Directed Translation

• Postfix notation (2.3.1)
• Definition of postfix notation
• If E is a variable or constant, then the postfix
  notation for E is E itself.
• If E is an expression of the form E1 op E2, where
  op is any binary operator, then the postfix
  notation for E is E1 E2 op , where E1 and E2
  are the postfix notations for E1 and E2,
  respectively.
• If E is a parenthesized expression of the form
  (E1), then the postfix notation for E is the same
  as the postfix notation for E1.
• Example 2.8
• (9-5)2 ? 95-2
• 9-(52) ? 952-
• Example 2.
• 9523
• Scanning from left to right

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2.3 Syntax-Directed Translation

• Synthesized attributes (2.3.2)
• A syntax-directed definition associates
• With each grammar symbol, a set of attributes,
  and
• With each production, a set of semantic rules for
  computing the values of the attributes associated
  with the symbols appearing in the production.

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2.3 Syntax-Directed Translation

• Simple syntax-directed definitions (2.3.3)
• A syntax-directed definition with the following
  property is termed simple.
• the string representing the translation of the
  nonterminal at the head of each production is the
  concatenation of the translations of the
  nonterminals in the production body, in the same
  order as in the production, with some optional
  additional string interleaved
• For example, 2.10

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2.3 Syntax-Directed Translation

• Tree traversal (2.3.4)
• Tree traversal is used for describing attribute
  evaluation and for specifying the execution of
  code fragments in a translation scheme.

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2.3 Syntax-Directed Translation

• Translation schemes (2.3.5)
• A syntax-directed translation scheme is a
  notation for specifying a translation by
  attaching program fragments (called semantic
  actions) to productions in a grammar.
• A translation scheme is like a syntax-directed
  definition, except that the order of evaluation
  of the semantic rules is explicitly specified.
• Example 2.12

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2.4 Parsing
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2.5 A Translator for Simple Expressions
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2.6 Lexical Analysis
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2.7 Symbol Tables
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2.8 Intermediate Code Generation