Download chapter 2 - A Simple Syntax-Directed Translator

2.2 A Simple Syntax-Directed
Translator
1
2
•
•
•
•
•
•
•
2.3 Syntax-Directed Translation
2.4 Parsing
2.5 A Translator for Simple Expressions
2.6 Lexical Analysis
2.7 Symbol Tables
2.8 Intermediate Code Generation
2.9 Summary of Chapter 2
3
Syntax-Directed Translation
• Syntax-directed translation is done by
attaching rules or program fragments to
productions in a grammar.
• For example, consider an expression expr
generated by the production:
• We can translate expr by exploiting its
structure, as in the following pseudo-code:
4
Concepts of syntax-directed
translation
• Attributes.
– An attribute is any quantity associated with a
programming construct. Examples of attributes are
data types of expressions, the number of instructions
in the generated code, or the location of the first
instruction in the generated code for a construct ,
among many other possibilities.
• (Syntax- directed) translation schemes.
– A translation scheme is a notation for attaching
program fragments to the productions of a grammar.
The program fragments are executed when the
production is used during syntax analysis.
5
Postfix Notation
• The postfix notation for an expression E can
be defined inductively as follows:
6
Postfix Notation
• Example 2.8 :
The postfix notation for (9-5)+2
• What about??
9- (5+2)
• Evaluate??
7
Postfix Notation
• How to evaluate??
– The "trick" is to repeatedly scan the postfix string
from the left , until you find an operator.
– Then, look to the left for the proper number of
operands,
– group this operator with its operands.
– Evaluate the operator on the operands, and
replace them by the result.
– repeat the process, continuing to the right and
searching for another operator.
8
Postfix Notation
• Example 2.9 : evaluate the following postfix
notation:
952+-3*
9
Synthesized Attributes
• We associate attributes with nonterminals and
terminals. Then, we attach rules to the
productions of the grammar; these rules
describe how the attributes are computed at
those nodes of the parse tree where the
production in question is used to relate a node
to its children.
10
11
12
Tree Traversals
• Tree traversals will be used for describing
attribute evaluation and for specifying the
execution of code fragments in a translation
scheme.
– A traversal of a tree starts at the root and visits each
node of the tree in some order.
– A depth-first traversal starts at the root and
recursively visits the children of each node in any
order, not necessarily from left to right . It is called
"depth first“ because it visits an unvisited child of a
node whenever it can, so it visits nodes as far away
from the root (as "deep" ) as quickly as it can.
13
14
Translation Schemes
• Preorder and Postorder Traversals
15
16
17
Semantic actions
18
Example
Actions for postfix translating 9-5+2 into 95-2+
19
2.4 Parsing
• Parsing is the process of determining how a
string of terminals can be generated by a
grammar.
– Top-Down Parsing
20
steps
21
22
Use ϶-Productions
• Predictive Parsing (self study)
• When to Use ϶-Productions
– predictive parser uses an ϶ -production as a
default when no other production can be used
23
24
2.5 A Translator for Simple Expressions
• Using the techniques of the last three sections, we
now construct a syntax directed translator
– A syntax-directed translation scheme often serves as the
specification for a translator.
– The scheme in Fig. 2.21 (repeated from Fig. 2.15) defines
the translation to be performed here
25
Abstract and Concrete Syntax
• A useful starting point for designing a translator is a
data structure called an abstract syntax tree.
• In an abstract syntax tree for an expression, each
interior node represents an operator; the children of
the node represent the operands of the operator.
26
Example
27
28
syntax tree vs. parse tree
• in the syntax tree, interior nodes represent
programming constructs while in the parse
tree,
the
interior
nodes
represent
nonterminals.
29
30
2.6 Lexical Analysis
• A lexical analyzer reads characters from the input and groups
them into "token objects.“
• The lexical analyzer in this section allows numbers, identifiers,
and "white space" (blanks, tabs, and newlines) to appear
within expressions.
31
Reading Ahead
• A lexical analyzer may need to read ahead some
characters before it can decide on the token to be
returned to the parser.
• For example, a lexical analyzer for C or Java must
read ahead after it sees the character >.
– If the next character is =, then > is part of the
character sequence >=, the lexeme for the token for
the "greater than or equal to" operator.
– Otherwise > itself forms the "greater than" operator,
and the lexical analyzer has read one character too
many.
32
Constants
33
Recognizing Keywords and Identifiers
• Most languages use fixed character strings such
as for, do, and if , as punctuation marks or to
identify constructs.
• Such character strings are called keywords.
• Character strings are also used as identifiers to
name variables, arrays, functions, and the like.
• Grammars routinely treat identifiers as terminals
to simplify the parser, which can then expect the
same terminal, say id, each time any identifier
appears in the input. For example, on input
34
Recognizing Keywords and Identifiers
• the parser works with the terminal stream id = id + id. The
token for id has an attribute that holds the lexeme.
• Writing tokens as tuples, we see that the tuples for the
input stream (2.6) are
• Keywords generally satisfy the rules for forming identifiers,
so a mechanism is needed for deciding when a lexeme
forms a keyword and when it forms an identifier.
• The problem is easier to resolve if keywords are reserved;
i.e., if they cannot be used as identifiers. Then, a character
string forms an identifier only if it is not a keyword.
35
2.7 Symbol table
• Symbol tables are data structures that are used by
compilers to hold information about source-program
constructs.
• The information is collected incrementally by the
analysis phases of a compiler and used by the synthesis
phases to generate the target code.
• Entries in the symbol table contain information about
an identifier such as its character string (or lexeme) , its
type, its position in storage, and any other relevant
information. Symbol tables typically need to support
multiple declarations of the same identifier within a
program.
36
Scope
• Symbol Table Per Scope: The term "scope of
identifier x' really refers to the scope of a
particular declaration of x.
• Scopes are important, because the same
identifier can be declared for different
purposes in different parts of a program.
• If blocks can be nested, several declarations of
the same identifier can appear within a single
block.
37
38
2.8 Intermediate Code Generation
• Two Kinds of Intermediate Representations
39
Construction of
Syntax Trees
40
L-values and R-values
• There is a distinction between the meaning of
identifiers on the left and right sides of an
assignment. In each of the assignments the
right side specifies an integer value, while the
left side specifies where the value is to be
stored.
41
Type Checking
42
Three-Address Code
43
Example
44
2.9 Summary o f Chapter 2
45