Chapt. 3 Language Translation

1
Chapt. 3 Language Translation

• Syntax and Semantics
• Translation phases
• Formal translation models

2
Syntax

• What is a valid string of the language?
• First pass of a compiler
• Error messages (are they helpful?)
• Compiler compiler (generator) such as YACC can
  automatically generate parser from BNF

3
Good syntax criteria

• Assist in Readability
• COBOL as self documenting
• Comments
• Length of identifiers
• Overloading of names
• Examples of poor features for readability
• blank as concatenation operation in SNOBOL
• Identifier names in Basic
• X1, Y
• Implicit typing
• Late binding

4
Good syntax criteria(cont.)

• Assist in Writeability
• Few and concise statements
• Rich librarycreated by language and user
• Support of abstraction
• Orthogonality
• Examples of poor features for writeability
• Large number of constructs
• Lack of necessary constructs
• Redundancy
• Ambiguity
• Ex if statement
• Case sensitivity??

5
Syntactic elements

• Character set
• 5, 6, 7, 8, 16 bit encoding schemes
• Identifiers
• Symbols such as letters, digits, , _, blank
• Length limitation
• Operation symbols various examples
• LISP prefix identifiers (ex PLUS)
• APL special Greek characters
• FORTRAN - .EQ., .GT.
• C - ,
• Java and , and

6
Syntactic elements (cont.)

• Keyword
• identifier used as part of primitive program unit
  (ex if, then, else, case)
• Reserved word
• Keyword that cannot be assigned by programmer
• READ is not a reserved word in Pascal
• Adding new reserved words to an update of a
  language can make old programs incorrect (upward
  compatability)

7
Syntactic elements (cont.)

• Noise words
• Used to improve readability-optional
• Ex perform 5 times
• Comments
• Used for documentation readability
• Blanks
• Completely ignored in FORTRAN
• Do 10 I 1.5
• Delimiters and brackets
• Spaces, , paired () begin end
• Fixed format vs free format

8
Program Structure

• Expression
• Precedence rules
• Statements
• structured programming
• Modules/ functions/ subprograms/ classes
• Nested units
• Static checks, efficient code for nonlocal
  references
• Separate unit compilation.
• Data and operations are compiled as a unit in
  classes
• Interface issues function specification to
  allow static checks (prototypes)
• Specifications (.h files) separate from
  implementations

9
Translation I- Lexical Analysis

• Byte stream organized into lexemes, each of which
  is identified (tagged)
• Numbers may be converted to binary
• Identifiers are stored in symbol table
• Tokens are output for syntactic analysis

10
Translation II parsing syntactic analysis

• Tokens organized into expressions, statements,
  etc.
• Is the input a valid string in the language?
• Generates parse tree, tables
• Produces error messages for invalid strings

11
Translation III semantic analysis

• Produces error messages for invalid constructs
• Ex identifier not declared type mismatch
• Compiled languages use and discard symbol table
• Reference to variable as offset from data
  sections
• Information must be stored together with
  identifier (ex type, range limitations)
• Macro substitutions
• Compiler directives
• define
• ifndef
• Pragma suppress range_checks

12
Translation IV optimization

• Semantic analysis output is typically one
  statement at a time
• Compiler can optimize code to optain results as
  efficient as assembly code
• ExSave intermediate results in registers
• remove constant operations from loop
• Change 2-dimensional array storage
• Code generations
• Linking and Loading

13
BNF (Backus Normal/Naur Form)

• Metalanguage
• defined as
• alternative
• ltgt nonterminal
• later introduced for iteration
• for optional
• sequence is implicit
• ex
• ltunsigned integergt ltdigitgtltunsigned integergt
  ltdigitgt
• ltdigitgt 0123456789

14
Context Free Grammars

• For balanced parenthesis
• S ? SS (S) ()
• Problem generate a parse tree for a string
  such as (()(()))((())()) from above
• Some language definition issues are context
  sensitive, such as each identifier must be
  declared before use
• Implementation issues such as
• Pass by value or reference

15
Syntax Charts

• Term at top left is defined by the following
  graph
• Graph branches for alternative
• Empty branch for optional
• Box around string for nonterminal
• Circle for terminal
• Arrow back for iteration
• ex p. 96 in text
• Sequence is explicit

16
Finite-State Automata

• Table used for lexical analysis
• Ex valid floating point number
• (note that limitations on range and precision are
  not specified)
• (whole part)? (decimal) ? (fractional) ?
• (exp) ? (exp value)
• Where whole part, fractional, and exp value have
  a looping arrow
• Digit is input to whole part
• . is input leading to decimal
• Digit leads from decimal to fractional
• E leads from fractional to exp
• Digit leads from exp to exp value