Chapter 3 Program translation 1

Chapt. 3 Language Translation

• Syntax and Semantics

• Translation phases

• Formal translation models

Chapter 3 Program translation 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

Chapter 3 Program translation 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

Chapter 3 Program translation 4

Good syntax criteria(cont.)

• Assist in Writeability– Few and concise statements– Rich library–created 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??

Chapter 3 Program translation 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 ||

Chapter 3 Program translation 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)

Chapter 3 Program translation 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

Chapter 3 Program translation 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

Chapter 3 Program translation 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

Chapter 3 Program translation 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

Chapter 3 Program translation 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

Chapter 3 Program translation 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– Ex:Save intermediate results in registers– remove constant operations from loop– Change 2-dimensional array storage

• Code generations• Linking and Loading

Chapter 3 Program translation 13

BNF (Backus Normal/Naur Form)

• Metalanguage::= defined as| alternative<> nonterminal{} later introduced for iteration[] for optionalsequence is implicit

ex: <unsigned integer> ::= <digit>|<unsigned integer> <digit><digit> ::= 0|1|2|3|4|5|6|7|8|9

Chapter 3 Program translation 14

Context Free Grammars

For balanced parenthesisS 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 asPass by value or reference

Chapter 3 Program translation 15

Syntax Charts

Term at top left is defined by the following graph

Graph branches for alternativeEmpty branch for optional

Box around string for nonterminalCircle for terminal

Arrow back for iteration ex: p. 96 in text

Sequence is explicit

Chapter 3 Program translation 16

Finite-State Automata

Table used for lexical analysisEx: 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 arrowDigit is input to whole part . is input leading to decimalDigit leads from decimal to fractionalE leads from fractional to expDigit leads from exp to exp value