Translating High Level Languages

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Translating High Level Languages

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Stages of translation

• Lexical analysis
• Syntactic analysis
• Code generation
• Linking
• Before
• Execution

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Lexical analysis

• Translate stream of characters into lexemes
• Lexemes belong to categories called tokens
• Token identity of lexemes is used at the next
  stage of syntactic analysis

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Examples tokens and lexemes

• Some token categories contain only one lexeme
  semi-colon
• Some tokens categorize many lexemes identifier
  count, maxCost,

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Tokens and Lexemes
yVal x 450 min ( 100, 4xVal ))
identifier
illegal lexeme
equal_sign
left_paren

• Lexical analysis
• identifies lexemes and their token type
• recognizes illegal lexemes (4xVal)
• does NOT identify syntax error ) )

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Syntax or Grammar of Language

• rules for
• generating (used by programmer) or
• recognizing (used by syntactic analyzer in
  translation
• a valid sequence of lexemes

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Grammars

• 4 categories of grammars (Chomsky)
• Two categories are important in computing
• Regular expressions (pattern matching)
• Context-free grammars (programming languages)

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Context-free grammar

• Meta-language for describing languages
• States rules or productions for what lexeme
  sequences are correct in the language
• Written in Backus-Naur Form (BNF)

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Example of BNF rule

• PROBLEM how to recognize all these as correct?
• y x
• f rVec.length 1
• button4.label Exit
• RULE for defining assignment statement
• ltassigngt ltvariablegt ltexpressiongt
• Assumes other rules for ltvariablegt, ltexpressiongt

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BNF rules

• Non-terminal and terminal symbols
• Non-terminals are defined by at least one rule
• Terminals are tokens or lexemes
• ltassignmentgt
• lt vargt ltexpressiongt

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Simple sample grammar(p.113)
ltassigngt ltidgt ltexprgt ltidgt A B
C // lexical ltexprgt ltidgt ltexprgt
ltidgt ltexprgt (
ltexprgt) ltidgt Assumes other
rules for ltvariablegt, ltexpressiongt
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Simple sample production
ltassigngt ltidgt ltexprgt lt- apply one rule at
each step B ltexprgt to leftmost
non-terminal B ltidgt ltexprgt B A
ltexprgt B A ( ltexprgt ) B A ( ltidgt
ltexprgt ) B A ( C ltexprgt ) B A ( C
ltidgt ) B A ( C C )
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Sample parse tree
ltassigngt
ltexprgt
ltidgt

ltexprgt
ltidgt

B
ltexprgt
)
A
(

ltexprgt
ltidgt
ltidgt
C
C
Leaves represent the sentence of lexemes
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Ambiguous grammar

• Different parse trees for same sentence
• Different translations for same sentence
• Different machine code for same source code!

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Grammars for human conventions

• Putting features of languages into grammars
• expression any length
• precedence - an extra non-terminal
• associativity - order in recursive rules
• nested if statements - dangling else problem
  p. 119

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Forms for grammars

• Backus-Naur form (BNF)
• Extended Backus-Naur fomr (EBNF)
• -shortens set of rules
• Syntax graphs
• -easier to read for learning language

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EBNF

• optional zero or one occurrence
• ltexprgt -gt ltexprgt lttermgt
• optional zero or more occurrences
• ltexprgt -gt lttermgt lttermgt
• or choice of alternative symbols
• lttermgt -gt lttermgt (/) lttermgt

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Syntax Graph - basic structures
expr
term

term
factor
term
/
term
factor

factor
/
expr
term

term
-
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BNF (p. 121)
EBNF
ltexprgt -gt ltexprgtlttermgt
ltexprgt-lttermgt lttermgt lttermgt -gt
lttermgtltfactorgt
lttermgt/ltfactorgt ltfactorgt
ltexprgt -gt ltexprgt (-) lttermgt lttermgt -gt lttermgt
(\/) ltfactorgt
ltexprgt -gt lttermgt (-) lttermgt lttermgt -gt
ltfactorgt (/)ltfactorgt
Syntax Graph
expr
term

term
-
term
factor

factor
/
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Attribute grammars

• Problem context-free grammars cannot describe
  some features needed in programming
• e.g. rules for using data types