Introduction to Compilers

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• Introduction to Compilers
• Set 1

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What is a Compiler?

• A compiler is software (a program) that
  translates a high-level programming language to
  machine language
• A simple representation would beSource Code
  ----gt Compiler -----gt Machine Language (Object
  File)
• It's not as simple as an assembler translator.
• A compiler has to perform several steps to
  produce an object file in machine code form.

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Analysis of the source code

• Lexical Analysis using a program called a SCANNER
• Syntax Analysis using a program called a PARSER
• Semantic Analysis using CODE GENERATION routines

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Lexical Analysis

• Scan the input source code to identify tokens of
  the programming language. 
• Tokens are basic units (keywords, identifier
  names, etc.) from which the source code is
  constructed
• The program to do this is called a SCANNER
•  

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Syntax Analysis

• Group the tokens identified by the scanner into
  grammatical phrases that will be used by the
  compiler to generate the output code. 
• This process is called parsing and is performed
  by a parser.

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Synthesis of the target program

• Generate an intermediate representation of the
  source program
• Code Optimization
• Code Generation

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Generate an intermediate representation

• This is performed by some compilers, but not
  necessarily all.
•  
• It should be easy to produce and easy to
  translate into the target program.

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Code Optimization

• Improve the intermediate code to produce a faster
  running machine code in the final translation.  
• Not all compilers include the code optimization
  step, which can require a lot of time.

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Linking the programs produced by the compiler

• The linker program links the object files from
  the program modules, and any additional library
  files to create the executable program. 
• This requires the use of relocatable addresses
  within the program to allow the program to run in
  different memory locations.

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Grammar 1
1.  SENTENCE -gt NOUNPHRASE VERB NOUNPHRASE 2. 
NOUNPHRASE -gt the ADJECTIVE NOUN 3.  NOUNPHRASE
-gt the NOUN 4.  VERB -gt pushed 5.  VERB -gt
helped 6.  ADJECTIVE -gt pretty 7.  ADJECTIVE -gt
poor 8.  NOUN -gt man 9.  NOUN -gt boy 10. NOUN -gt
cat
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• Grammar 1 is an example of a context-free grammar
  (the only kind we will deal with).
• The grammar consist of 10 productions e.g.
  production 3 is
• nounphrase -gt the noun
• Here nounphrase is referred to as the lefthand
  side (lhs) and the noun is referred to as the
  righthand side (rhs)

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• The set of all lhss constitutes the set of
  nonterminals of the grammar.
• In this case they are SENTENCE,
  NOUNPHRASE,VERB, ADJECTIVE, NOUN
• All the other symbols occurring in the grammar
  (i.e. in some rhs, but never as any lhs) are the
  terminals of the grammar.
• In this case the,pushed,helped,pretty,poor,

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• The lhs of the first production is called the
  goal symbol, in this case sentence.
• A derivation of a string in the grammar is a list
  of strings starting with the goal symbol, in
  which each string, except the first, is obtained
  from the preceding one by applying a substitution
  of one of its symbols using one of the
  productions as a substitution rule

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• A string which has a derivation is said to be
  derivable.
• Derivable strings that consist entirely of
  terminal symbols are called sentences of the
  grammar. E.g.
• the man helped the poor boy
• is a sentence of Grammar 1.
• The set of all sentences of a grammar is called
  the language defined by the grammar

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Grammar 1 (Cont.1)
Derivation of the sentence  "the man helped the
poor boy 1.      SENTENCE                      
(goal symbol)  2. gt  NOUNPHRASE VERB
NOUNPHRASE     (by Rule 1) 3. gt  the NOUN VERB
NOUNPHRASE       (Rule 3) 4. gt  the man VERB
NOUNPHRASE        (Rule 8) 5. gt  the man helped
NOUNPHRASE 6. gt  the man helped the ADJECTIVE
NOUN 7. gt  the man helped the poor NOUN 8. gt 
the man helped the poor boy (this derivation
shows that "the man helped the poor boy is a
sentence in the language defined by the grammar.)
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Grammar 1 (Cont.2)
This derivation may also be represented
diagrammatically by a syntax tree        
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Typical format of a grammar for a programming
language
PROGRAM -gt PROGRAM STATEMENT PROGRAM -gt
STATEMENT STATEMENT -gt ASSIGNMENT-STATEMENT STATEM
ENT -gt IF-STATEMENT STATEMENT -gt
DO-STATEMENT ... ASSIGNMENT-STATEMENT -gt
... ... IF-STATEMENT -gt ... ... DO-STATEMENT -gt
... ...
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Grammar 2
A simple grammar for arithmetic statements
1.    E -gt E T 2.    E -gt T 3.    T -gt T
a 4.    T -gt a
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Grammar 2 (Cont.1)
Derivation of  a a a 1.        E
Goal Symbol 2. gt    E T Rule 1 3.
gt    E T a Rule 3 4. gt    E a a
Rule 4 5. gt    T a a Rule 2 6.
gt    a a a Rule 4
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Grammar 2 (Cont.2)
Derivation of a a a written in reverse
1.   a a a Given sentential
form 2.   T a a Rule 4 in
reverse 3.   E a a Rule 2 in
reverse 4.   E T a Rule 4 5.   E T
Rule 3 in reverse 6.   E
Rule 1