What is a compiler?

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• What is a compiler?
• A program that reads a program written in one
  language (source language) and translates it into
  an equivalent program in another language (target
  language).
• Traditionally, the source language is a high
  level language and the target language is a low
  level language (machine code).

Target program
Source program
compiler
Error message
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• Why do we need to learn how a compiler works?
• It is very unlikely for anyone to write another
  C/C/java compiler.
• But
• Parsing a small language occurs quite often
  scripts languages such as python, parsing a set
  of commands for a mail server, etc.
• For systems people, the compiler knowledge is the
  basic requirement.
• A new processor is fast because it runs code
  generated by a compiler fast.
• If you implement a new operating system, you need
  to know how compiler interacts with the OS. An OS
  is fast because it runs code generated by a
  compiler fast.
• New architectures keep coming new compiler
  optimizations are always needed.

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• Knowledge required to write a good compiler
• Formal language (lexical analysis and parsing)
• Programming language (activation record, scope,
  type checking, etc)
• Algorithm (optimization, register allocation,
  etc)
• Graph theory (optimization, data flow analysis)
• Computer architecture (target language, machine
  dependent optimizations)
• Software engineering (large amount of code)

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Source program with macros
A typical compilation process
Preprocessor
Source program
Compiler
Target assembly program
Try gcc with v, -E, -S flags On linprog.
assembler
Relocatable machine code
Loader/link-editor
Absolute machine code
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Compiler Phases
Source program
Front End
Lexical analyzer
Syntax analyzer
Symbol table manager
Error handler
Semantic analyzer
Intermediate code generator
Code optimizer
Backend
Code generator
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• Compiler phases
• source program is a stream of characters pos
  initrate60
• lexical analysis groups characters into
  non-separatable units, called token, and
  generates token stream Id1 id2 id3 const
• the information about the identifiers must be
  stored somewhere (symbol table).
• Syntax analysis checks whether the token stream
  meets the grammatical specification of the
  language and generates the syntax tree.
• Semantic analysis checks whether the program has
  a meaning (e.g. if pos is a record and init and
  rate are integers then the assignment does not
  make sense).

Id1

id2

id3
60
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• Compiler phases
• intermediate code generation, intermediate code
  is something that is both close to the final
  machine code and easy to manipulate (for
  optimization). One example is the three-address
  code
• dst op1 op op2
• The three-address code for the assignment
  statement
• temp1 60
• temp2 id3 temp1
• temp3 id2 temp2
• id1 temp3
• Code optimization produces better/semantically
  equivalent code.
• temp1 id3 60.0
• id1 id2 temp1
• code generator generates assembly
• MOVF id3, R2
• MULF 60.0, R2
• MOVF id2, R1
• ADDF R2, R1
• MOVF R1, id1

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• The use of intermediate code can also reduce the
  complexity of compilers
• m front ends and n backends share a common
  intermediate code.

1
1
2
2
IC
m
n
Backends
Front Ends
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• Compiler construction tools
• scanner generator (lex)
• parser generator (yacc)
• syntax-directed translation engines
• automatic code generators
• data-flow engines