Compiler Construction

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Compiler Construction

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Objectives

• Given a context-free grammar, G, and the
  grammar-independent functions for a
  recursive-descent parser, complete the
  recursive-descent parser by adding the
  grammar-dependent functions.
• Given naive intermediate code for a C loop,
  hand-optimize the code, reducing by at least 20
  the number of intermediate code instructions
  needed while maintaining semantic correctness of
  the program.
• Define "register assignment" in the context of a
  compiler. Explain why register assignment is an
  important compiler optimization.

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Compiler Tasks
Code Generation
Tokens
Source
Intermediate
Intermediate
Lexor
Parser
Optimizer
Assembler or Binary or Source
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CFGs Example

• Simplified description of English grammar
• G
• S ? NP VP
• NP ? N Adj NP
• N ? car dog
• Adj ? big green
• VP ? V V NP
• V ? is eats
• Example derivation
• S ?G NP VP ?G Adj NP VP ?G Adj N VP ?G Adj N V
• ?G Adj N eats ?G big N eats ?G big dog eats

Production form LHS variable. RHS string of
variables and/or terminals.
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CFGs Formal Definition

• G (V, S, P, S)
• V variables a finite set
• S alphabet or terminals a finite set
• P productions a finite set
• S start variable S?V
• Productions form, where A?V, a?(V?S)
• A ? a

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CFGs Example 1

• an bn n?0
• S ? e a S b
• Formally G (S, a,b,
• S ? e, S ? a S b, S)

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CFGs CFLs Example 2

• all strings of balanced parentheses
• A core idea of most programming languages.

P ? e ( P ) P P
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CFGs Example 2

• Language of Assignment Statements
• P ? S P
• P ? S
• S ? id E
• E ? expressions (expressions left as exercise)

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Example Languages

• Set of strings over a,b with an even number of
  as
• Arithmetic expressions using , , /, -, (,)
  Assume that all operands are called id

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Recursive Descent Parser

• Built from a context-free grammar
• Simple rules build recursive program
• Grammar independent part (see pdf file)
• Grammar dependent
• Include a function for each non-terminal (lhs)
• Generate code for each rule, left to right
• Call non-terminals
• Match terminals
• Use if statement to choose between multiple
  rules with the same lhs (non-terminal) symbol