compilation backus naur form bnf and context free grammars cfgs
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Compilation: Backus-Naur Form (BNF) and Context Free Grammars (CFGs). We care about. Completeness of specification Determination of legal expressions Resolution of ambiguities Avoid small mistakes causing major errors. BNF/CFG. Backus-Naur Form Context Free Grammars

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we care about
We care about
  • Completeness of specification
  • Determination of legal expressions
  • Resolution of ambiguities
  • Avoid small mistakes causing major errors
bnf cfg
BNF/CFG
  • Backus-Naur Form
  • Context Free Grammars
  • Similar mechanisms for specifying legal syntax
  • Both focused on production rules
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phases of compilation
Phases of compilation

lexical analysis (tokenization) – grouping the characters into tokens (int, {, x, etc) (linear scan)

Syntax analysis (parsing) – grouping tokens into expressions or statements ( int x = 10;) (recursion; CFG)

semantic analysis (Syntax-directed translation), - type checking, implicit typecasting, check indices to arrays, variables declared before use, etc. Necessary because most programming languages can\'t be completely captured by CFG

code generation (next) – actually generating assembler code

code optimization – looking for ways to make the assembler faster

code generation using the parse tree to generate assembler code
Code generation: Using the parse tree to generate assembler code

To prune the leaves of a parse tree means to eliminate all the leaves of a node, replacing the leaves (and their parent) with the intended “meaning” (a value or a chunk of code)

After code generation, code optimizer

parse trees are central to compiler theory
Parse trees are central to compiler theory
  • They allow us to identify the production rule corresponding to the chunk of code, and from there replace that chunk of code with a chunk of assembly.
ambiguity
Ambiguity
  • A grammar that produces more than one derivation for the same sentence is ambiguous
  • Eg: E E+E | E*E | (E) | -E | id
    • There are two derivations for id+id*id – find them both.
ambiguity continued
Ambiguity continued
  • There are some languages for which there is no unambiguous grammar
  • HOWEVER there are some rules of thumb we can use to help us deal with many ambiguous grammars
  • Ambiguity often arises if the right side of a production rule contains 2 or more occurrences of the same non-terminal
  • Ambiguity can cause problems if the grammar needs to observe rules of precedence or associativity
slide12

E -> E+T | T

  • T -> T*F | F
  • F -> -P | P
  • P ->(E) | id
derivations
Derivations
  • A leftmost derivation is one in which only the leftmost non-terminal in a sentential form is replaced at each step.
  • A rightmost derivation is one in which only the rightmost non-terminal in a sentential form is replaced at each step
  • Remember ambiguity? Leftmost and rightmost derivations are usually unique
    • Generally pick leftmost or rightmost and stick with it
    • if they generate different parse trees the language is ambiguous (not iff)
  • In general, proving a grammar is unambiguous is undecidable
2 3 1 2 4
2*3+(1+2)*4
  • E -> E+T | T
  • T -> T*F | F
  • F -> -P | P
  • P ->(E) | id
compilation vs interpretation
Compilation vs interpretation

An interpreted language is a programming language for which most of its implementations execute instructions directly, without previously compiling a program into machine-language instructions. The interpreter executes the program directly, translating each statement into a sequence of one or more subroutines already compiled into machine code.

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