1. Chapter 2: A Simple One Pass Compiler

2. The Entire Compilation Process
Grammars for Syntax Definition
Syntax-Directed Translation
Parsing - Top Down & Predictive
Pulling Together the Pieces
The Lexical Analysis Process
Symbol Table Considerations
A Brief Look at Code Generation
Concluding Remarks/Looking Ahead

3. Grammars for Syntax Definition
A Context-free Grammar (CFG) Is Utilized to Describe the Syntactic Structure of a Language
A CFG Is Characterized By:
1. A Set of Tokens or Terminal Symbols
2. A Set of Non-terminals
3. A Set of Production Rules Each Rule Has the Form NT  {T, NT}*
4. A Non-terminal Designated As the Start Symbol

4. Grammars for Syntax Definition Example CFG
list  list + digit
list  list - digit
list  digit
digit  0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
(the “|” means OR)
(So we could have written
list  list + digit | list - digit | digit )

5. Grammars are Used to Derive Strings:
Using the CFG defined on the previous slide, we can derive the string: as follows:
list  list + digit
 list - digit + digit
 digit - digit + digit
 9 - digit + digit
 digit 
P1 : list  list + digit
P2 : list  list - digit
P3 : list  digit
P4 : digit  9
P4 : digit  5
P4 : digit  2

6. Grammars are Used to Derive Strings:
This derivation could also be represented via a Parse Tree
(parents on left, children on right)
list  list + digit
 list - digit + digit
 digit - digit + digit
 9 - digit + digit
 digit 
list
digit 9 5 2 - +

7. A More Complex Grammar What is this grammar for ?
block  begin opt_stmts end
opt_stmts  stmt_list | 
stmt_list  stmt_list ; stmt | stmt
What is this grammar for ?
What does “” represent ?
What kind of production rule is this ?

8. Defining a Parse Tree
More Formally, a Parse Tree for a CFG Has the Following Properties:
Root Is Labeled With the Start Symbol
Leaf Node Is a Token or 
Interior Node (Now Leaf) Is a Non-Terminal
If A  x1x2…xn, Then A Is an Interior; x1x2…xn Are Children of A and May Be Non-Terminals or Tokens

9. Other Important Concepts Ambiguity
Two derivations (Parse Trees) for the same token string.
string - 9 + 5 2
string + 2 - 5 9
Grammar:
string  string + string | string – string | 0 | 1 | …| 9
Why is this a Problem ?

10. Other Important Concepts Associativity of Operators
Left vs. Right
right
letter c b a =
list
digit 9 5 2 - +
list  list + digit |
| list - digit | digit
digit  0 | 1 | 2 | …| 9
right  letter = right | letter
letter  a | b | c | …| z

11. Embedding Associativity
The language of arithmetic expressions with + -
(ambiguous) grammar that does not enforce associativity
string  string + string | string – string | 0 | 1 | …| 9
non-ambiguous grammar enforcing left associativity (parse tree will grow to the left)
string  string + digit | string - digit | digit
digit  0 | 1 | 2 | …| 9
non-ambiguous grammar enforcing right associativity (parse tree will grow to the right)
string  digit + string | digit - string | digit

12. Other Important Concepts Operator Precedence
What does
9 + 5 * 2
mean?
( )
* /
+ -
is precedence order
Typically
This can be
incorporated
into a grammar
via rules:
expr  expr + term | expr – term | term
term  term * factor | term / factor | factor
factor  digit | ( expr )
digit  0 | 1 | 2 | 3 | … | 9
Precedence Achieved by:
expr & term for each precedence level
Rules for each are left recursive or associate to the left