Simple One-Pass Compiler

1
Simple One-Pass Compiler

• Natawut Nupairoj, Ph.D.
• Department of Computer Engineering
• Chulalongkorn University

2
Outline

• Translation Scheme.
• Annotated Parse Tree.
• Parsing Fundamental.
• Top-Down Parsers.
• Abstract Stack Machine.
• Simple Code Generation.

3
Simple One-Pass Compiler
Scanner
Source program (text stream)
m
a
i
n
(
)

Parser
Object Code (text stream)
4
Sample Grammar

• expr ? expr term
• expr ? expr - term
• expr ? term
• term ? 0 1 2 ... 9

5
Derivation

• String 9 5 2
• expr ? expr term
• ? expr term term
• ? term term term
• ? 9 term term
• ? 9 5 term
• ? 9 5 2
• leftmost/rightmost derivation

6
Parse Tree
expr expr term expr term term 9
- 5 2
7
Translation Scheme

• Context-free Grammar with Embedded Semantic
  Actions.
• expr expr term
• expr expr term
• expr term
• term 0
• term 1
• ...
• term 9

print() print(-) print(0)
print(1) print(9)
emitting (???) a translation
8
Parse Tree with Semantic Actions
expr print()
expr term - print(-) 2
print(2) expr term term 5
print(5) 9 print(9)
Depth-first traversal
Input 9 5 2 Output
9
5
-
2

9
Location of Semantic Actions

• Semantic Actions can be placed anywhere on the
  RHS.
• expr print() expr term
• expr print(-) expr term
• expr term
• term 0 print(0)
• term 1 print(1)
• ...
• term 9 print(9)

10
Parsing Approaches

• Top-down parsing
• build parse tree from start symbol
• match result terminal string with input stream
• simple but limit in power
• Bottom-up parsing
• start from input token stream
• build parse tree from terminal until get start
  symbol
• complex but powerful

11
Top Down vs. Bottom Up
start here
result
match
start here
result
input token stream
input token stream
Top-down Parsing
Bottom-up Parsing
12
Example

• type simple
• id
• array simple of type
• simple integer
• char
• num dotdot num
• Input Token String
• array num dotdot num of integer

13
Top-Down Parsing with Left-to-right Scanning of
Input Stream

• type
• array simple of type
• Input array num dotdot num of integer

lookahead token
14
Backtracking(Recursive-Descent Parsing)

• simple
• integer char num
• Input array num dotdot num of integer

lookahead token
15
Predictive Parsing

• type simple
• id
• array simple of type
• simple integer
• char
• num dotdot num

type array simple of
type Input array num dotdot num of integer
lookahead token
16
The Program for Predictive Parser
match (scanner)
Input (text stream)
a
r
r
a
y

OK
match(array)
Predictive Parser
Output
17
The Program for Predictive Parsing

• procedure match ( t token ) procedure
  simple
• begin begin
• if lookahead t then if lookahead
  integer then
• lookahead nexttoken match ( integer )
• else error else if lookahead char
  then
• end match ( char )
• else if lookahead num then begin
• procedure type match ( num )
• match ( dotdot )
• match ( num )
• begin end
• if lookahead is in integer, char, num
  then else error
• simple end
• else if lookahead then begin
• match ( ) match ( id )
• end
• else if lookahead array then begin
• match ( array ) match ( ) simple
  match ( ) match ( of ) type
• end

18
Mapping Between Production and Parser Codes

• type -gt arrary simple of type
• match(array) match() simple match()
  match(of) type

scanner
parser
parsing (recognition) of simple
19
Lookahead Symbols

• A -gt a
• FIRST( a ) set of fist token in strings
• generated from a
• FIRST(simple) integer, char, num
• FIRST( id )
• FIRST(array simple of type) array

20
Rules for Predictive Parser

• If A -gt a and A -gt b then
• FIRST(a) and FIRST(b) are disjoint
• e-production
• stmt -gt begin opt_stmts end
• opt_stmts -gt stmt_list opt_stmts e

21
Left Recursion

• Left Recursion gt Parser loops forever
• A -gt Aa b
• expr -gt expr term term
• Rewriting...
• A -gt b R
• R -gt a R e

22
Example

• expr ? expr term
• expr ? expr - term
• expr ? term
• term ? 0 1 2 ... 9

expr ? term rest rest ? term rest -
term rest e term ? 0 1 2 ... 9
23
Semantic Actions
expr ? term rest rest ? term print()
rest - term print(-) rest e term
? 0 print(0) 1 print(1) ...
24
expr ? term rest rest ? term print()
rest - term print(-) rest e term
? 0 print(0) ...

• procedure rest
• begin
• if lookahead then
• begin
• match()
• term()
• print()
• rest()
• else if lookahead - then begin
• match(-)
• term()
• print(-)
• rest()
• end
• end

procedure expr begin term() rest() end
25
Abstract Stack Machines

• Primitive Arithmetic
• Operator
• 1 3 5
• Stack 1
• Stack 3
• Add
• Stack 5
• Multiply

• L-values R-values
• i i 1
• ADDRESS OF i VALUE OF i
• p q

26
Stack Manipulation

• push v push v on top of stack
• pop v pop v from top of stack
• rvalue l push content of data l
• lvalue l push address of data l
• put r-value on top to l-value below and pop
  both from top
• copy push copy of top

27
Translation of Expression

• day (1461 y) div 4 (153 m 2) div 5
  d
• lvalue day push 153 div
• push 1461 rvalue m
• rvalue y rvalue d
• push 2
• push 4
• div push 5

28
Translation Scheme for Assignment Statement

• stmt -gt id emit(lvalue, id.lexeme)
• expr emit()

29
Control Flow

• label l target for jump
• goto l jump to lable l
• gofalse l pop top jump if zero
• gotrue l pop top jump if nonzero

30
Translation of if Statement

• stmt -gt if expr out newlabel emit (
  gofalse, out )
• then stmt1 emit ( label, out )
• code for expr
• gofalse out
• code for stmt1
• label out