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Chapter 2-2
A Simple One-Pass Compiler
2
Syntax-directed translation
Syntax-directed definition translation of a construct in terms of attributes associated with its syntactic components– Syntactic structure :: context-free grammar
– Grammar symbol :: a set of attributes and with each production, a set of semantic rules for computing values of the attributes associated with the symbols appearing in the production
Translation input-output mapping Annotated parse tree a parse tree showing the attribute
values at each node
3
Synthesized Attributes
An attribute is said to be synthesized if its value at a parse-tree node is determined from attribute values at the children of node
예제 설명
4
PRODUCTION SEMANTIC RULE
expr expr1 + term
expr expr1 - term
expr term
term 0
term 1
. . .
term 9
expr.t := expr1.t || term.t || ‘+’
expr.t := expr1.t || term.t || ‘-’
expr.t := term.t
term.t := ‘0’
term.t := ‘1’
. . .
term.t := ‘9’
Syntax-directed definition for infix to postfix translation.
5
expr.t = 95 - 2 +
term.t = 2
+ 2-
term.t = 5expr.t = 9
term.t = 9
9 5
expr.t = 95 -
Attribute values at nodes in a parse tree.
6
Depth first traversals
Robot positioning ::– seq seq instr | begin
instr east | north | west | south Depth-first traversals Translation Schemes
– Context-free grammar in which program fragments called semantic actions are embedded
Emitting a Translation
7
Annotated parse tree for begin west south.
seq.x = -1seq.y = -1
instr.dx = 0instr.dy = -1
seq.x = -1seq.y = 0
seq.x = 0seq.y = 0
instr.dx = -1instr.dy = 0 south
westbegin
8
Syntax-directed definition of the robot’s position.
PRODUCTION SEMANTIC RULES
seq begin seq.x := 0
seq.y := 0
seq seq1 instr seq.x := seq.x1 + instr.dx
seq.y := seq.y1 + instr.dy
instr eastinstr.dx := 1
instr.dy := 0
instr northinstr.dx := 0
instr.dy := 1
instr westinstr.dx := -1
instr.dy := 0
instr southinstr.dx := 0
instr.dy := -1
9
expr expr1 + term
expr expr1 - term
expr term
term 0
term 1
. . .
term 9
{ print (‘+’) }
{ print (‘-’) }
{ print (‘0’) }
{ print (‘1’) }
{ print (‘9’) }
Actions translating expressions into postfix notation.
10
Actions translating 9-5+2 into 95-2+
expr
{print('+')}
term
{print('2')}
+
expr
{print('- ')}
term
{print('5')}
-
expr
term
{print('9')}9
2
5
11
Top-down parsing
Lookahead … the current token being scanned
Array[ num dotdot num ] of integer– 과정 설명
12
(2.8)
type simple
| id
| array [ simple ] of typ
e
Simple integer
| char
| num dotdot num
13
(a) type type
Array [ simple ] of type type
(c) Array [ simple ] of type
num dotdot num type
(d) Array [ simple ] of type
num dotdot num simple
type
(e) Array [ simple ] of type
num dotdot num simple integer
Steps in the top-down construction of a parse tree
14
Predictive parsing
First– 예제로 설명– ∈-production
Designing a Predictive Parser Left Recursion
– expr expr + term– 일반화
15
expr term rest
rest + term { print(‘+’) } rest | - term { print(‘-’) } rest |
term 0 { print (‘0’) }
term 1 { print (‘1’) }
. . .
term 9 { print (‘9’) }
(2.14)
16
Translation of 9 – 5 + 2 into 95 – 2 +.
expr
term rest
9 {print(‘9’)} - term {print(‘-’)} rest
5 {print(‘5’)} + term {print(‘+’)} rest
2 {print(‘2’)}
17
expr(){
term(); rest();}
rest(){
if(lookahead == ‘+’) {match(‘+’); term(); putchar(‘+’); rest();
}else if (lookahead == ‘-’) {
match(‘-’); term(); putchar(‘-’); rest();}else;
} term(){
if (isdigit(lookahead)) {putchar(lookahead); match(lookahead);
}else error;
}
Fig. 2. 22. Functions for the nonterminals expr, rest, and term.
18
Replacement for functions expr and rest of Fig. 2.22.
expr(){
term();while(1)
if(lookahead == ‘+’) {match(‘+’); term(); putchar(‘+’);
}else if (lookahead == ‘-’) {
match(‘-’); term(); putchar(‘-’);}else break;
}
19
Implementing the interactions in Fig. 2. 25.
uses getchar() return token to read character to caller
pushes back c using ungetc(c, stdin)
sets global variable to attribute val
ue
lexan() lexical analyzer
tokenval
20
Symbol table and array for storing strings.
ARRAY symtable
attributes
0
1
2
3
4
lexptr token
ARRAY lexemes
d i v EOS m o d EOS c o u n t EOS i EOS
div
mod
id
id
21
Code layout for conditional and while statements.
label test
code for expr
gofalse out
code for stmt1
goto test
label out
WHILEIF
stmt if expr then stmt1 { out := newlable; stmt.t := expr.t || ‘gofalse’ out || stmt1.t || ‘label’ out
code for expr
gofalse out
code for stmt1
label out
(2.18)
22
Fig. 4.15. Parsing table M for grammar (4.11)
NONTER-MINAL
INPUT SYMBOL
id + * ( ) $
E
E'
T
T'
F
E TE'
T FT'
F id
E' +TE'
T' T' *FT'
E TE'
T FT'
F (E)
E'
T'
E'
T'
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