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Chapter 2-2
A Simple One-Pass Compiler
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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
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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
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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.
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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
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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
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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
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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.
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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
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Top-down parsing
Lookahead … the current token being scanned
Array[ num dotdot num ] of integer– 과정 설명
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(2.8)
type simple
| id
| array [ simple ] of typ
e
Simple integer
| char
| num dotdot num
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(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
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Predictive parsing
First– 예제로 설명– ∈-production
Designing a Predictive Parser Left Recursion
– expr expr + term– 일반화
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expr term rest
rest + term { print(‘+’) } rest | - term { print(‘-’) } rest |
term 0 { print (‘0’) }
term 1 { print (‘1’) }
. . .
term 9 { print (‘9’) }
(2.14)
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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’)}
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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.
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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;
}
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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
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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
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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)
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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'
