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Chapter 2-2 A Simple One-Pass Compiler

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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

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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

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4 PRODUCTION SEMANTIC RULE expr expr 1 + term expr expr 1 - term expr term term 0 term 1... term 9 expr.t := expr 1.t || term.t || + expr.t := expr 1.t || term.t || - expr.t := term.t term.t := 0 term.t := 1... term.t := 9 Syntax-directed definition for infix to postfix translation.

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5 Attribute values at nodes in a parse tree.

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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

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7 Annotated parse tree for begin west south.

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8 Syntax-directed definition of the robots position. PRODUCTIONSEMANTIC RULES seq begin seq.x := 0 seq.y := 0 seq seq 1 instr seq.x := seq.x 1 + instr.dx seq.y := seq.y 1 + instr.dy instr east instr.dx := 1 instr.dy := 0 instr north instr.dx := 0 instr.dy := 1 instr west instr.dx := -1 instr.dy := 0 instr south instr.dx := 0 instr.dy := -1

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9 expr expr 1 + term expr expr 1 - 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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10 Actions translating into 95-2+

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11 Top-down parsing Lookahead … the current token being scanned Array[ num dotdot num ] of integer –

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12 (2.8) type simple | id | array [ simple ] of type Simple integer | char | num dotdot num

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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

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14 Predictive parsing First – – -production Designing a Predictive Parser Left Recursion – expr expr + term –

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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)

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16 Translation of 9 – 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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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 Functions for the nonterminals expr, rest, and term.

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18 Replacement for functions expr and rest of Fig expr() { term(); while(1) if(lookahead == + ) { match( + ); term(); putchar( + ); } else if (lookahead == - ) { match( - ); term(); putchar( - ); } else break; }

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19 Implementing the interactions in Fig uses getchar()return token to read character to caller pushes back c using ungetc(c, stdin) sets global variable to attribute value lexan() lexical analyzer tokenval

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20 Symbol table and array for storing strings.

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21 Code layout for conditional and while statements. label test code for expr gofalse out code for stmt 1 goto test label out WHILE IF stmt if expr then stmt 1 { out := newlable; stmt.t := expr.t || gofalse out || stmt 1.t || label out code for expr gofalse out code for stmt 1 label out (2.18)

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22 Fig Parsing table M for grammar (4.11) NONTER- MINAL INPUT SYMBOL id+*()$ EE'TT'FEE'TT'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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