Compilers: Attr. Grammars/8 1 Compiler Structures Objective – –describe semantic analysis with attribute grammars, as applied in yacc and recursive descent parsers , Semester 1, Attribute Grammars

Compilers: Attr. Grammars/8 2 Overview 1. What is an Attribute Grammar? 2. Parse Tree Evaluation 3. Attributes 4. Attribute Grammars and yacc 5. A Grid Grammar 6.Recursive Descent and Attributes

Compilers: Attr. Grammars/8 3 In this lecture Source Program Target Lang. Prog. Semantic Analyzer Syntax Analyzer Lexical Analyzer Front End Code Optimizer Target Code Generator Back End Int. Code Generator Intermediate Code concentrating on attribute grammars

Compilers: Attr. Grammars/ What is an Attribute Grammar? An attribute grammar is a context free grammar with semantic actions attached to some of the productions – –semantic = meaning An action specifies the meaning of a production in terms of its body terminals and nonterminals.

Compilers: Attr. Grammars/8 5 Example Attribute Grammar L  E E  E + T E  T T  T * F T  F F  ( E ) F  num printf(E body.val) E.val := E body.val + T body.val E.val := T body.val T.val := T body.val * F body.val T.val := F body.val F.val := E body.val F.val := value(num) ProductionSemantic Action

Compilers: Attr. Grammars/ Parse Tree Evaluation One way of understanding semantic actions is as extra information (attributes) attached to the nodes of the parse tree for the input. The semantic action specifies the parent node attribute in terms of the attributes of its children.

Compilers: Attr. Grammars/8 7 Basic Parse Tree Input: 9 * L  E E  E + T E  T T  T * F T  F F  ( E ) F  num L E T E+ * T F 9 F 5 F 2 T

Compilers: Attr. Grammars/8 8 Adding Meaning to the Tree What is the meaning of "9 * 5 + 2"? – –the answer is to evaluate it, to get 47 Add attributes to the tree, starting from the leaves and working up to the root – –use the semantic actions to get the attribute values

Compilers: Attr. Grammars/8 9 Parse Tree with Actions L E T E+ * T F 9 F 5 F 2 T printf(E body.val) E.val := E body.val + T body.val E.val := T body.val T.val := T body.val * F body.val T.val := F body.val F.val := E body.val F.val := value(num) printf 2 2 evaluate bottom-up 5

Compilers: Attr. Grammars/ Attributes Attribute values can be – –numbers, strings, any data structures, code, assembly language instructions It's not always necessary to build a parse tree in order to evaluate the grammar's action.

Compilers: Attr. Grammars/8 11 Kinds of Attribute There are two main kinds of attribute evaluation: – –synthesized and inherited attributes The value of a synthesized attribute is calculated by using its body values – –as in the previous example

Compilers: Attr. Grammars/8 12 Synthesized Attributes in a Tree Example: ProductionSemantic Action T  T * FT.val := T body.val * F body.val * T F T evaluate bottom-up

Compilers: Attr. Grammars/8 13 Inherited Attributes An inherited attribute for a body symbol (i.e. terminal, non-terminal) gets its value from the other body symbols and the parent value – –often used for evaluating more complex programming language features

Compilers: Attr. Grammars/8 14 Inherited Attributes in a Tree X.x := function(A.a, Y.y) Y.y := function(A.a, X.x) A.a X.xY.y A.a X.xY.y Direction of evaluation Two examples:

Compilers: Attr. Grammars/ Attribute Grammars and yacc yacc supports (synthesized) attribute grammars – –yacc actions are semantic actions – –no parse tree is needed, since yacc evaluates the actions using the parser's built-in stack

Compilers: Attr. Grammars/8 16 expr.y Again %token NUMBER % exprs: expr '\n' { printf("Value = %d\n", $1); } | exprs expr '\n' { printf("Value = %d\n", $2); } ; expr: expr '+' term { $$ = $1 + $3; } | expr '-' term { $$ = $1 - $3; } | term { $$ = $1; } ; continued declarations actions attributes

Compilers: Attr. Grammars/8 17 term: term '*' factor { $$ = $1 * $3; } | term '/' factor{ $$ = $1 / $3; } /* integer division */ | factor ; factor: '(' expr ')' { $$ = $2; } | NUMBER ; continued more actions

Compilers: Attr. Grammars/8 18 $$ #include "lex.yy.c" int yyerror(char *s) { fprintf(stderr, "%s\n", s); return 0; } int main(void) { yyparse(); // the syntax analyzer return 0; } c code

Compilers: Attr. Grammars/8 19 Evaluation in yacc Stack $ $ 3 $ F $ T $ T * $ T * 5 $ T * F $ T $ E $ E + $ E + 4 $ E + F $ E + T $ E $ E \n $ Es Input 3*5+4\n$ *5+4\n$ *5+4\n$ *5+4\n$ 5+4\n$ +4\n$ +4\n$ +4\n$ +4\n$ 4\n$ \n$ \n$ \n$ \n$ $ $ Stack Action shift reduce F  num reduce T  F shift shift reduce F  num reduce T  T * F reduce E  T shift shift reduce F  num reduce T  F reduce E  E + T shift reduce Es  E \n accept val _ Semantic Action $$ = $1 (implicit) $$ = $1 (implicit) $$ = $1 (implicit) $$ = $1 * $3 $$ = $1 (implicit) $$ = $1 (implicit) $$ = $1 (implicit) $$ = $1 + $3 printf $1 Input: 3 * 5 + 4\n

Compilers: Attr. Grammars/ A Grid Grammar A robot starts at (0,0) on a grid, and is given compass directions: – –n = north, s = south, e = east, w = west Evaluate the sequence of directions to work out the final position of the robot.

Compilers: Attr. Grammars/8 21 Example The robot receives the directions: – –n e e n n w – –what is the 'meaning' (semantics) of the directions? – –the 'meaning' is the final robot position, (1,3) start final n e w s

Compilers: Attr. Grammars/ Grid Grammar Input: n w s s robot  path path  path step | e step  e | w | s | n robot path step s path step s path step w path step n e

Compilers: Attr. Grammars/8 23 Grid Attribute Grammar robot  path path  path step path  e step  e step  w step  s step  n printf( path body.(x,y) ) path.x := path body.x + step body.dx path.y := path body.y + step body.dy path.(x,y) = (0,0) step.(dx,dy) := (1,0) step.(dx,dy) := (-1,0) step.(dx,dy) := (0,-1) step.(dx,dy) := (0,1) ProductionSemantic Actions

Compilers: Attr. Grammars/8 24 Data Types The path rules use (x,y), the position of the robot. The step rules use (dx,dy), the step taken by the robot. Implementing these data types requires new features of yacc. (x,y) dx,dy

Compilers: Attr. Grammars/8 25 Parse Tree with Actions Input: n w s s robot path step s path step s path step w path step n e (0,0) (0,1) (-1,1) (-1,0) (-1,-1) 0,1 -1,0 0,-1 printf (-1,-1) evaluate bottom-up

Compilers: Attr. Grammars/ Non-integer Yacc Attributes The default yacc attributes (e.g. $$, $1, etc) are integers. We want data structures for (x,y) and (dx,dy), coded as two struct types.

Compilers: Attr. Grammars/8 27 Defining New Types The new types are collected together inside a %union in the yacc definitions section: %union{ type1 name1; type2 name2;... } For the grid: %union{ struct (int x, int y; } pos; struct (int dx, int dy; } offset; }

Compilers: Attr. Grammars/8 28 The non-terminals that return the new types must be listed. Any tokens that use the types must be listed. For the grid: % type step % type path Using the Types these non-terminals return values of the specified type

Compilers: Attr. Grammars/8 29 Using Typed Variables If an attribute (variable) is a record, then dotted-name notation is used to refer to its fields – –e.g. $$.dx, $1.y The default action ($$ = $1) will cause an error if $$ and $1 are not the same type.

Compilers: Attr. Grammars/ Grid Compiler $ flex grid.l $ bison grid.y $ gcc grid.tab.c -o gridEval grid.l, a flex file grid.y, a bison file bison flexlex.yy.c grid.tab.c gcc gridEval, c executable #include

Compilers: Attr. Grammars/8 31 Usage $./gridEval nwss Robot is at (-1,-1) $./gridEval n n n w w w s e Robot is at (-2,2) $ I typed these lines. I typed ctrl-D

Compilers: Attr. Grammars/8 32 grid.l % [nN]{return NORTH;} [sS]{return SOUTH;} [eE] {return EAST;} [wW]{return WEST;} [ \n\t]; % int yywrap(void) { return 1; }

Compilers: Attr. Grammars/8 33 grid.y %union{ struct { int x; int y; } pos; struct { int dx; int dy; } offset; } %token EAST WEST NORTH SOUTH %type step %type path % continued type definitions types use by the non-terminals

Compilers: Attr. Grammars/8 34 robot: path { printf("Robot is at (%d,%d)\n", $1.x, $1.y); } ; path: path step {$$.x = $1.x + $2.dx; $$.y = $1.y + $2.dy;} | {$$.x = 0; $$.y = 0;} ; step: EAST {$$.dx = 1; $$.dy = 0;} | WEST {$$.dx = -1; $$.dy = 0;} | SOUTH {$$.dx = 0; $$.dy = -1;} | NORTH {$$.dx = 0; $$.dy = 1;} ; % continued

Compilers: Attr. Grammars/8 35 #include "lex.yy.c" int yyerror(char *s) { fprintf(stderr, "%s\n", s); return 0; } int main(void) { yyparse(); return 0; }

Compilers: Attr. Grammars/ Recursive Descent and Attributes It is easy to add semantic actions to a recursive descent parser – –in many cases, there's no need for the parser to build a parse tree in order to evaluate the attributes The basic translation strategy: – –each production becomes a function continued

Compilers: Attr. Grammars/8 37 The function (e.g. f()) calls other functions representing its body non-terminals – –those functions return values (attributes) to f() – –f() combines the values, and returns a value (attribute)

Compilers: Attr. Grammars/ The Expressions Parser Again The basic LL(1) grammar: Stats => ( [ Stat ] \n )* Stat => let ID = Expr | Expr Expr => Term ( (+ | - ) Term )* Term => Fact ( (* | / ) Fact ) * Fact => '(' Expr ')' | Int | Id

Compilers: Attr. Grammars/8 39 An Expressions Program (test3.txt)  give answer let x = 2  declare variable 3 + ( (x*y)/2) // comments // y let x = 5 let y = x /0  error // comments

Compilers: Attr. Grammars/8 40 exprParse1.c is a recursive descent parser using the expressions language. It differs from exprParse0.c by having semantic actions attached to its productions – –these actions evaluate the expressions, and assign values to expression variables 6.2. Parsing with Actions

Compilers: Attr. Grammars/8 41 Grammar with Actions ProductionsActions Stats => ( [ Stat ] \n )* --- Stat => let ID = Expr add id to symbol table; id.val = expr.val; print( id.val ); Stat => Exprprint( expr.val ); continued

Compilers: Attr. Grammars/8 42 Expr => Term ( (+ | - ) Term )* return term 1.val (+| -) term 2.val (+| -)... term n.val; Term => Fact ( (* | / ) Fact ) * return fact 1.val (*| /) fact 2.val (*| /)... fact n.val; continued

Compilers: Attr. Grammars/8 43 Fact => '(' Expr ')return expr.val; Fact => Int return int.val; Fact => Idlookup id; if not found then add (id, 0) to table; return id.val;

Compilers: Attr. Grammars/8 44 The Symbol Table The symbol table is a data structure used to store expression variables and their values. In exprParse1.c, it's an array of structs, with each struct holding the name of the variable and its current integer value... id value syms[]

Compilers: Attr. Grammars/ Usage $ gcc -Wall -o exprParse1 exprParse1.c $./exprParse1 < test3.txt == 11 x being declared x = 2 y being declared == 3 x = 5 Error: Division by zero; using 1 instead y = 5 $

Compilers: Attr. Grammars/ exprParse1.c Callgraph same as in exprParse0.c symbol table (new) generated from grammar (now with actions)

Compilers: Attr. Grammars/ Symbol Table Data Structures #define MAX_SYMS 15 // max no of variables typedef struct SymInfo { char *id; // name of variable int value; // value (an integer) } SymbolInfo; int symNum = 0; // number of symbols stored SymbolInfo syms[MAX_SYMS];.. id value syms[] 01214

Compilers: Attr. Grammars/8 48 Symbol Table Functions SymbolInfo *getIDEntry(void) /* find _OR_ create symbol table entry for current tokString; return a pointer to it */ { SymbolInfo *si = NULL; if ((si = lookupID(tokString)) != NULL) // already declared return si; // add id to table printf("%s being declared\n", tokString); return addID(tokString, 0); //0 is default value } // end of getIDEntry()

Compilers: Attr. Grammars/8 49 SymbolInfo *lookupID(char *nm) /* is nm in the symbol table? return pointer to struct or NULL */ { int i; for(i=0; i<symNum; i++) if (!strcmp(syms[i].id, nm)) return &syms[i]; return NULL; } // end of lookupID()

Compilers: Attr. Grammars/8 50 SymbolInfo *addID(char *nm, int value) /* add nm and value to the symbol table; return pointer to struct */ { if (symNum == MAX_SYMS) { printf("Symbol table full; cannot add %s\n", nm); exit(1); } syms[symNum].id = (char *) malloc(strlen(nm)+1); strcpy(syms[symNum].id, nm); syms[symNum].value = value; SymbolInfo *si = &syms[symNum]; symNum++; return si; } // end of addID()

Compilers: Attr. Grammars/8 51 Using the Symbol Table The grammar functions use the symbol table via the matchID() function. SymbolInfo *matchId(void) // checks current ID with symbol table { SymbolInfo *si; dprint("Parsing ident\n"); if ((si = getIDEntry()) == NULL) { printf("Error: id is NULL on line %d\n",lineNum); exit(1); } match(ID); // ok, so consume ID token return si; } // end of matchId()

Compilers: Attr. Grammars/ Translating the Grammar Rules The same translation is carried out as before, but the code is augmented with actions. The semantic actions are translated into extra C code in the grammar functions.

Compilers: Attr. Grammars/8 53 The Grammar Functions main() and statements() are unchanged from exprParse0.c since they don't have any semantic actions. Functions with extra actions: – –statement(), expression(), term(), factor()

Compilers: Attr. Grammars/8 54 int main(void) { nextToken(); statements(); match(SCANEOF); return 0; } void statements(void) // statements ::= { // [ statement] '\n' } { dprint("Parsing statements\n"); dprint("Parsing statements\n"); while (currToken != SCANEOF) { while (currToken != SCANEOF) { if (currToken != NEWLINE) if (currToken != NEWLINE) statement(); statement(); match(NEWLINE); match(NEWLINE); } } // end of statements() Unchanged Functions

Compilers: Attr. Grammars/8 55 statement() Before and After void statement(void) // statement ::= ( 'let' ID '=' EXPR ) | EXPR { if (currToken == LET) { match(LET); match(ID); match(ASSIGNOP); expression(); } else expression(); } // end of statement() with no semantic actions

Compilers: Attr. Grammars/8 56 void statement(void) // statement ::= ( 'let' ID '=' EXPR ) | EXPR { SymbolInfo *si; int value; dprint("Parsing statement\n"); if (currToken == LET) { match(LET); si = matchId(); // was match(ID); match(ASSIGNOP); value = expression(); si->value = value; printf("%s = %d\n", si->id, value); } else { // expression value = expression(); printf("== %d\n", value); } Actions: add id to table; id.val = expr.val; print( id.val ); or print( expr.val );

Compilers: Attr. Grammars/8 57 expression() Before and After void expression(void) // expression ::= term ( ('+'|'-') term )* { term(); while((currToken == PLUSOP) || (currToken == MINUSOP)) { match(currToken); term(); } } // end of expression() with no semantic actions

Compilers: Attr. Grammars/8 58 int expression(void) // expression ::= term ( ('+'|'-') term )* { int result, v2; int isAddOp; dprint("Parsing expression\n"); result = term(); while((currToken == PLUSOP) || (currToken == MINUSOP)) { isAddOp = (currToken == PLUSOP) ? 1 : 0; match(currToken); v2 = term(); if (isAddOp == 1) // addition result += v2; else // subtraction result -= v2; } return result; } // end of expression() Action: return term 1.val (+| -) term 2.val (+| -)... term n.val;

Compilers: Attr. Grammars/8 59 term() Before and After void term(void) // term ::= factor ( ('*'|'/') factor )* { factor(); while((currToken == MULTOP) || (currToken == DIVOP)) { match(currToken); factor(); } } // end of term() with no semantic actions

Compilers: Attr. Grammars/8 60 int term(void) // term ::= factor ( ('*'|'/') factor )* { int result, v2; int isMultOp; dprint("Parsing term\n"); result = factor(); while((currToken == MULTOP) || (currToken == DIVOP)) { isMultOp = (currToken == MULTOP) ? 1 : 0; match(currToken); v2 = factor(); if (isMultOp == 1) // multiplication result *= v2; else { // division if (v2 == 0) printf("Error: Division by zero; using 1 instead\n"); else result = result / v2; } return result; } // end of term() Action: return fact 1.val (*| / ) fact 2.val (*| / )... fact n.val;

Compilers: Attr. Grammars/8 61 factor() Before and After void factor(void) // factor ::= '(' expression ')' | INT | ID { if(currToken == LPAREN) { match(LPAREN); expression(); match(RPAREN); } else if(currToken == INT) match(INT); else if (currToken == ID) match(ID); else syntax_error(currToken); } // end of factor() with no semantic actions

Compilers: Attr. Grammars/8 62 int factor(void) // factor ::= '(' expression ')' | INT | ID { int result = 0; dprint("Parsing factor\n"); if(currToken == LPAREN) { match(LPAREN); result = expression(); match(RPAREN); } else if(currToken == INT) { match(INT); result = currTokValue; } else if (currToken == ID) { SymbolInfo *si = matchId(); result = si->value; } else syntax_error(currToken); return result; } // end of factor() Actions: return expr.val; or return int.val; or add id to table (if new); return id.val;