Compiler Structures

Compiler Structures 241-437, Semester 1, 2011-2012 • Objective • extend the expressions language compiler to generate a parse tree for the input program, and then evaluate it 9. Creating and Evaluating a Parse Tree

Overview 1. The Expressions Grammar 2. exprParse2.c 3. Parse Tree Data Structures 4. Revised Parse Functions 5. Tree Building 6. Tree Printing 7. Tree Evaluation

Source Program In this lecture Lexical Analyzer Front End Syntax Analyzer Semantic Analyzer Int. Code Generator concentrating on parse tree generation and evaluation Intermediate Code Code Optimizer Back End Target Code Generator Target Lang. Prog.

An Expressions Program (test3.txt) 5 + 6 let x = 2 3 + ( (x*y)/2) // comments // y let x = 5 let y = x /0 // comments

exprParse2.c • A recursive descent parser using the expressions language. • This version of the parser differs from exprParse1.c by having the parse functions (e.g. statements(), statement()) create a parse tree as they execute. continued

There's a new printTree() function which prints the final tree, and evalTree() which evaluates it. • Usage: $ gcc -Wall -o exprParse2 exprParse2.c$ ./exprParse2 < test1.txt

Output for test1.txt let x = 2 let y = 3 + x > exprParse2 < test1.txt \n \n NULL = x 2 = y + 3 x \n \n = printed tree; same as y + = NULL x 2 3 x continued

x being declared x = 2 == 2 y being declared y = 5 == 5 > evaluation of the parse tree

3. Parse Tree Data Structures typedef struct TreeNode { Token operTok; union { char *id; int value; struct {struct TreeNode *left, *right;} branches; } u; } Tree; A tree is made from TreeNodes.

Graphically one of ID, INT,NEWLINE,ASSIGNOP,PLUSOP, MINUSOP, MULTOP, DIVOP TreeNode operTok id variable name (for ID) OR a union, u value integer (for INT) OR children pointers of this node (used by NEWLINE, ASSIGNOP,PLUSOP, MINUSOP, MULTOP, DIVOP) left right branches

Macros for Using TreeNode Fields #define TreeOper(t) ((t)->operTok) #define TreeID(t) ((t)->u.id) #define TreeValue(t) ((t)->u.value) #define TreeLeft(t) ((t)->u.branches.left) #define TreeRight(t) ((t)->u.branches.right)

4. Revised Parse Functions • The parse functions have the same 'shape' as the ones in exprParse0.c, but now call tree building functions, and return a Tree result. • Functions: • main(), statements(), statement(), expression(), term(), factor()

int main(void) { nextToken(); statements(); match(SCANEOF); return 0; } int main(void) // parse, then print and evaluate the resulting tree { Tree *t; nextToken(); t = statements(); match(SCANEOF); printTree(t, 0); printf("\n\n"); evalTree(t); return 0; } main() Before and After

statements() Before and After with no semantic actions void statements(void) // statements ::= { [ statement] '\n' } { dprint("Parsing statements\n"); while (currToken != SCANEOF) { if (currToken != NEWLINE) statement(); match(NEWLINE); } } // end of statements()

Tree *statements(void) { Tree *t, *left, *statTree; left = NULL; dprint("Parsing statements\n"); while (currToken != SCANEOF) { if (currToken != NEWLINE) statTree = statement(); else statTree = NULL; match(NEWLINE); if (statTree != NULL) { t = makeTreeNode(NEWLINE, left, statTree); left = t; } } return left; } // end of statements()

Tree Structure for statements • A statements sequence: s1 \n1 s2 \n2 s3 \n3 becomes: \n3 \n2 s3 s2 \n1 s1 NULL

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

Tree *statement(void) { Tree *t, *idTree, *exprTree; dprint("Parsing statement\n"); if (currToken == LET) { match(LET); idTree = matchId(); // build tree node, not symbol table entry match(ASSIGNOP); exprTree = expression(); t = makeTreeNode(ASSIGNOP, idTree, exprTree); } else // expression t = expression(); return t; } // end of statement()

Tree Structures for statement = or expr tree ID node expr tree

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

Tree *expression(void) { Tree *t, *left, *right; int isAddOp; dprint("Parsing expression\n"); left = term(); while((currToken == PLUSOP)||(currToken == MINUSOP)) { isAddOp = (currToken == PLUSOP) ? 1 : 0; nextToken(); right = term(); if (isAddOp == 1) // addition t = makeTreeNode(PLUSOP, left, right); else // subtraction t = makeTreeNode(MINUSOP, left, right); left = t; } return left; } // end of expression()

Tree Structure for expression • An expression sequence: t1 +1 t2 - t3 +2 t4 becomes: +2 - t4 t3 +1 t2 t1

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

Tree *term(void) { Tree *t, *left, *right; int isMultOp; dprint("Parsing term\n"); left = factor(); while((currToken == MULTOP) || (currToken == DIVOP)) { isMultOp = (currToken == MULTOP) ? 1 : 0; nextToken(); right = factor(); if (isMultOp == 1) // multiplication t = makeTreeNode(MULTOP, left, right); else // division t = makeTreeNode(DIVOP, left, right); left = t; } return left; } // end of term()

Tree Structure for term • An term sequence: f1 *1 f2 / f3 *2 f4 becomes: *2 / f4 f3 *1 f2 f1

factor() Before and After with no semantic actions 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()

Tree *factor(void) { Tree *t = NULL; dprint("Parsing factor\n"); if(currToken == LPAREN) { match(LPAREN); t = expression(); match(RPAREN); } else if(currToken == INT) { t = makeIntLeaf(currTokValue); match(INT); } else if (currToken == ID) t = matchId();// do not access symbol table else syntax_error(currToken); return t; } // end of factor()

Match an ID (Extended) Tree *matchId(void) { Tree *t; if (currToken == ID) t = makeIDLeaf(tokString); match(ID); return t; } // end of matchID()

Tree Structure for factor • There are three possible nodes: tree node INT node ID node or or

5. Tree Building TreeNode • The nodes in a parse tree are connected by the parse functions. • A tree node can have three different shapes: operTok id OR value a union OR left right branches

Making a Tree Node Tree *treeMalloc(void) // a tree node with no fields specified { Tree *t; t = (Tree *) malloc( sizeof(Tree) ); if(t == NULL) { /* out of memory? */ perror("Tree Node not made; out of memory"); exit(1); } return t; } // end of treeMalloc()

Making an ID Node operTok ID "id str" id Tree *makeIDLeaf(char *id) { Tree *t; t = treeMalloc(); TreeOper(t) = ID; TreeID(t) = (char *) malloc(strlen(id)+1); strcpy(TreeID(t), id); return t; } // end of makeIDLeaf() no symbol table entry created yet

Making an INT Node operTok INT Tree *makeIntLeaf(int value) { Tree *t; t = treeMalloc(); TreeOper(t) = INT; TreeValue(t) = value; return t; } // end of makeIntLeaf() integer value

Making a Node with Children Tree *makeTreeNode(Token op, Tree *left, Tree *right) /* Build an internal tree node, which contains an operator and points to two subtrees.*/ { Tree *t; t = treeMalloc(); TreeOper(t) = op; TreeLeft(t) = left; TreeRight(t) = right; return t; } // end of makeTreeNode() operTok op branches left right

6. Tree Printing • The printTree() function recurses over the tree, and does three different things depending on the three possible 'shapes' for a tree node. • It includes an indent counter, which is used to print spaces (indents) in front of the node information.

void printTree(Tree *t, int indent) // print a tree, indenting by indent spaces { printIndent(indent); if (t == NULL) { printf("NULL\n"); return; } : continued

Token tok = TreeOper(t); if (tok == INT) printf("%d\n", TreeValue(t)); else if (tok == ID) printf("%s\n", TreeID(t)); else { // operator if (tok == NEWLINE) printf("\\n\n"); // show the \n else printf("%s\n", tokSyms[tok]); printTree(TreeLeft(t), indent+2); printTree(TreeRight(t), indent+2); } } // end of printTree()

void printIndent(int n) { int spaces; for(spaces = 0; spaces != n; spaces++) putchar(' '); } // end of printIndent()

> exprParse2 < test2.txt \n \n \n NULL = x56 2 = bing_BONG - * 27 2 x56 * 5 / 67 3 Tree Printing Examples let x56 = 2 let bing_BONG = (27 * 2) - x56 5 * (67 / 3)

Graphically \n \n * / 5 \n = 67 3 bing_BONG - S3 = NULL x56 * x56 2 27 2 S1 S2

test3.txt 5 + 6 let x = 2 3 + ( (x*y)/2) // comments // y let x = 5 let y = x /0 // comments

> exprParse2 < test3.txt \n \n \n \n \n NULL + 5 6 = x 2 + 3 / * x y 2 = x 5 = y / x 0

7. Tree Evaluation • Tree evaluation works in two stages: • evalTree() searches over the tree looking for subtrees which start with an operator which is not NEWLINE • these subtrees are evaluated by eval(), using the operators in their nodes

Finding non-NEWLINEs \n evalTree() used here \n * / 5 \n = 67 3 bing_BONG - = NULL x56 * x56 2 27 2 eval() used here

Code void evalTree(Tree *t) { if (t == NULL) return; Token tok = TreeOper(t); if (tok == NEWLINE) { evalTree( TreeLeft(t)); evalTree( TreeRight(t)); } else printf("== %d\n", eval(t)); } // end of evalTree()

The operator can be one of ID, INT,ASSIGNOP,PLUSOP, MINUSOP, MULTOP, DIVOP int eval(Tree *t) { SymbolInfo *si; if (t == NULL) return 0; Token tok = TreeOper(t); if (tok == ID) { si = getIDEntry( TreeID(t) ); // lookup ID in symbol table return si->value; } : 7 possibilities continued

else if (tok == INT) return TreeValue(t); else if (tok == ASSIGNOP) { // id = expr si = evalID(TreeLeft(t)); //add ID to sym. table int result = eval(TreeRight(t)); si->value = result; printf("%s = %d\n", si->id, result); return result; } else if (tok == PLUSOP) return eval(TreeLeft(t)) + eval(TreeRight(t)); else if (tok == MINUSOP) return eval(TreeLeft(t)) - eval(TreeRight(t)); :

else if (tok == MULTOP) return eval(TreeLeft(t)) * eval(TreeRight(t)); else if (tok == DIVOP) { int right = eval(TreeRight(t)); if (right == 0) { printf("Error: Div by 0; using 1 instead\n"); return eval(TreeLeft(t)); } else return eval(TreeLeft(t)) / right; } return 0; // shouldn't reach here } // end of eval()

SymbolInfo *evalID(Tree *t) { char *id = TreeID(t); return getIDEntry(id); // create sym. table entry for id } // end of evalID() this function finds or creates a symbol table entry for the id, and return a pointer to the entry (same as in exprParse1.c)

Compiler Structures