CSE 431S Code Generation. Washington University Spring 2013

Transcription

1 CSE 431S Code Generation Washington University Spring 2013

2 Code Generation Visitor The code generator does not execute the program represented by the AST It outputs code to execute the program

3 Source to AST Source if (i > 431) { a = b + c; AST IF_STATEMENT GREATER_THAN VAR_USE IDENTIFIER (i) (SymbolInfo: INT, lv = 0) INTEGER_LITERAL (431) BLOCK EXPRESSION_STATEMENT ASSIGN IDENTIFIER (a) (SymbolInfo: INT, lv = 1) ADDITION VAR_USE IDENTIFIER (b) (SymbolInfo: INT, lv = 2) VAR_USE IDENTIFIER (c) (SymbolInfo: INT, lv = 3)

4 AST to Code AST IF_STATEMENT GREATER_THAN VAR_USE IDENTIFIER (i) (SymbolInfo: INT, lv = 0) INTEGER_LITERAL (431) BLOCK EXPRESSION_STATEMENT ASSIGN IDENTIFIER (a) (SymbolInfo: INT, lv = 1) ADDITION VAR_USE IDENTIFIER (b) (SymbolInfo: INT, lv = 2) VAR_USE IDENTIFIER (c) (SymbolInfo: INT, lv = 3) Code iload 0 ldc 431 if_icmpgt label3 iconst_0 goto label4 label3: iconst_1 label4: ifeq label1 iload 2 iload 3 iadd istore 1 goto label2 label1: label2:

5 Break It Down IF_STATEMENT node Create two labels (will be needed later) Visit first child Code for boolean test expression should be generated Code for the boolean expression should leave 0 (for false) or 1 (for true) on top of stack Output code that compares top of stack to 0 and jump to label for else block (to be output later) if 0 Visit second child Code for then block should be generated Output code that jumps over else block and output label to start else block Visit third child (if it exists) Code for else block should be generated Output label at end of else block

6 IF_STATEMENT private void visitifstatementnode(astnode node) throws Exception { String elselabel = generatelabel(); String endlabel = generatelabel(); node.getchild(0).accept(this); // visit first child stream.println(" ifeq " + elselabel); node.getchild(1).accept(this); // visit second child stream.println(" goto " + endlabel); stream.println(elselabel + ":"); ASTNode elseblock = node.getchild(2); if (elseblock!= null) { elseblock.accept(this); // visit third child stream.println(endlabel + ":");

7 GREATER_THAN GREATER_THAN node Create two labels (will be needed later) Visit first child Code should leave value on top of operand stack Visit second child Code should leave value on top of operand stack Output code to compare top two values on stack and push 0 or 1 for false or true Code is type-dependent, but we're using just integer types

8 GREATER_THAN Node private void visitgreaterthannode(astnode node) throws Exception { String truelabel = generatelabel(); String endlabel = generatelabel(); node.getchild(0).accept(this); // visit first child node.getchild(1).accept(this); // visit second child stream.println(" if_icmpgt " + truelabel); stream.println(" iconst_0"); stream.println(" goto endlabel); stream.println(truelabel + ":"); stream.println(" iconst_1"); stream.println(endlabel + ":");

9 VAR_USE Node VAR_USE node Outputs code to load a local var onto the stack Code is type-dependent, but we're using just integer types The LocalVarMapVisitor, which runs before the CodeGenVisitor, has already assigned local var slots to the Identifier nodes private void visitvaruse(astnode node) throws Exception { IdentifierNode idnode = (IdentifierNode) node.getchild(0); SymbolInfo si = idnode.getsymbolinfo(); int lvindex = si.getlocalvarindex(); stream.println(" iload " + lvindex);

10 INTEGER_LITERAL Node INTEGER_LITERAL node Outputs code to load an int on the operand stack private void visitintegerliteralnode(astnode node) throws Exception { int value = ((IntegerLiteralNode) node).getvalue(); stream.println( "ldc " + value);

11 ASSIGN Node ASSIGN node Do not visit the first child Code to store will be output after everything else Visit second child Code should leave value on top of operand stack Output code to store value on top of stack into local var slot specified by first child Code is type-dependent, but we're using just integer types

12 ASSIGN Node private void visitassignnode(astnode node) throws Exception { node.getchild(1).accept(this); // visit second child IdentifierNode idnode = (IdentifierNode) node.getchild(0); SymbolInfo si = idnode.getsymbolinfo(); int lvindex = si.getlocalvarindex(); stream.println(" istore " + lvindex);

13 ADDITION Node ADDITION node Visit first child Code should leave value on top of operand stack Visit second child Code should leave value on top of operand stack Output code to add values on top of stack Code is type-dependent, but we're using just integer types private void visitadditionuse(astnode node) throws Exception { node.getchild(0).accept(this); // visit first child node.getchild(1).accept(this); // visit second child stream.println(" iadd");