Problem 4

1. Problem 4 SC Java Byte Code Generator Cheng-Chia Chen

2. Goals • Build a back end for SC to generate Java byte codes from the AST created in Problem 3. • Use the skeleton back end in the file ClassFile.java, inserting calls to routines from the BCEL libraries to generate a sequence of Java byte codes. • Use information from your project 3 symbol table to assign storage for variables in your program.

3. > type Mysc.c int x; int[] y; int m1(int a) { …} void m2() { …} main(){…} global vars ==> static fields procedure ==> static method main() ==> main() > type Mysc.java public class mysc { public static int x; public static int[] y; public static int m1(int 1){…} public static void m2(int 1){…} public static main(String[]) {…} } Mapping from sc programs to java classes

5. project files • Files you need to modify • mysc.lex Regular expressions & actions for scanner (should require no changes from project 3) • mysc.cup Grammar & actions for parser (only change should be uncommenting a few lines) • ClassFile.java :Class to handle byte code generation (most if not all of your code changes)

6. Project Files • (Files you should not need to modify) • Const.java Constants for mycc • symTabEntry.java Symbol table entries • symTab.java Symbol tables • astNode.java AST nodes • astNodeList.java list of AST nodes • expNode.java expression nodes • sym.java produced by CUP from mysc.cup • parser.java produced by CUP from mysc.cup • Yylex.java produced by JLex

7. Project Files • go.bat • script for building compiler • go1.bat <prog> • script for testing compiler on one file <prog.c> • goTest.bat • script for testing compiler on several test files • toy*.c • toy input files that the skeleton compiler can compile • test*.c • test input files for project • test*.out • example compiler output files from working project

8. main functions to be implemented • Documentation on Java Byte code • Document on BCEL : http://jakarta.apache.org/bcel/ • Main methods you should implement in classFile.java • genProcCode() : Generate code for a procedure. Allocate storage for local variables, then create code as a list of instructions. • genTreeCode() : Generate code for a SC astNode representing TREE_BLOCK, TREE_IF or TREE_WHILE statements. • genExpCode() : Generate code for a SC expNode representing a single TREE_INSTRUCTION • genOpCode(): Generate code for a SC expNode representing an operand

9. Main data structures used • InstructionList • instances of which you would used most often. • maintains a list of Java byte code instruction equivalents that make up the output of the compiler back end. • New instructions may be added to the list, and other InstructionLists may also be appended to existing list. • New java byte codes instructions are created through constructors and added to an InstructionList. • Ex: given il x and expNode (plus op1 op2) => • genExpCode(x, op1);// generate instructions for first operand, append to x • genExpCode(x, op2);// generate instructions for 2nd operand, append to x • x.append(new IADD());// generate IADD instruction, append to x

10. The append() function returns an InstructionHandle (label) that can be used as targets of other generated conditional branch instructions: • InstructionList x, y; • InstructionHandle label; • label = x.append(new NOP()); // returns label for conditional branch targets • y.append(new IFEQ(labe1)); // create new conditional branch instruction

11. From mysc.cup • main(String[]) {… String cname = fname.substring(0, fname.length()-2); FileReader fp = new FileReader(fname); addPrintFunctions(); // add print functions in the global symbol table new parser(new Yylex(fp)).parse(); // parse input code.print(); // ast saved in code; print AST ClassFile class_file = new ClassFile(cname); // prepare codegen ClassFile.generate_code(code, globalSymTab); // generate code }