Minor details

Author: dibyendumajumdar

registervm/src/main/java/com/compilerprogramming/ezlang/bytecode/BasicBlock.java

@@ -7,8 +7,8 @@
 public class BasicBlock {
     public final int bid;
     public final boolean loopHead;
-    public List<BasicBlock> successors = new ArrayList<>(); // successors
-    List<BasicBlock> predecessors = new ArrayList<>();
+    public final List<BasicBlock> successors = new ArrayList<>(); // successors
+    public final List<BasicBlock> predecessors = new ArrayList<>();
     public final List<Instruction> instructions = new ArrayList<>();
 
     public BasicBlock(int bid, boolean loopHead) {
@@ -25,7 +25,6 @@ public void addSuccessor(BasicBlock successor) {
         successors.add(successor);
         successor.predecessors.add(this);
     }
-
     public static StringBuilder toStr(StringBuilder sb, BasicBlock bb, BitSet visited)
     {
         if (visited.get(bb.bid))
@@ -41,5 +40,4 @@ public static StringBuilder toStr(StringBuilder sb, BasicBlock bb, BitSet visite
         }
         return sb;
     }
-
 }

registervm/src/main/java/com/compilerprogramming/ezlang/bytecode/FunctionBuilder.java

@@ -11,14 +11,22 @@
 
 public class FunctionBuilder {
 
-    BasicBlock entry;
-    BasicBlock exit;
-    int bid = 0;
-    BasicBlock currentBlock;
-    BasicBlock currentBreakTarget;
-    BasicBlock currentContinueTarget;
-
-    List<Operand> virtualStack = new ArrayList<>();
+    public BasicBlock entry;
+    public BasicBlock exit;
+    private int bid = 0;
+    private BasicBlock currentBlock;
+    private BasicBlock currentBreakTarget;
+    private BasicBlock currentContinueTarget;
+
+    /**
+     * We essentially do a form of abstract interpretation as we generate
+     * the bytecode instructions. For this purpose we use a virtual operand stack.
+     *
+     * This is similar to the technique described in
+     * Dynamic Optimization through the use of Automatic Runtime Specialization
+     * by John Whaley
+     */
+    private List<Operand> virtualStack = new ArrayList<>();
 
     public FunctionBuilder(Symbol.FunctionTypeSymbol functionSymbol) {
         AST.FuncDecl funcDecl = (AST.FuncDecl) functionSymbol.functionDecl;
@@ -66,7 +74,7 @@ private void compileReturn(AST.ReturnStmt returnStmt) {
             if (indexed)
                 codeIndexedLoad();
             if (virtualStack.size() == 1)
-                code(new Instruction.Move(pop(), new Operand.ReturnRegisterOperand(0)));
+                code(new Instruction.Move(pop(), new Operand.ReturnRegisterOperand()));
             else if (virtualStack.size() > 1)
                 throw new CompilerException("Virtual stack has more than one item at return");
         }

registervm/src/main/java/com/compilerprogramming/ezlang/bytecode/Operand.java

@@ -37,17 +37,22 @@ public String toString() {
         }
     }
 
+    /**
+     * Represents the return register, which is the location where
+     * the caller will expect to see any return value. The VM must map
+     * this to appropriate location.
+     */
     public static class ReturnRegisterOperand extends Operand {
-        public final int regnum;
-        public ReturnRegisterOperand(int regnum) {
-            this.regnum = regnum;
-        }
+        public ReturnRegisterOperand() {}
         @Override
-        public String toString() {
-            return "R" + regnum;
-        }
+        public String toString() { return "RET"; }
     }
 
+    /**
+     * Represents a temp register, maps to a location on the
+     * virtual stack. Temps start at offset 0, but this is a relative
+     * register number from start of temp area.
+     */
     public static class TempRegisterOperand extends Operand {
         public final int regnum;
         public TempRegisterOperand(int regnum) {
@@ -66,7 +71,6 @@ public LoadIndexedOperand(Operand arrayOperand, Operand indexOperand) {
             this.arrayOperand = arrayOperand;
             this.indexOperand = indexOperand;
         }
-
         @Override
         public String toString() {
             return arrayOperand + "[" + indexOperand + "]";