Small fix

src/main/java/chocopy/pa3/CodeGenImpl.java

@@ -814,7 +814,7 @@ public class CodeGenImpl extends CodeGenBase
                     node.right.dispatch(this);
                     backend.emitSW(Register.A0, Register.FP, -sp_off*wordSize, "Push right operand on stack slot "+sp_off);
                     incSp(1);
-                    backend.emitADDI(Register.SP,Register.FP,-sp_off*wordSize,"Set SP to last argument");
+                    backend.emitADDI(Register.SP,Register.FP,-(sp_off-1)*wordSize,"Set SP to last argument");
                     backend.emitJAL(concatLabel, "Call concatenation routine");
                     backend.emitADDI(Register.SP, Register.FP, "-"+size_label, "Set SP to stack frame");
                     return Register.A0;
@@ -1095,19 +1095,19 @@ public class CodeGenImpl extends CodeGenBase
             {
                 backend.emitLW(vacantReg, FP, -sp_off * wordSize, String.format("Pop stack slot %d", sp_off));
                 final Label bp = generateLocalLabel();
-                backend.emitBNEZ(listObj, bp, "Ensure not None");
+                backend.emitBNEZ(vacantReg, bp, "Ensure not None");
                 backend.emitJ(errorNone, "Go to error handler");
                 backend.emitLocalLabel(bp, "Not None");
                 final Label bt = generateLocalLabel();
-                backend.emitLW(temp, listObj, getAttrOffset(listClass, "__len__"), "Load attribute: __len__");
+                backend.emitLW(temp, vacantReg, getAttrOffset(listClass, "__len__"), "Load attribute: __len__");
                 backend.emitBLTU(index, temp, bt, "Ensure 0 <= index < len");
                 backend.emitJ(errorOob, "Go to error handler");
                 backend.emitLocalLabel(bt, "Index within bounds");
                 backend.emitADDI(index, index, 4, "Compute list element offset in words");
                 backend.emitLI(temp, wordSize, "Word size in bytes");
                 backend.emitMUL(index, index, temp, "Compute list element offset in bytes");
-                backend.emitADD(index, listObj, index, "Pointer to list element");
-                backend.emitLW(vacantReg, index, 0, "Set list element");
+                backend.emitADD(index, vacantReg, index, "Pointer to list element");
+                backend.emitLW(Register.A0, index, 0, "Set list element");
             } 
             else 
             {