Implement short-circuit evaluation for the null coalescing operator

engine/compiler.c

@@ -4648,6 +4648,25 @@ static sxi32 PH7_GenStateEmitExprCode(
 		/* All done */
 		return SXRET_OK;
 	}
+	if(iVmOp == PH7_OP_NULLC) {
+		/* The parser stores the source LHS in pRight for this right-associative operator. */
+		rc = PH7_GenStateEmitExprCode(&(*pGen), pNode->pRight, iFlags);
+		if(rc != SXRET_OK) {
+			return rc;
+		}
+		/* Preserve a non-empty LHS without converting it to boolean. */
+		PH7_VmEmitInstr(pGen->pVm, pGen->pIn->nLine, PH7_OP_JMPNZ, 2, 0, 0, &nJmpIdx);
+		PH7_VmEmitInstr(pGen->pVm, pGen->pIn->nLine, PH7_OP_POP, 1, 0, 0, 0);
+		rc = PH7_GenStateEmitExprCode(&(*pGen), pNode->pLeft, iFlags);
+		if(rc != SXRET_OK) {
+			return rc;
+		}
+		pInstr = PH7_VmGetInstr(pGen->pVm, nJmpIdx);
+		if(pInstr) {
+			pInstr->iP2 = PH7_VmInstrLength(pGen->pVm);
+		}
+		return SXRET_OK;
+	}
 	/* Generate code for the left tree */
 	if(pNode->pLeft) {
 		if(iVmOp == PH7_OP_CALL) {

engine/vm.c

@@ -2344,11 +2344,18 @@ static sxi32 VmByteCodeExec(
 			 *
 			 * Take the jump if the top value is not zero (TRUE jump).Pop the top most
 			 * entry in the stack if P1 is zero.
+			 * If P1 is 2, test for a non-empty value without converting or popping it.
 			 */
 			case PH7_OP_JMPNZ:
 				if(pTos < pStack) {
 					goto Abort;
 				}
+				if(pInstr->iP1 == 2) {
+					if(!PH7_MemObjIsNull(pTos)) {
+						pc = pInstr->iP2 - 1;
+					}
+					break;
+				}
 				/* Get a boolean value */
 				if((pTos->nType & MEMOBJ_BOOL) == 0) {
 					PH7_MemObjToBool(pTos);