3 files changed, 19 insertions, 6 deletions
diff --git a/source/slang/slang-emit-vm.cpp b/source/slang/slang-emit-vm.cpp
index 1602fbfb8..80d3762aa 100644
--- a/source/slang/slang-emit-vm.cpp
+++ b/source/slang/slang-emit-vm.cpp
@@ -247,6 +247,17 @@ public:
             operand = addConstantValue(constantInst);
             mapInstToOperand[inst] = operand;
         }
+        else if (auto constantVector = as<IRMakeVector>(inst))
+        {
+            SLANG_ASSERT(constantVector->getOperandCount() > 0);
+            operand = ensureInst(constantVector->getOperand(0));
+            for (UInt i = 1; i < constantVector->getOperandCount(); i++)
+            {
+                ensureInst(constantVector->getOperand(i));
+            }
+            operand.size *= (uint32_t)constantVector->getOperandCount();
+            mapInstToOperand[inst] = operand;
+        }
         else
         {
             SLANG_UNEXPECTED("unsupported global inst for vm bytecode emit");
diff --git a/source/slang/slang-ir-lower-existential.cpp b/source/slang/slang-ir-lower-existential.cpp
index 076ed57bd..ea7f906a9 100644
--- a/source/slang/slang-ir-lower-existential.cpp
+++ b/source/slang/slang-ir-lower-existential.cpp
@@ -68,13 +68,15 @@ struct ExistentialLoweringContext
         auto witnessTableIdType = cast<IRWitnessTableIDType>(tupleType->getOperand(1));
         auto anyValueType = cast<IRAnyValueType>(tupleType->getOperand(2));
 
-        // Create a null value for `rttiObject` for now since it will not be used.
+        // Create a standin value for `rttiObject` for now since it will not be used
+        // other than test for null in the case of `Optional<IFoo>`.
         auto uint2Type = builder->getVectorType(
             builder->getUIntType(),
             builder->getIntValue(builder->getIntType(), 2));
+        IRInst* standinVal = builder->getIntValue(builder->getUIntType(), 0xFFFFFFFF);
         IRInst* zero = builder->getIntValue(builder->getUIntType(), 0);
-        IRInst* zeroVectorArgs[] = {zero, zero};
-        IRInst* rttiObject = builder->emitMakeVector(uint2Type, 2, zeroVectorArgs);
+        IRInst* standinRTTIVectorArgs[] = {standinVal, zero};
+        IRInst* rttiObject = builder->emitMakeVector(uint2Type, 2, standinRTTIVectorArgs);
 
         // Pack the user provided value into `AnyValue`.
         IRInst* packedValue = inst->getValue();
diff --git a/source/slang/slang-ir-lower-generics.cpp b/source/slang/slang-ir-lower-generics.cpp
index be21a66ba..8677973f5 100644
--- a/source/slang/slang-ir-lower-generics.cpp
+++ b/source/slang/slang-ir-lower-generics.cpp
@@ -23,11 +23,11 @@
 namespace Slang
 {
 // Replace all uses of RTTI objects with its sequential ID.
-// Currently we don't use RTTI objects at all, so all of them
-// are 0.
+// Currently we don't use RTTI objects other than check for null/invalid value,
+// so all of them are 0xFFFFFFFF.
 void specializeRTTIObjectReferences(SharedGenericsLoweringContext* sharedContext)
 {
-    uint32_t id = 0;
+    uint32_t id = 0xFFFFFFFF;
     for (auto rtti : sharedContext->mapTypeToRTTIObject)
     {
         IRBuilder builder(sharedContext->module);