More arcana about MPEs carrying class existentials

The first word in a class existential is the class pointer itself.
This pointer exposes spare bits differently depending
on the platform, which becomes apparent when you try to reflect
an Optional carrying such an MPE.

Add new test cases and some logic to zero out the first
word of spare bit information only on platforms with 8-byte pointers.

Author: tbkka

stdlib/public/RemoteInspection/TypeLowering.cpp

@@ -437,14 +437,24 @@ BitMask RecordTypeInfo::getSpareBits(TypeConverter &TC, bool &hasAddrOnly) const
     // Mask the rest of the fields as usual...
     break;
   }
-  case RecordKind::ClassExistential:
-    // Class existential is a bunch of pointers that expose spare bits
-    // ... so we can fall through ...
+  case RecordKind::ClassExistential: {
+    // First pointer in a Class Existential is the class pointer
+    // itself, which can be tagged or have other mysteries on 64-bit, so
+    // it exposes no spare bits from the first word there...
+    auto pointerBytes = TC.targetPointerSize();
+    if (pointerBytes == 8) {
+      auto zeroPointerSizedMask = BitMask::zeroMask(pointerBytes);
+      mask.andMask(zeroPointerSizedMask, 0);
+    }
+    // Otherwise, it's the same as an Existential Metatype
+    DISPATCH_FALLTHROUGH;
+  }
   case RecordKind::ExistentialMetatype: {
-    // A bunch of pointers that expose spare bits
+    // All the pointers in an Existential Metatype expose spare bits...
+    auto pointerBytes = TC.targetPointerSize();
     auto mpePointerSpareBits = TC.getBuilder().getMultiPayloadEnumPointerMask();
-    auto mpePointerSpareBitMask = BitMask(TC.targetPointerSize(), mpePointerSpareBits);
-    for (int offset = 0; offset < (int)getSize(); offset += TC.targetPointerSize()) {
+    auto mpePointerSpareBitMask = BitMask(pointerBytes, mpePointerSpareBits);
+    for (int offset = 0; offset < (int)getSize(); offset += pointerBytes) {
       mask.andMask(mpePointerSpareBitMask, offset);
     }
     return mask;

validation-test/Reflection/reflect_Enum_values10.swift

@@ -147,6 +147,30 @@ reflect(enumValue: Q2.e(C()))
 // CHECKALL-NEXT: (enum reflect_Enum_values10.Q2)
 // CHECKALL-NEXT: Value: .e(_)
 
+reflect(enumValue: Optional<Q2>.some(.a(C())))
+
+// CHECKALL: Reflecting an enum value.
+// CHECKALL-NEXT: Type reference:
+// CHECKALL-NEXT: (bound_generic_enum Swift.Optional
+// CHECKALL-NEXT:   (enum reflect_Enum_values10.Q2))
+// CHECKALL-NEXT: Value: .some(.a(_))
+
+reflect(enumValue: Optional<Q2>.some(.e(C())))
+
+// CHECKALL: Reflecting an enum value.
+// CHECKALL-NEXT: Type reference:
+// CHECKALL-NEXT: (bound_generic_enum Swift.Optional
+// CHECKALL-NEXT:   (enum reflect_Enum_values10.Q2))
+// CHECKALL-NEXT: Value: .some(.e(_))
+
+reflect(enumValue: Optional<Q2>.none)
+
+// CHECKALL: Reflecting an enum value.
+// CHECKALL-NEXT: Type reference:
+// CHECKALL-NEXT: (bound_generic_enum Swift.Optional
+// CHECKALL-NEXT:   (enum reflect_Enum_values10.Q2))
+// CHECKALL-NEXT: Value: .none
+
 doneReflecting()
 
 // CHECKALL: Done.