Merge pull request swiftlang#18637 from jckarter/share-c-and-uninhabited-enum-value-witness

[WIP] IRGen: Use prefab value witness tables for uninhabited and @objc enums.

Author: jckarter

lib/IRGen/GenValueWitness.cpp

@@ -983,18 +983,28 @@ getAddrOfKnownValueWitnessTable(IRGenModule &IGM, CanType type) {
 
   if (auto nom = type->getAnyNominal()) {
     // TODO: Generic metadata patterns relative-reference their VWT, which won't
-    // work if it's in a different module without supporting indirection through
-    // the GOT.
+    // work if the VWT is in a different module without supporting indirection
+    // through the GOT.
     if (nom->isGenericContext())
       return nullptr;
-    // TODO: Enums need additional value witnesses for their tag manipulation.
-    if (isa<EnumDecl>(nom))
-      return nullptr;
+    // TODO: Non-C enums have extra inhabitants and also need additional value
+    // witnesses for their tag manipulation (except when they're empty, in
+    // which case values never exist to witness).
+    if (auto enumDecl = dyn_cast<EnumDecl>(nom))
+      if (!enumDecl->isObjC() && !type->isUninhabited())
+        return nullptr;
   }
-  
+ 
+  auto &C = IGM.Context;
+
   type = getFormalTypeInContext(type);
 
   auto &ti = IGM.getTypeInfoForUnlowered(AbstractionPattern::getOpaque(), type);
+
+  // Empty types can use empty tuple witnesses.
+  if (ti.isKnownEmpty(ResilienceExpansion::Maximal))
+    return IGM.getAddrOfValueWitnessTable(TupleType::getEmpty(C));
+
   // We only have witnesses for fixed type info.
   auto *fixedTI = dyn_cast<FixedTypeInfo>(&ti);
   if (!fixedTI)
@@ -1003,7 +1013,6 @@ getAddrOfKnownValueWitnessTable(IRGenModule &IGM, CanType type) {
   CanType witnessSurrogate;
 
   // Handle common POD type layouts.
-  auto &C = type->getASTContext();
   ReferenceCounting refCounting;
   if (fixedTI->isPOD(ResilienceExpansion::Maximal)
       && fixedTI->getFixedExtraInhabitantCount(IGM) == 0) {

test/IRGen/empty_enum.swift

@@ -0,0 +1,6 @@
+// RUN: %target-swift-frontend -primary-file %s -emit-ir | %FileCheck %s
+
+// CHECK: @"$S10empty_enum6JamaisOMf" =
+//   CHECK-SAME: @"$SytWV"
+
+enum Jamais {}

test/IRGen/objc_ns_enum.swift

@@ -8,9 +8,9 @@
 import Foundation
 import gizmo
 
-// CHECK: @"$SSo16NSRuncingOptionsVWV" = linkonce_odr hidden constant
 // CHECK: @"$SSo16NSRuncingOptionsVMn" = linkonce_odr hidden constant
 // CHECK: @"$SSo16NSRuncingOptionsVN" = linkonce_odr hidden constant
+//   CHECK-SAME: @"$SBi{{[0-9]+}}_WV"
 // CHECK: @"$SSo16NSRuncingOptionsVSQSCMc" = linkonce_odr hidden constant %swift.protocol_conformance_descriptor { {{.*}}@"$SSo16NSRuncingOptionsVSQSCWa
 // CHECK: @"$SSo28NeverActuallyMentionedByNameVSQSCWp" = linkonce_odr hidden constant