[Runtime] Add entry point to compare conformance descriptors.

include/swift/AST/ASTContext.h

@@ -653,6 +653,11 @@ class ASTContext final {
   /// for the target platform.
   AvailabilityContext getCompareTypeContextDescriptorsAvailability();
 
+  /// Get the runtime availability of the
+  /// swift_compareProtocolConformanceDescriptors entry point for the target
+  /// platform.
+  AvailabilityContext getCompareProtocolConformanceDescriptorsAvailability();
+
   /// Get the runtime availability of features introduced in the Swift 5.2
   /// compiler for the target platform.
   AvailabilityContext getSwift52Availability();

include/swift/Runtime/Metadata.h

@@ -423,6 +423,21 @@ const WitnessTable *swift_getAssociatedConformanceWitness(
                                   const ProtocolRequirement *reqBase,
                                   const ProtocolRequirement *assocConformance);
 
+/// Determine whether two protocol conformance descriptors describe the same
+/// conformance of a type to a protocol.
+///
+/// Runtime availability: Swift 5.4
+///
+/// \param lhs The first protocol conformance descriptor to compare.
+/// \param rhs The second protocol conformance descriptor to compare.
+///
+/// \returns true if both describe the same conformance, false otherwise.
+SWIFT_RUNTIME_EXPORT
+SWIFT_CC(swift)
+bool swift_compareProtocolConformanceDescriptors(
+    const ProtocolConformanceDescriptor *lhs,
+    const ProtocolConformanceDescriptor *rhs);
+
 /// Fetch a uniqued metadata for a function type.
 SWIFT_RUNTIME_EXPORT
 const FunctionTypeMetadata *

include/swift/Runtime/RuntimeFunctions.def

@@ -735,6 +735,18 @@ FUNCTION(GetAssociatedConformanceWitness,
               ProtocolRequirementStructTy->getPointerTo()),
          ATTRS(NoUnwind, ReadNone))
 
+// SWIFT_RUNTIME_EXPORT
+//     SWIFT_CC(swift) bool swift_compareProtocolConformanceDescriptors(
+//         const ProtocolConformanceDescriptor *lhs,
+//         const ProtocolConformanceDescriptor *rhs);
+FUNCTION(CompareProtocolConformanceDescriptors, 
+         swift_compareProtocolConformanceDescriptors, SwiftCC, 
+         CompareProtocolConformanceDescriptorsAvailability,
+         RETURNS(Int1Ty),
+         ARGS(ProtocolConformanceDescriptorPtrTy, 
+              ProtocolConformanceDescriptorPtrTy),
+         ATTRS(NoUnwind, ReadNone))
+
 // Metadata *swift_getMetatypeMetadata(Metadata *instanceTy);
 FUNCTION(GetMetatypeMetadata, swift_getMetatypeMetadata, C_CC, AlwaysAvailable,
          RETURNS(TypeMetadataPtrTy),

lib/AST/Availability.cpp

@@ -299,6 +299,11 @@ AvailabilityContext ASTContext::getCompareTypeContextDescriptorsAvailability() {
   return getSwiftFutureAvailability();
 }
 
+AvailabilityContext
+ASTContext::getCompareProtocolConformanceDescriptorsAvailability() {
+  return getSwiftFutureAvailability();
+}
+
 AvailabilityContext ASTContext::getSwift52Availability() {
   auto target = LangOpts.Target;
 

lib/IRGen/IRGenModule.cpp

@@ -660,6 +660,16 @@ namespace RuntimeConstants {
     }
     return RuntimeAvailability::AlwaysAvailable;
   }
+
+  RuntimeAvailability
+  CompareProtocolConformanceDescriptorsAvailability(ASTContext &Context) {
+    auto featureAvailability =
+        Context.getCompareProtocolConformanceDescriptorsAvailability();
+    if (!isDeploymentAvailabilityContainedIn(Context, featureAvailability)) {
+      return RuntimeAvailability::ConditionallyAvailable;
+    }
+    return RuntimeAvailability::AlwaysAvailable;
+  }
 } // namespace RuntimeConstants
 
 // We don't use enough attributes to justify generalizing the

stdlib/public/runtime/Metadata.cpp

@@ -4993,6 +4993,12 @@ const WitnessTable *swift::swift_getAssociatedConformanceWitness(
                                                    assocConformance);
 }
 
+bool swift::swift_compareProtocolConformanceDescriptors(
+    const ProtocolConformanceDescriptor *lhs,
+    const ProtocolConformanceDescriptor *rhs) {
+  return MetadataCacheKey::compareProtocolConformanceDescriptors(lhs, rhs) == 0;
+}
+
 /***************************************************************************/
 /*** Recursive metadata dependencies ***************************************/
 /***************************************************************************/

stdlib/public/runtime/MetadataCache.h

@@ -386,6 +386,14 @@ class MetadataCacheKey {
 
     auto *aDescription = awt->getDescription();
     auto *bDescription = bwt->getDescription();
+    return compareProtocolConformanceDescriptors(aDescription, bDescription);
+  }
+
+public:
+  /// Compare two conformance descriptors, checking their contents if necessary.
+  static int compareProtocolConformanceDescriptors(
+      const ProtocolConformanceDescriptor *aDescription,
+      const ProtocolConformanceDescriptor *bDescription) {
     if (aDescription == bDescription)
       return 0;
 
@@ -405,6 +413,7 @@ class MetadataCacheKey {
                            bDescription->getProtocol());
   }
 
+private:
   /// Compare the content from two keys.
   static int compareContent(const void * const *adata,
                             const void * const *bdata,