Merge pull request #9429 from jckarter/dont-resort-protocols-at-runtime

Runtime: swift_getExistentialTypeMetadata should trust the compiler's ordering of protocols in compositions.

stdlib/public/runtime/Demangle.cpp

@@ -161,14 +161,10 @@ swift::_swift_buildDemanglingForMetadata(const Metadata *type,
     auto proto_list = Dem.createNode(Node::Kind::ProtocolList);
     proto_list->addChild(type_list, Dem);
 
-    // Sort the protocols by their mangled names.
-    // The ordering in the existential type metadata is by metadata pointer,
-    // which isn't necessarily stable across invocations.
-    std::sort(protocols.begin(), protocols.end(),
-          [](const ProtocolDescriptor *a, const ProtocolDescriptor *b) -> bool {
-            return strcmp(a->Name, b->Name) < 0;
-          });
-
+    // The protocol descriptors should be pre-sorted since the compiler will
+    // only ever make a swift_getExistentialTypeMetadata invocation using
+    // its canonical ordering of protocols.
+
     for (auto *protocol : protocols) {
       // The protocol name is mangled as a type symbol, with the _Tt prefix.
       StringRef ProtoName(protocol->Name);

stdlib/public/runtime/Metadata.cpp

@@ -2501,8 +2501,9 @@ swift::swift_getExistentialTypeMetadata(ProtocolClassConstraint classConstraint,
                                         const ProtocolDescriptor **protocols)
     SWIFT_CC(RegisterPreservingCC_IMPL) {
 
-  // Sort the protocol set.
-  std::sort(protocols, protocols + numProtocols);
+  // We entrust that the compiler emitting the call to
+  // swift_getExistentialTypeMetadata always sorts the `protocols` array using
+  // a globally stable ordering that's consistent across modules.
 
   ExistentialCacheEntry::Key key = {
     superclassConstraint, classConstraint, numProtocols, protocols

unittests/runtime/Metadata.cpp

@@ -449,41 +449,6 @@ TEST(MetadataTest, getExistentialMetadata) {
       return b;
     });
 
-  // protocol compositions are order-invariant
-  RaceTest_ExpectEqual<const ExistentialTypeMetadata *>(
-    [&]() -> const ExistentialTypeMetadata * {
-
-      const ProtocolDescriptor *protoList4[] = {
-        &ProtocolA,
-        &ProtocolB
-      };
-
-      const ProtocolDescriptor *protoList5[] = {
-        &ProtocolB,
-        &ProtocolA
-      };
-
-      auto ab = swift_getExistentialTypeMetadata(ProtocolClassConstraint::Any,
-                                                /*superclass=*/nullptr,
-                                                2, protoList4);
-      auto ba = swift_getExistentialTypeMetadata(ProtocolClassConstraint::Any,
-                                                /*superclass=*/nullptr,
-                                                2, protoList5);
-      EXPECT_EQ(ab, ba);
-      EXPECT_EQ(MetadataKind::Existential, ab->getKind());
-      EXPECT_EQ(2U, ab->Flags.getNumWitnessTables());
-      EXPECT_EQ(ProtocolClassConstraint::Any, ab->Flags.getClassConstraint());
-      EXPECT_EQ(2U, ab->Protocols.NumProtocols);
-      EXPECT_TRUE(
-           (ab->Protocols[0]==&ProtocolA && ab->Protocols[1]==&ProtocolB)
-        || (ab->Protocols[0]==&ProtocolB && ab->Protocols[1]==&ProtocolA));
-      EXPECT_EQ(SpecialProtocol::None,
-                ab->Flags.getSpecialProtocol());
-      EXPECT_EQ(nullptr,
-                ab->getSuperclassConstraint());
-      return ab;
-    });
-
   const ProtocolDescriptor *protoList6[] = {
     &ProtocolClassConstrained,
   };
@@ -865,10 +830,8 @@ TEST(MetadataTest, getExistentialTypeMetadata_subclass) {
       EXPECT_EQ(ProtocolClassConstraint::Class,
                 ex2->Flags.getClassConstraint());
       EXPECT_EQ(2U, ex2->Protocols.NumProtocols);
-      EXPECT_TRUE((ex2->Protocols[0] == &OpaqueProto1 &&
-                   ex2->Protocols[1] == &ClassProto1) ||
-                  (ex2->Protocols[0] == &ClassProto1 &&
-                   ex2->Protocols[1] == &OpaqueProto1));
+      EXPECT_TRUE(ex2->Protocols[0] == &OpaqueProto1 &&
+                  ex2->Protocols[1] == &ClassProto1);
       EXPECT_EQ(&MetadataTest2, ex2->getSuperclassConstraint());
       return ex2;
     });