Runtime: Optional-like enums with single-refcounted payloads can share the refcounted value witnesses.

swift_retain and objc_retain both handle nil already, so we can bypass the generic Optional value witnesses when instantiating Optional on a class type, which happens kind of a lot.

Swift SVN r23613

include/swift/Runtime/Metadata.h

@@ -2190,14 +2190,6 @@ swift_getTupleTypeMetadata3(const Metadata *elt0, const Metadata *elt1,
                             const Metadata *elt2, const char *labels,
                             const ValueWitnessTable *proposedWitnesses);
 
-/// Replace entries of a freshly-instantiated value witness table with more
-/// efficient common implementations where applicable.
-///
-/// For instance, if the value witness table represents a POD type, this will
-/// insert POD value witnesses into the table. The vwtable's flags must have
-/// been initialized before calling this function.
-void swift_installCommonValueWitnesses(ValueWitnessTable *vwtable);
-
 /// Initialize the value witness table and struct field offset vector for a
 /// struct, using the "Universal" layout strategy.
 extern "C" void swift_initStructMetadata_UniversalStrategy(size_t numFields,

stdlib/runtime/Enum.cpp

@@ -18,6 +18,7 @@
 #include "swift/Basic/Fallthrough.h"
 #include "swift/Runtime/Metadata.h"
 #include "swift/Runtime/Enum.h"
+#include "Private.h"
 #include <cstring>
 #include <algorithm>
 
@@ -73,7 +74,22 @@ swift::swift_initEnumValueWitnessTableSinglePayload(ValueWitnessTable *vwtable,
   vwtable->stride = llvm::RoundUpToAlignment(size, align);
   
   // Substitute in better common value witnesses if we have them.
-  swift_installCommonValueWitnesses(vwtable);
+  // If the payload type is a single-refcounted pointer, and the enum has
+  // a single empty case, then we can borrow the witnesses of the single
+  // refcounted pointer type, since swift_retain and objc_retain are both
+  // nil-aware. Most single-refcounted types will use the standard
+  // value witness tables for NativeObject or UnknownObject. This isn't
+  // foolproof but should catch the common case of optional class types.
+  auto payloadVWT = payload->getValueWitnesses();
+  if (emptyCases == 1
+      && (payloadVWT == &_TWVBo
+          || payloadVWT == &_TWVBO)) {
+#define COPY_PAYLOAD_WITNESS(NAME) vwtable->NAME = payloadVWT->NAME;
+    FOR_ALL_FUNCTION_VALUE_WITNESSES(COPY_PAYLOAD_WITNESS)
+#undef COPY_PAYLOAD_WITNESS
+  } else {
+    installCommonValueWitnesses(vwtable);
+  }
                        
   // If the payload has extra inhabitants left over after the ones we used,
   // forward them as our own.

stdlib/runtime/Metadata.cpp

@@ -1217,7 +1217,7 @@ static OpaqueValue *pod_direct_initializeArrayWithTakeFrontToBack(
 #define pod_indirect_initializeArrayWithTakeBackToFront \
   pod_direct_initializeArrayWithTakeFrontToBack
 
-void swift::swift_installCommonValueWitnesses(ValueWitnessTable *vwtable) {
+void swift::installCommonValueWitnesses(ValueWitnessTable *vwtable) {
   auto flags = vwtable->flags;
   if (flags.isPOD()) {
     // Use POD value witnesses.
@@ -1230,6 +1230,7 @@ void swift::swift_installCommonValueWitnesses(ValueWitnessTable *vwtable) {
       FOR_ALL_FUNCTION_VALUE_WITNESSES(INSTALL_POD_INDIRECT_WITNESS)
 #undef INSTALL_POD_INDIRECT_WITNESS
     }
+    return;
   }
   
   if (vwtable->flags.isBitwiseTakable()) {
@@ -1251,9 +1252,16 @@ void swift::swift_installCommonValueWitnesses(ValueWitnessTable *vwtable) {
       vwtable->initializeArrayWithTakeBackToFront
         = pod_indirect_initializeArrayWithTakeBackToFront;
     }
+    return;
   }
 
-  return;
+  if (!vwtable->flags.isInlineStorage()) {
+    // For values stored out-of-line, initializeBufferWithTakeOfBuffer is
+    // always a memcpy.
+    vwtable->initializeBufferWithTakeOfBuffer
+      = pod_indirect_initializeBufferWithTakeOfBuffer;
+    return;
+  }
 }
 
 /*** Structs ***************************************************************/
@@ -1275,7 +1283,7 @@ void swift::swift_initStructMetadata_UniversalStrategy(size_t numFields,
   vwtable->stride = layout.stride;
   
   // Substitute in better value witnesses if we have them.
-  swift_installCommonValueWitnesses(vwtable);
+  installCommonValueWitnesses(vwtable);
 
   // We have extra inhabitants if the first element does.
   // FIXME: generalize this.

stdlib/runtime/Private.h

@@ -99,6 +99,17 @@ namespace swift {
   /// where there is no common root class. rdar://problem/18987058
   const ClassMetadata *getRootSuperclass();
   
+  /// Replace entries of a freshly-instantiated value witness table with more
+  /// efficient common implementations where applicable.
+  ///
+  /// For instance, if the value witness table represents a POD type, this will
+  /// insert POD value witnesses into the table. The vwtable's flags must have
+  /// been initialized before calling this function.
+  ///
+  /// Returns true if common value witnesses were used, false otherwise.
+  void installCommonValueWitnesses(ValueWitnessTable *vwtable);
+
+
 } // end namespace swift
 
 #endif /* SWIFT_RUNTIME_PRIVATE_H */