Runtime: Fill in the value witness table of a class when doing singleton metadata initialization

On Windows the image format does not support cross-image absolute
data symbol references. One case where we emit these is in class
metadata, because the value witness table always points at the
value witness table for Builtin.NativeObject, defined in the
runtime.

Instead, fill in the value witness table at runtime when doing
singleton metadata initialization.

Another change that will come later is to force use of singleton
metadata initialization on Windows, even if the class is otherwise
completely fixed.

Author: slavapestov

lib/IRGen/GenMeta.cpp

@@ -2478,17 +2478,6 @@ namespace {
       B.add(metadata);
     }
 
-    /// The runtime provides a value witness table for Builtin.NativeObject.
-    void addValueWitnessTable() {
-      ClassDecl *cls = Target;
-      
-      auto type = (cls->checkObjCAncestry() != ObjCClassKind::NonObjC
-                   ? IGM.Context.TheUnknownObjectType
-                   : IGM.Context.TheNativeObjectType);
-      auto wtable = IGM.getAddrOfValueWitnessTable(type);
-      B.add(wtable);
-    }
-
     void addDestructorFunction() {
       if (auto ptr = getAddrOfDestructorFunction(IGM, Target)) {
         B.add(*ptr);
@@ -2658,6 +2647,14 @@ namespace {
       B.addInt(IGM.SizeTy, getClassFieldOffset(IGM, baseType, var).getValue());
     }
 
+    void addValueWitnessTable() {
+      auto type = (Target->checkObjCAncestry() != ObjCClassKind::NonObjC
+                   ? IGM.Context.TheUnknownObjectType
+                   : IGM.Context.TheNativeObjectType);
+      auto wtable = IGM.getAddrOfValueWitnessTable(type);
+      B.add(wtable);
+    }
+
     void addFieldOffsetPlaceholders(MissingMemberDecl *placeholder) {
       llvm_unreachable("Fixed class metadata cannot have missing members");
     }
@@ -2691,6 +2688,11 @@ namespace {
       B.addInt(IGM.SizeTy, 0);
     }
 
+    void addValueWitnessTable() {
+      // The runtime fills in the value witness table for us.
+      B.add(llvm::ConstantPointerNull::get(IGM.WitnessTablePtrTy));
+    }
+
     void addFieldOffsetPlaceholders(MissingMemberDecl *placeholder) {
       for (unsigned i = 0,
                     e = placeholder->getNumberOfFieldOffsetVectorEntries();

stdlib/public/runtime/Metadata.cpp

@@ -89,7 +89,29 @@ Metadata *TargetSingletonMetadataInitialization<InProcess>::allocate(
     auto *classDescription = cast<ClassDescriptor>(description);
     return swift_relocateClassMetadata(classDescription, pattern);
   }
-  return IncompleteMetadata.get();
+
+  // Otherwise, we have a static template that we can initialize in-place.
+  auto *metadata = IncompleteMetadata.get();
+
+  // If this is a class, we have to initialize the value witness table early
+  // so that two-phase initialization can proceed as if this metadata is
+  // complete for layout purposes when it appears as part of an aggregate type.
+  if (auto *classMetadata = dyn_cast<ClassMetadata>(metadata)) {
+    auto *fullMetadata = asFullMetadata(metadata);
+
+    // Begin by initializing the value witness table; everything else is
+    // initialized by swift_initClassMetadata().
+#if SWIFT_OBJC_INTEROP
+    fullMetadata->ValueWitnesses =
+      (classMetadata->Flags & ClassFlags::UsesSwiftRefcounting)
+         ? &VALUE_WITNESS_SYM(Bo)
+         : &VALUE_WITNESS_SYM(BO);
+#else
+    fullMetadata->ValueWitnesses = &VALUE_WITNESS_SYM(Bo);
+#endif
+  }
+
+  return metadata;
 }
 
 /// Copy the generic arguments into place in a newly-allocated metadata.

test/IRGen/dllimport.swift

@@ -42,14 +42,12 @@ public func g() {
 // CHECK-NO-OPT-DAG: declare dllimport %swift.refcounted* @swift_retain(%swift.refcounted* returned)
 // CHECK-NO-OPT-DAG: @"$s9dllexport1pMp" = external dllimport global %swift.protocol
 // CHECK-NO-OPT-DAG: @"$sytN" = external dllimport global %swift.full_type
-// CHECK-NO-OPT-DAG: @"$sBoWV" = external dllimport global i8*
 // CHECK-NO-OPT-DAG: declare dllimport swiftcc i8* @"$s9dllexport2ciAA1cCvau"()
 // CHECK-NO-OPT-DAG: declare dllimport swiftcc %swift.refcounted* @"$s9dllexport1cCfd"(%T9dllexport1cC* swiftself)
 // CHECK-NO-OPT-DAG: declare dllimport swiftcc %swift.metadata_response @"$s9dllexport1cCMa"(i32)
 // CHECK-NO-OPT-DAG: declare dllimport void @swift_deallocClassInstance(%swift.refcounted*, i32, i32)
 
 // CHECK-OPT-DAG: declare dllimport %swift.refcounted* @swift_retain(%swift.refcounted* returned) local_unnamed_addr
-// CHECK-OPT-DAG: @"$sBoWV" = external dllimport global i8*
 // CHECK-OPT-DAG: @"__imp_$s9dllexport1pMp" = external externally_initialized constant %swift.protocol*
 // CHECK-OPT-DAG: declare dllimport swiftcc i8* @"$s9dllexport2ciAA1cCvau"()
 // CHECK-OPT-DAG: declare dllimport swiftcc %swift.metadata_response @"$s9dllexport1cCMa"(i32)

test/IRGen/generic_vtable.swift

@@ -38,6 +38,10 @@ public class Concrete : Derived<Int> {
 //// Type metadata for 'Base' has a static vtable.
 
 // CHECK-LABEL: @"$s14generic_vtable4BaseCMf" = internal global
+// -- destructor
+// CHECK-SAME: void (%T14generic_vtable4BaseC*)* @"$s14generic_vtable4BaseCfD"
+// -- value witness table
+// CHECK-SAME: i8** @"$sBoWV"
 // -- vtable entry for 'm1()'
 // CHECK-SAME: void (%T14generic_vtable4BaseC*)* @"$s14generic_vtable4BaseC2m1yyF"
 // -- vtable entry for 'm2()'
@@ -110,6 +114,10 @@ public class Concrete : Derived<Int> {
 //// Type metadata for 'Concrete' has a static vtable.
 
 // CHECK-LABEL: @"$s14generic_vtable8ConcreteCMf" = internal global <{{.*}}> <{
+// -- destructor
+// CHECK-SAME: void (%T14generic_vtable8ConcreteC*)* @"$s14generic_vtable8ConcreteCfD",
+// -- value witness table is filled in at runtime
+// CHECK-SAME: i8** null,
 // -- nominal type descriptor
 // CHECK-SAME: @"$s14generic_vtable8ConcreteCMn",
 // -- vtable entry for 'm1()'