[clang] Implement [[msvc::no_unique_address]] (PR #65675)

[Eli Friedman via cfe-commits]

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@@ -2937,14 +2964,97 @@ void MicrosoftRecordLayoutBuilder::layoutNonVirtualBase(
       BaseOffset = CharUnits::Zero();
     } else {
       // Otherwise, lay the base out at the end of the MDC.
-      BaseOffset = Size = Size.alignTo(Info.Alignment);
+      BaseOffset = DataSize = Size = Size.alignTo(Info.Alignment);
     }
+
+    // Place in EmptySubobjects map but don't check the position? MSVC seems to
+    // not allow fields to overlap at the end of a virtual base, but they can
+    // overlap with other bass.
+    EmptySubobjects->CanPlaceBaseAtOffset(Base, BaseOffset);
   }
+
   Bases.insert(std::make_pair(BaseDecl, BaseOffset));
   Size += BaseLayout.getNonVirtualSize();
+  DataSize = Size;
   PreviousBaseLayout = &BaseLayout;
 }
 
+BaseSubobjectInfo *MicrosoftRecordLayoutBuilder::computeBaseSubobjectInfo(
+    const CXXRecordDecl *RD, bool IsVirtual, BaseSubobjectInfo *Derived) {
+  // This is copied directly from ItaniumRecordLayoutBuilder::ComputeBaseSubobjectInfo.
+  BaseSubobjectInfo *Info;
+
+  if (IsVirtual) {
+    // Check if we already have info about this virtual base.
+    BaseSubobjectInfo *&InfoSlot = VirtualBaseInfo[RD];
+    if (InfoSlot) {
+      assert(InfoSlot->Class == RD && "Wrong class for virtual base info!");
+      return InfoSlot;
+    }
+
+    // We don't, create it.
+    InfoSlot = new (BaseSubobjectInfoAllocator.Allocate()) BaseSubobjectInfo;
+    Info = InfoSlot;
+  } else {
+    Info = new (BaseSubobjectInfoAllocator.Allocate()) BaseSubobjectInfo;
+  }
+
+  Info->Class = RD;
+  Info->IsVirtual = IsVirtual;
+  Info->Derived = nullptr;
+  Info->PrimaryVirtualBaseInfo = nullptr;
+
+  const CXXRecordDecl *PrimaryVirtualBase = nullptr;
+  BaseSubobjectInfo *PrimaryVirtualBaseInfo = nullptr;
+
+  // Check if this base has a primary virtual base.
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efriedma-quic wrote:

Are primary virtual bases actually an thing in the MSVC ABI?  It seems like PrimaryVirtualBase and PrimaryVirtualBaseInfo aren't actually used anywhere.

https://github.com/llvm/llvm-project/pull/65675