CodeGen: Resize LifetimeExtendedCleanupHeader to avoid alignment issues

The LifetimeExtendedCleanupHeader is carefully fit into 32 bytes,
meaning that cleanups on the LifetimeExtendedCleanupStack are *always*
allocated at a misaligned address and cause undefined behaviour.

There are two ways to solve this - add padding after the header when
we allocated our cleanups, or just simplify the header and let it use
64 bits in the first place. I've opted for the latter, and added a
static assert to avoid the issue in the future.

llvm-svn: 241133

clang/lib/CodeGen/CodeGenFunction.h

@@ -297,12 +297,12 @@ public:
   /// Header for data within LifetimeExtendedCleanupStack.
   struct LifetimeExtendedCleanupHeader {
     /// The size of the following cleanup object.
-    unsigned Size : 29;
+    unsigned Size;
     /// The kind of cleanup to push: a value from the CleanupKind enumeration.
-    unsigned Kind : 3;
+    CleanupKind Kind;
 
-    size_t getSize() const { return size_t(Size); }
-    CleanupKind getKind() const { return static_cast<CleanupKind>(Kind); }
+    size_t getSize() const { return Size; }
+    CleanupKind getKind() const { return Kind; }
   };
 
   /// i32s containing the indexes of the cleanup destinations.
@@ -402,6 +402,8 @@ public:
     LifetimeExtendedCleanupStack.resize(
         LifetimeExtendedCleanupStack.size() + sizeof(Header) + Header.Size);
 
+    static_assert(sizeof(Header) % llvm::AlignOf<T>::Alignment == 0,
+                  "Cleanup will be allocated on misaligned address");
     char *Buffer = &LifetimeExtendedCleanupStack[OldSize];
     new (Buffer) LifetimeExtendedCleanupHeader(Header);
     new (Buffer + sizeof(Header)) T(A...);