diff --git a/llvm/include/llvm/IR/Metadata.h b/llvm/include/llvm/IR/Metadata.h
--- a/llvm/include/llvm/IR/Metadata.h
+++ b/llvm/include/llvm/IR/Metadata.h
@@ -952,7 +952,9 @@
   /// For resizable MDNodes, the space for the storage vector is also allocated
   /// immediately before the header, overlapping with the operands.
   struct Header {
-    bool IsResizable : 1;
+    // Explicity set alignment on z/OS because bitfields by default
+    // have an alignment of 1.
+    bool IsResizable [[gnu::aligned(alignof(size_t))]] : 1;
     bool IsLarge : 1;
     size_t SmallSize : 4;
     size_t SmallNumOps : 4;
@@ -1032,6 +1034,9 @@
     }
   };
 
+  static_assert(__builtin_offsetof(Header, NumUnresolved) == sizeof(size_t),
+                "NumUnresolved offset is incorrect");
+
   Header &getHeader() { return *(reinterpret_cast<Header *>(this) - 1); }
 
   const Header &getHeader() const {