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
@@ -951,7 +951,9 @@
   /// The operands are in turn located immediately before the header.
   /// For resizable MDNodes, the space for the storage vector is also allocated
   /// immediately before the header, overlapping with the operands.
-  struct Header {
+  struct alignas(alignof(size_t)) Header {
+    // Explicity set alignment on z/OS because bitfields by default
+    // have an alignment of 1.
     bool IsResizable : 1;
     bool IsLarge : 1;
     size_t SmallSize : 4;