[SystemZ][z/OS] Force alignment to fix build failure on z/OS

[abhina.sreeskantharajan]

Summary

The following commit https://reviews.llvm.org/D125998 added a static_assert which was triggered on z/OS because bitfields are always aligned to 1 regardless of type.

error: static_assert failed due to requirement 'alignof(llvm::SmallVector<llvm::MDOperand, 0>) <= alignof(llvm::MDNode::Header)' "LargeStorageVector too strongly aligned"

The solution was to force the alignment to be size_t.

[abhina.sreeskantharajan]

empty commit

[wolfgangp]

llvm/include/llvm/IR/Metadata.h

959

This fails for me on Windows builds using Visual Studio (at least VS19), since it does not support this attribute. I would suggest using alignas() in the struct declaration instead, like

struct alignas(alignof(size_t)) Header {

and see if this works for you.

1037

This is not guaranteed on all platforms, because some compilers don't necessarily pack the bitfields tightly. In fact, I'll need to reexamine this for Visual Studio builds and perhaps adjust the bitfield types. I would ask you to not put this assert in for now.

[abhina.sreeskantharajan]

Use alignas and remove assert.

[abhina.sreeskantharajan]

Update comment

[abhina.sreeskantharajan]

Thanks, I have changed it to alignas and removed the static_assert.

[wolfgangp]

LGTM.

llvm/include/llvm/IR/Metadata.h

class MDNode : public Metadata {
   friend class LLVMContextImpl;
   friend class DIArgList;
 
   /// The header that is coallocated with an MDNode along with its "small"
   /// operands. It is located immediately before the main body of the node.
   /// 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 {
+  /// Explicity set alignment because bitfields by default have an
+  /// alignment of 1 on z/OS.
+  struct alignas(alignof(size_t)) Header {
     bool IsResizable : 1;
     bool IsLarge : 1;
     size_t SmallSize : 4;
     size_t SmallNumOps : 4;
     size_t : sizeof(size_t) * CHAR_BIT - 10;
 
     unsigned NumUnresolved = 0;
     using LargeStorageVector = SmallVector<MDOperand, 0>;
 
     static constexpr size_t NumOpsFitInVector =
         sizeof(LargeStorageVector) / sizeof(MDOperand);
struct alignas(alignof(size_t)) Header {
     ArrayRef<MDOperand> operands() const {
       if (IsLarge)
         return getLarge();
       return makeArrayRef(reinterpret_cast<const MDOperand *>(this) - SmallSize,
                           SmallNumOps);
     }
   };
 
   Header &getHeader() { return *(reinterpret_cast<Header *>(this) - 1); }
 
   const Header &getHeader() const {
     return *(reinterpret_cast<const Header *>(this) - 1);
   }
 
   ContextAndReplaceableUses Context;
 
 protected: