[NFC] Define move and copy constructors and copy assignment operators for MDOperand.

[wolfgangp]

Summary

Preparation for a patch to introduce a resizing functionality for MDNodes. See D124548 for a more detailed discussion.

Using a SmallVector<MDOperand> as a large storage container seems to require these constructors/operators to be defined.

[dexonsmith]

Can you add a unit test for this? Ideally something that could expose errors if the tracking logic isn’t right, so probably a TempMDTuple or something assigned/moved between operands.

llvm/include/llvm/IR/Metadata.h

778

There shouldn’t be a const here, and Op should probably be reset. Is there a retrack() function for that operation?

782

I don’t think we need this and it seems better to error here. I think we should keep the copy constructor/operator deleted and make this move-only.

[dexonsmith]

llvm/include/llvm/IR/Metadata.h

793

Looking at this, is it even safe to use the standard move operators? Seems like you don’t have a way to pass in the owner…

[wolfgangp]

llvm/include/llvm/IR/Metadata.h

782

In that case I'm a bit stuck because the implementation of SmallVector uses std::fill_n() and std::uninitialized_fill_n() which assign and copy the value type and want the copy constructor (at least on gcc-9). Do you have any suggestions of should we roll our own hung-off storage class after all?

793

IIUC (and I might be wrong there), only operands of uniqued nodes have an owner? Since we're only resizing non-uniqued nodes it seems it would probably work but how would one enforce the destination node always being non-uniqued?

[dexonsmith]

llvm/include/llvm/IR/Metadata.h

778

Yeah, there is; TrackingMDRef(TrackingMDRef &&X) seems to call it.

782

I suspect that's because the move constructor/operator had the wrong signature (an incorrect const). The following seems to work for me locally:

struct MoveOnly {
  MoveOnly() = default;
  MoveOnly(MoveOnly &&) = default;
  MoveOnly &operator=(MoveOnly &&) = default;
  MoveOnly(const MoveOnly &) = delete;
  MoveOnly &operator=(const MoveOnly &) = delete;
};
SmallVector<MoveOnly> X;
X.resize(10);
X.push_back(MoveOnly());

793

Looks like MDNode doesn't expose non-const MDOperand& so it's probably safe as-is!

[wolfgangp]

llvm/include/llvm/IR/Metadata.h

782

Ah, I had the equivalent of

SmallVector<MoveOnly> X(NumOps);

which doesn't compile for me. Anyway that's the workaround.

[dexonsmith]

llvm/include/llvm/IR/Metadata.h

782

Huh! We should probably fix that to make it work at some point! Curious that one works and not the other.

[wolfgangp]

Only define the move constructor and move assignment operator. Use retrack() instead of relying on the destructor to untrack the source op.

[dexonsmith]

In D125994#3526014, @dexonsmith wrote:

Can you add a unit test for this? Ideally something that could expose errors if the tracking logic isn’t right, so probably a TempMDTuple or something assigned/moved between operands.

(Not sure if you were posting for review yet, but I took a look and noticed this comment hadn’t been addressed yet.)

[wolfgangp]

Added a unit test to make sure the move constructor and the move assignment op of MDOperand adjust tracking info.

[dexonsmith]

LGTM.

llvm/include/llvm/IR/Metadata.h

 /// re-unique itself.
 ///
 /// In particular, this is used by \a MDNode.
 class MDOperand {
   Metadata *MD = nullptr;
 
 public:
   MDOperand() = default;
-  MDOperand(MDOperand &&) = delete;
-  MDOperand(const MDOperand &) = delete;
-  MDOperand &operator=(MDOperand &&) = delete;
-  MDOperand &operator=(const MDOperand &) = delete;
+  MDOperand(const MDOperand &&Op) {
+    MD = Op.MD;
+    track(nullptr);
+  }
+  MDOperand(const MDOperand &Op) {
+    MD = Op.MD;
+    track(nullptr);
+  }
+  MDOperand &operator=(const MDOperand &&Op) {
+    reset(Op.MD, nullptr);
+    return *this;
+  }
+  MDOperand &operator=(const MDOperand &Op) {
+    reset(Op.MD, nullptr);
+    return *this;
+  }
   ~MDOperand() { untrack(); }
 
   Metadata *get() const { return MD; }
   operator Metadata *() const { return get(); }
   Metadata *operator->() const { return get(); }
   Metadata &operator*() const { return *get(); }
 
   void reset() {
     untrack();
     MD = nullptr;
   }
   void reset(Metadata *MD, Metadata *Owner) {
     untrack();
     this->MD = MD;
     track(Owner);
   }
 
 private:
   void track(Metadata *Owner) {
     if (MD) {