Fix possible self assign issue for DIEValue

[XinWang10]

Summary

In DIEValue's operator assignment constructor, it didn't identify if
the two obj is the same.
I add code to identify them so that it will work correctly when we do
self assign here.

[skan]

llvm/include/llvm/CodeGen/DIE.h

467

Shouldn't we compare this with &X instead?

[XinWang10]

• resolve comment

[skan]

llvm/include/llvm/CodeGen/DIE.h

469

https://stackoverflow.com/questions/12015156/what-is-wrong-with-checking-for-self-assignment-and-what-does-it-mean

Use copy-and-swap method?

DIEValue &operator=(DIEValue X) {
  swap(*this, X);
  return *this;
}

[XinWang10]

llvm/include/llvm/CodeGen/DIE.h

469

This reported a lot of errors to me, I think swap will also call operator assign?
From my point of view, the logic here for assign is correct if we just remove destroyVal(), since the Val is on stack, we don't need to destroy it and it will be overwrite by X. But I'm not sure if there is some concern here.

When I read this code, another question occured to me, why do we need the explicit copy and assign constructor, this class don't maintain any dynamic memory.
I think if I remove the copy and assign constructor, it will also work for me.
But I don't want to make that big change :)

[XinWang10]

llvm/include/llvm/CodeGen/DIE.h

469

Seems can pass check-all locally without copy and assign construction function.

[skan]

llvm/include/llvm/CodeGen/DIE.h

469

swap does not call assignment operator, and it swaps the members of two classes.
It seems copyVal will dynamically allocate memory.

What's the error?

[XinWang10]

llvm/include/llvm/CodeGen/DIE.h

469

swap can not get a rvalue as parm.
copyVal calls construct which just calls it's constructor.

[skan]

LGTM

llvm/include/llvm/CodeGen/DIE.h

 public:
   DIEValue() = default;
 
   DIEValue(const DIEValue &X) : Ty(X.Ty), Attribute(X.Attribute), Form(X.Form) {
     copyVal(X);
   }
 
   DIEValue &operator=(const DIEValue &X) {
+    if (this == &X)
+      return *this;
     destroyVal();
     Ty = X.Ty;
     Attribute = X.Attribute;
     Form = X.Form;
     copyVal(X);
     return *this;
   }
 
   ~DIEValue() { destroyVal(); }