Fix DebugInfo replaceAllUsesWith.

[friss]

Summary

replaceAllUsesWith had been modified to allow a DbgNode value to be
replaced by itself. In that case a new node is created by copying the
current DbgNode and the copy is used as replacement value.

When that copying happens, the value stored in this->DbgNode at the end
of RAUW would be a reference to the Node that has just been deleted.

This doesn't produce any bug right now, because the DI node on which we
call RAUW won't be used again.

[aprantl]

Good catch! We should probably add to the doxygen comment that this DIType is guaranteed to be valid, after this operation, and that it will wrap a new MDNode or D's MDNode, respectively.

[aprantl]

I did not get why rauw'ing a node with itself isn't a noop. Why is the copy necesary?

[friss]

You'd have to ask David if he remembers the details. I dug the commit history of this stuff, and he is the one that introduced it.

The commits are about memory leaks, thus I believe it was related to getting read of node allocated with MDNode::getTemporary(). In the following commits of the forward decl work, I definitely use it this way. If I have a real definition to replace the temporary forward decl, then RAUW with that, otherwise, RAUW with itself and it turns the temporary node in a permanent one avoiding memory leaks.

[dblaikie]

Thanks for the catch

[friss]

Closed by commit rL217749 (authored by @friss).

llvm/trunk/lib/IR/DebugInfo.cpp

for (size_t i = 0; i != Ops.size(); ++i)
       Ops[i] = DbgNode->getOperand(i);
     DN = MDNode::get(VMContext, Ops);
   }
 
   MDNode *Node = const_cast<MDNode *>(DbgNode);
   const Value *V = cast_or_null<Value>(DN);
   Node->replaceAllUsesWith(const_cast<Value *>(V));
   MDNode::deleteTemporary(Node);
-  DbgNode = D;
+  DbgNode = DN;
 }
 
 /// replaceAllUsesWith - Replace all uses of the MDNode used by this
 /// type with the one in D.
 void DIDescriptor::replaceAllUsesWith(MDNode *D) {
 
   assert(DbgNode && "Trying to replace an unverified type!");
   assert(DbgNode != D && "This replacement should always happen");