[LLVMdev] We have a pervasive problem in the SelectionDAG: the CSEMap.

[Chandler Carruth]

There are a large number of places where we use pointer-identity for
SDNodes through code paths which at times use ReplaceAllUsesWith. When
coupled with the CSEMap which re-canonicalizes nodes at every step, this
forms really bad ABA problems: we will often end up deleting a node because
we build a new-but-identical node during RAUW. When this happens, the
pointers cease to be valid and can lead to ABA.

The RAUW code itself and the DAG combiner fix this by using a DAG Update
Listener which snoops on all node deletions and tries to update every
datastructure that might be holding an iterator or pointer key that has
become invalidated. But there are a lot more datastructures key-ing on
'SDNode *' than there are DAG update listeners, so I suspect we have latent
bugs here already.

Does anyone have good ideas about how to solve this? I thought I did until
I realized how fundamental the conflict is. I've now given up, but I wanted
to raise the issue in case others are hitting it or have good ideas about
how to get to a more predictable world.
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