llvm dev - Mar 2016 - Should SelectionDAG Chain represent all memory dependies

Hi all:

I'm attempting to improve the way that stores are merged in the Selection
DAG and it looks like memory-memory dependencies are not completely stored
in the Chain, and one must also consider data dependencies in the analysis.
While this makes for a less complex DAG, it also means that that when we
attempt to parallelize Memory operations must search through all possible
predecessor nodes in the DAG for potential aliases (expensive) or we must
be very conservative.
Our current store merge analysis partially works around this by special
casing some common patterns, but if we could efficiently check through all
necessary dependencies during alias analysis we could have a more complete
and simpler merge step.

The obvious fix is to change the DAG to represent all such information in
the Chain and deal with the messier graph, but I wanted to first see the
list's thoughts on the matter.

-Nirav
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