[PATCH] D70079: [SelectionDAG] Combine U{ADD,SUB}O diamonds into {ADD,SUB}CARRY

[David Zarzycki via Phabricator via llvm-commits]

davezarzycki marked 4 inline comments as done and an inline comment as not done.
davezarzycki added inline comments.


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Comment at: lib/CodeGen/SelectionDAG/DAGCombiner.cpp:2873
+    return Merged.getValue(1);
+  DAG.ReplaceAllUsesOfValueWith(Carry1.getValue(0), Merged.getValue(0));
+  return DAG.getConstant(0, DL, MVT::i1);
----------------
lebedev.ri wrote:
> I'm not following this bit, why do we do this only for `ISD::AND`?
> 
As the comment above tries to explain, we can prove in this diamond UADDO/USUBO scenario that it is impossible for both UADDO/USUBO nodes to overflow at the same time. Either one does, or the other, therefore using AND to merge the carry flags will always return zero.


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  https://reviews.llvm.org/D70079/new/

https://reviews.llvm.org/D70079