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

Wed Nov 20 06:34:58 PST 2019

lebedev.ri marked an inline comment as done.
lebedev.ri added inline comments.

================
Comment at: lib/CodeGen/SelectionDAG/DAGCombiner.cpp:2815
+//            |   /    |
+//     (uaddo *, *)    \________
+//       |  \                   \
----------------
davezarzycki wrote:
> lebedev.ri wrote:
> > Since you are drawing `add`, you could swap it's operands, and thus CarryIn wouldn't have to intercept PartialCarryOutX. Just a thought, not sure it matters.
> I'd really rather not. The current diagram matches the UADDO/USUBO generated by CodeGenPrepare from plain IR. It also matches the UADDO/USUBO that clang emits as a part of its builtin addc/subc intrinsics. Also, if one substitutes usubo for uaddo, the diagram still holds true, whereas if the add operands were swapped, the diagram would be wrong for usubo.
Sure, that was only an observation.

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

https://reviews.llvm.org/D70079