[PATCH] D28853: [SelectionDAG] Improve knownbits handling of UMIN/UMAX (PR31293)
Simon Pilgrim via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Thu Jan 19 14:43:21 PST 2017
RKSimon added a comment.
Comment at: lib/CodeGen/SelectionDAG/SelectionDAG.cpp:2712-2715
+ // UMAX - we know that the result will have the maximum of the
+ // known one leading bits of the inputs.
+ unsigned LeadOne = KnownOne.countLeadingOnes();
+ LeadOne = std::max(LeadOne, KnownOne2.countLeadingOnes());
> Should it also be the minimum of the known leading 0s?
If one input is 0xFFFFxxxx and the other is 0xFxxxxxxx then we know that the UMAX result is at least 0xFFFF0000.
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