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The patterns that set a mask register to 0/1 KXOR %kn, %kn, %kn / KXNOR %kn, %kn, %kn are replaced with KXOR %k0, %k0, %kn / KXNOR %k0, %k0, %kn - AVX-512 targets optimization. KNL does not recognize dependency-breaking idioms for mask registers, so kxnor %k1, %k1, %k2 has a RAW dependence on %k1. Using %k0 as the undef input register is a performance heuristic based on the assumption that %k0 is used less frequently than the other mask registers, since it is not usable as a write mask. Differential Revision: http://reviews.llvm.org/D15739 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256365 91177308-0d34-0410-b5e6-96231b3b80d8