craig.topper added a comment. The main reason that we used inline assembly is due to the fixed register allocation for these instructions. We would have had to write out the register rules in the backend as a special case as seen in the getInstrWFourImplicitOps in https://reviews.llvm.org/D44386 Since the register allocator had no freedom there was no performance advantage to an intrinsic. So we just did inline assembly to avoid needing to special case these instructions.
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