[llvm-dev] Reg units for unaddressable register parts?

[Krzysztof Parzyszek via llvm-dev]

On X86, the registers AX, EAX and RAX all share the exact same register 
units. In terms of units, there is no difference between these 
registers. This makes register units insufficient to track liveness, 
since live AX does not imply live EAX.

Would it make sense to have register units (and lane masks) for the 
parts of registers that are not individually addressable?

-Krzysztof

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