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

[Krzysztof Parzyszek via llvm-dev]

On 9/29/2016 9:45 AM, Krzysztof Parzyszek via llvm-dev wrote:
> In the x86-64 case, if EAX had an extra reg unit that it would share
> with RAX (for the unaddressable part extending from bit 16 upwards),
> then none of AL=, AH=, or AX= would invalidate the rest of EAX and RAX,
> while EAX= would, since it would store into the "hidden" reg unit.

Quentin,
If such units were something that targets could explicitly request via 
some construct in a .td file, would you find that acceptable?

-Krzysztof