llvm dev - Apr 2017 - callee-save for register aliases

We have vector registers and scalar registers that alias their low bits. That is

def ScalarSRI : SubRegIndex<32>
def Scalar<n> : Register<n>
def Vector<n, s> : Register<n> { let SubRegs = [s]; let
SubRegIndices = ScalarSRI; }

If a function only defs the scalar part of a callee-saved vector register, the
other bits are not affected and it would be desirable not to save and restore
them.  But determineCalleeSaves() uses isPhysRegModified(), which returns true
if that register or any of its aliases are defed.

Is there a way around this?

-Alan
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