Why high vsr registers [vsr32 - vsr63] are not used when -mvsx is specified on powerpc?

[Carrot Wei]

Hi

When I tried to build 444.namd with options "-O2 -m64 -mvsx
-mcpu=power7", I can see vsx instructions are actually used, there are
many xs- started instructions, but none of them use high registers
[vsr32 -vsr63], does anybody know the reason?

One example is function calc_pair_energy_fullelect in file
ComputeNonbondedUtil.o, there are many vsr register spilling but high
vsr registers are never used.

thanks
Carrot