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Fast duplicate lane

Note: This was originally posted on 17th April 2013 at http://forums.arm.com

hi.
I have a little problem.

I have as input a Dn register with 8 byte.
[a, b, c, d, e, f, g, h]

I'd like to have 2 Dn Register with
[a, a, b, b, c, c, d, d]
and
[e, e, f, f, g, g, h, h]

The purpose is to try to do that with a minimum number of NEON register.
for the moment the best Way I found is something like


vmovl.u8              Qn, Dn                 @ convert byte to half word
vmul.u16              Qn, Qn, Qx    @ Dx contain 8 * 257

I'm looking for a solution not using extra register !

do you nhave any idea ?
thank's