Always operating on just the first two elements of a 4-sized vector

[Matthias Kretz]

Hi Georgios,

I think I have the perfect solution for that coming up for Vc 1.0. The general 
idea is to use something like this:

typedef Vc::simd_array<double, 2> double2_v;

The implementation then is smart enough to figure out that - even when 
compiled for AVX - it should use an SSE register. The double_v type would 
still be the best for the target (in contrast to best for the problem :) ).

Until then, with Vc 0.7, you can only work around this by using a different 
translation unit for where you want to use SSE. Compile with -DVC_IMPL=SSE and 
Vc::double_v will be Vc::SSE::double_v.
BUT, be very careful what this means for the types. All Vc types will be 
different for differently compiled translation units then. You can only 
communicate between those via arrays of fundamental types.

If you want to have a better solution sooner than Vc 1.0 there is something 
you could do for Vc 0.8. In principle Vc 0.8 could support declaring all 
possible Vc implementations in the same translation unit. Then Vc::double_v 
would still behave as now, but in addition you could explicitly use the 
Vc::SSE::double_v type. (Of course this reduces portability - but only until 
Vc 1.0 when you can replace this with the above double2_v.)

Cheers,
	Matthias

-- 
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 Dipl.-Phys. Matthias Kretz

 Web:   http://compeng.uni-frankfurt.de/?mkretz

 SIMD easy and portable: http://compeng.uni-frankfurt.de/?vc
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