Re: [eigen] std::complex vectorization braindump

[Mark Borgerding <mark@xxxxxxxxxxxxxx>]

Benoit Jacob wrote:
> [snip]
>
> *** General problems ***
>
> vectorizing std::complex breaks at least 2 assumptions that Eigen is
> currently making:
> 1) that vectorized paths can assume real numbers
>  ---> For example in Dot.h the vectorized paths don't bother conjugating
> 2) that PacketSize==1 means no vectorization
>  ---> This breaks for complex<double>
>   
[snip]
> 5) The puzzle: What to do about ei_pabs() ? In the same vein it would
> be nice to introduce a ei_pabs2()... but we need to solve the
> question: what should they return, a half of a packet of reals???
>
>   

Is a packet limited to 128 bits long?
If longer is possible, this may have some advantages:
1. PacketSize remains > 1 ( minor benefit to avoid short-term code scrub)
2. Unrolls some loops. (see below)
3. The complex-to-real problem (e.g. abs2) goes away

I've found a certain amount of loop unrolling very beneficial to speed, 
even with SIMD.
e.g. loading 4 SIMD registers, working on them, then storing the results 
can be much faster than doing them one at a time.


-- Mark