Re: [boost] SIMD implementation of uBLAS

Subject: Re: [boost] SIMD implementation of uBLAS
From: Nasos Iliopoulos (nasos_i_at_[hidden])
Date: 2013-05-29 09:32:59

On 05/29/2013 09:05 AM, Joel Falcou wrote:
> On 29/05/2013 15:00, Nasos Iliopoulos wrote:
>> As a matter of fact I am surprised by the progress  of the compilers 
>> auto-vectorization facilities the last few years, that make me 
>> -doubt- the need for explicit vectorization any more. The GSOC 
>> project will make it clear for us. An added benefit on relying on 
>> compiler is that future vector instructions come for free. A 
>> disadvantage is of course the non-guarantee that auto-vectorization 
>> will work but I find this rarely the case.
>
> I beg to differ, you're in for some nasty surprises. It basically 
> works for simple operations on simple one-level loops with easily 
> inferred loop boundaries.
> Also, those stuff are very fragile and based on vendor willingness to 
> do whatever.
>
That's true. So what we are looking at is breaking down the certain 
algorithms to a state that the vectorizer can penetrate the patterns. A 
triple loop won't   be just vectorized but providing clear functional 
paths is working. I don't expect that it will work generally for the 
current expression templates back-end that's why the we encouraged the 
student to keep his proposal within the bounds of certain functions.

Additionally just injecting  explicit vectorization instructions is not 
gonna work; you need to alter your computational patterns that in the 
end come very close to what the compiler would optimize anyway.

> In multiple actual cases we had to deal with in both academic and 
> industrial context, the autovectorizer was rapidly confused even for 
> rather simple c++ code.
>
We are also worried with polluting the code with vectorization 
instructions that will make things quite unmanageable in the future. I 
also think boost libraries should stay closer to what standard C++ 
specifies and this fixation of mine may be hindering my willingness to 
support non-standard items.

This is a good discussion,
Nasos

>
> _______________________________________________
> Unsubscribe & other changes: 
> http://lists.boost.org/mailman/listinfo.cgi/boost