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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,

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