Re: [boost] Boost SIMD beta release

Subject: Re: [boost] Boost SIMD beta release
From: Dave Abrahams (dave_at_[hidden])
Date: 2012-12-20 14:34:55

on Thu Dec 20 2012, Mathias Gaunard <mathias.gaunard-AT-ens-lyon.org> wrote:

> On 20/12/12 19:42, Dave Abrahams wrote:
>>
>> on Thu Dec 20 2012, Mathias Gaunard <mathias.gaunard-AT-ens-lyon.org> wrote:
>>
>>> On 20/12/12 19:12, Dave Abrahams wrote:
>>>>
>
>>>> on Thu Dec 20 2012, Joel Falcou <joel.falcou-AT-gmail.com> wrote:
>>>>
>>>>
>>>>> Maybe we should have a simd::unaligned_input_iterator and/or make
>>>>> simd::transform accept non aligned data. Note that on any pre-Nehalem
>>>>> CPU, the unaligned load will be horrendsously slow.
>>>>
>>>> It's probably a bit late to bring this up, but did you consider a design
>>>> that uses something like Matt Austern's segmented iterator concept
>>>> (http://lafstern.org/matt/segmented.pdf) and imposes a hierarchical view
>>>> over regular memory that segments on aligned memory regions?
>>>
>>> The code needs to be the same for all segments in that model, unless
>>> I've missed something.
>>
>> I don't understand exactly what you mean, nor what problem you see with
>> that model.
>
> This model is good for B-trees and other similar things.

...like deques, circular buffers, blocked arrays, etc.

> I don't see how this can apply here at all.
>
> We need to iterate, load and store our data differently depending on
> the segment.

Yes, incomplete end segments are dealt with differently.  Did you have
something else in mind?

> We even have different types to manipulate on each of them.  In the
> segmented iterator model, all sibling iterators must be the same type,
> and therefore have the same code.

Yes; there's a runtime test.  

> The only reasonable thing to do would be to return a fusion sequence
> of ranges, and have algorithms deal with that. Even this still has a
> bit of overhead over a hand-written loop structure.

I'm afraid you have not been specific enough about the problem you claim
to see for me to be able to evaluate whether it exists or not.

-- 
Dave Abrahams
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