Re: [boost] interest in structure of arrays container?

Subject: Re: [boost] interest in structure of arrays container?
From: Larry Evans (cppljevans_at_[hidden])
Date: 2016-10-26 01:58:04

On 10/25/2016 11:07 PM, Michael Marcin wrote:
> On 10/25/2016 8:23 PM, Larry Evans wrote:
>>>
>>> At the very least support for the basic SSE 16 byte alignment of
>>> subarrays is crucial.
>>>
>>>
>>> My best idea so far is some magic wrapper type that gets special
>>> treatment. Like:
>>> using data_t = soa_block< float3, soa_align<float,16>, bool >;
>>
>> Something like:
>>
>>   template<typename T, std::size_t Alignment>
>>   struct alignas(Alignment) soa_align {
>>     T data;
>>   };
>>
>> Have you tried that yet.  If not, I might try.
>>
>
> The issue is you don't want to overalign all elements of the array, just
> the first element.
>

But aligning the first soa_align<T,A> is all that's needed because
sizeof(soa_align<T,A>)%A == 0, hence, all subsequent elements would
be aligned.  At least that's my understanding.  Am I missing something?

> I have a working solution (using Peter Dimov's mp11 library as I'm not
> well-versed in post cpp03 metaprogramming).
>
> I'm just trying to play around with implementation ideas at the moment.
>
> Basically it'd be a nice to store only a single pointer and cheap
> constant time member sub-array access.
>
> But with alignment concerns all I've managed so far are two
> implementations.
>
>  1. 1 pointer with linear time member array access
>  2. n-pointers with constant time member array access
>
> I feel like there should exist implementation that trades a bit of
> dynamic allocation size for a single pointer and constant time member
> array access.
>

I intended soa_block to fill that need (after all the tasks
shown in the **TODO** comments were done).  If you see some
flaw in the code, of course, I love to hear about it.
**TODO**

-regards,
Larry