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

26 Oct 2016

      On 10/26/2016 12:58 AM, Larry Evans wrote:
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On 10/25/2016 11:07 PM, Michael Marcin wrote:
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On 10/25/2016 8:23 PM, Larry Evans wrote:
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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, bool >;
Something like:
template
  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 is all that's needed because
sizeof(soa_align)%A == 0, hence, all subsequent elements would
be aligned.  At least that's my understanding.  Am I missing something?
Perhaps I'm misunderstanding.
Using your struct above:
std::array< soa_align, 4 > data;
std::cout << "align array: " << alignof(decltype(data)) << '\n'
     << "size element: " << sizeof( data[0] ) << '\n'
     << "size array: " << sizeof( data ) << '\n'
     << "offset[1]: " << (char*)&(data[1]) - (char*)data.data() << '\n';

align array: 16
size element: 16
size array: 64
offset[1]: 16

For data to work with SSE instructions this needs to report:

align array: 16
size element: 4
size array: 16
offset[1]: 4

i.e. 4 floats have to be contiguous in memory, and the *first* float has 
to be aligned to 16 bytes.
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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**
IIRC it had implemented roughly the #2 strategy, storing a pointer + an 
array of n+1 offsets to access n members in constant time.