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

On 10/26/2016 02:27 AM, Michael Marcin wrote:

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.

Ah! I had no clue :( Thanks for explaining. I should have read more about SSE. [snip]