Add P3242: Copy and fill for mdspan
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mdspan_copy/mdspan_copy.md
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| ```c++ | ||
| template<class SrcElementType, class SrcExtents, class SrcLayoutPolicy, class SrcAccessorPolicy, | ||
| class DstElementType, class DstExtents, class DstLayoutPolicy, class DstAccessorPolicy> | ||
| void copy(const mdspan<SrcElementType, SrcExtents, SrcLayoutPolicy, SrcAccessorPolicy> &src, const mdspan<DstElementType, DstExtents, DstLayoutPolicy, DstAccessorPolicy> &dst) |
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mdspans are views; would you therefore consider passing them by value, as is done in P1673?
| void copy(const mdspan<SrcElementType, SrcExtents, SrcLayoutPolicy, SrcAccessorPolicy> &src, const mdspan<DstElementType, DstExtents, DstLayoutPolicy, DstAccessorPolicy> &dst) | |
| void copy(mdspan<SrcElementType, SrcExtents, SrcLayoutPolicy, SrcAccessorPolicy> src, mdspan<DstElementType, DstExtents, DstLayoutPolicy, DstAccessorPolicy> dst); |
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revisiting this, because even though they are views, the size of the mdspan is proportional to the number of dynamic extents; so it's very possible to have mdspans that are not passible in registers. I'm happy to keep it as we have in other papers, but this particular aspect might get challenged
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@nmm0 I don't really mind either way. For std::linalg, all the mdspan are no more than rank 2, but here, they might have arbitrary rank.
I suspect LEWG would want to hear that argument. They might also want some performance measurements to justify a difference from the status quo.
mdspan_copy/mdspan_copy.md
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| However, existing standard library facilities are not sufficient here. Currently, `mdspan` does not have iterators or ranges that represent the span of the `mdspan`. Additionally, it's not entirely clear what this would entail. | ||
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| Moreover, the manner in which an `mdspan` is copied (or filled) is highly performance sensitive, particularly in regards to caching behavior when traversing mdspan memory. A naïve user implementation is easy to get wrong in addition to being tedious for higher rank `mdspan`s. Ideally, an implementation would be free to use information about the layout of the `mdspan` known at compile time to perform optimizations; e.g. a continuous span `mdspan` copy for trivial types could be implementeed with a `memcpy`. |
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Naive implementation is not tedious
for i0 in [0, extents(0)
for i1 in [0, extents(1)
...
for iK in [0, extents(K)
dst[i0, i1, ..., iK] = src[i0, i1, ..., iK];just (sometimes) really inefficient.
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Haha well, I think writing an 8-nested for loop is pretty tedious :P. I guess it's a matter of opinion
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I already submitted a PR to kokkos/mdspan that does generic iteration over extents of any rank. It's not really tedious, but would be really inefficient without at least the specializations for exhaustive layouts.
nmm0
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mdspan
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