[PATCH v6 2/7] libstdc++: Implement submdspan and submdspan_mapping for layout_left. [PR110352]

[Tomasz Kaminski]

On Tue, Dec 9, 2025 at 10:59 AM Jonathan Wakely <jwakely@redhat.com> wrote:

> On Tue, 9 Dec 2025 at 09:37, Tomasz Kamiński <tkaminsk@redhat.com> wrote:
> >
> > From: Luc Grosheintz <luc.grosheintz@gmail.com>
> >
> > Implements `submdspan` and `submdspan_mapping` for layout_left as
> > described in P3663 (Future proofing mdspan).
> >
> > When computing the offset of the submdspan, one must check that the
> > lower bound of the slice range isn't out-of-range. There's a few
> > cases when the lower bound is never out-of-range:
> >
> >   - full_extent and exts.extent(k) != 0,
> >   - collapsing slice types.
> >
> > If those conditions are known to hold, no checks are generated.
> >
> > Similarly, if all slices are full_extent, there's no need to call
> > mapping(0,...,0) for standardized mappings.
> >
> > The implementation prepares to use the symmetry between layout_left and
> > layout_right and introduces concepts like a "layout side", i.e. left,
> > right or unknown/strided.
> >
> > The tests use an iterator to replace nested for-loops. Which also makes
> > it easier to write the core test logic in a rank-independent manner.
> >
> >         PR libstdc++/110352
> >
> > libstdc++-v3/ChangeLog:
> >
> >         * include/std/mdspan (__mdspan::__is_submdspan_mapping_result)
> >         (__mdspan::__submdspan_mapping_result, __mdspan::__fwd_prod)
> >         (__mdspan::__acceptable_slice_type, __mdspan::__slice_begin)
> >         (__mdspan::__suboffset, __mdspan::_LayoutSide,
> __mdspan::__mapping_side)
> >         (__mdspan::_StridesTrait, __mdspan::__substrides_generic)
> >         (__mdspan::__substrides_standardized, __mdspan::__substrides)
> >         (__mdspan::__is_unit_stride_slice, __mdspan::_SliceKind)
> >         (__mdspan::__make_slice_kind, __mdspan::__make_slice_kind_array)
> >         (__mdspan::__is_block, __mdspan::__padded_block_begin_generic)
> >         (__mdspan::__padded_block_begin, __mpdspan::_SubMdspanMapping)
> >         (__mdspan::__submdspan_mapping_impl): Define.
> >         (__mdspan::__dynamic_slice_extent,
> __mdspan::__static_slice_extent)
> >         (__mdspan::__subextents): Move eariel in the file.
>
> "earlier"
>
> >         (layout_left::mapping::submdspan_mapping,
> __mdspan::__sliceable_mapping)
> >         (__mdspan::__submapping, submdspan): Define.
> >         * src/c++23/std.cc.in: Add submdspan.
> >         * testsuite/23_containers/mdspan/submdspan/generic.cc: New test.
> >         * testsuite/23_containers/mdspan/submdspan/selections/left.cc:
> >         Instantiate selection tests for layout_left.
> >         *
> testsuite/23_containers/mdspan/submdspan/selections/testcases.h: Generic
> >         tests different selections.
> >         * testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc:
> New test.
> >         * testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc: New
> test.
> >
> > Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
> > Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
> > ---
> > v7:
> >  - list all new entries in changes.
> >  - move submdspan_mapping poison pill before concept,
> >  - use const mapping as argument to concept
> >  - refactor __submdspan into __submapping funciton.
> >
> > Tested *mdspan* locally.
>
> We've reached the point where I no longer understand the <mdspan>
> code! But this looks OK for trunk, with the typo above fixed.

It's not that hard, especially with consteval functions for determining
what kind of
mapping to produce.
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