These are the revised submdspan_mapping related patches from v2.
A few comments:
- __any_past_the_end: the suggestion was to precheck for an empty
extent. I kept the current logic, because the additional complexity
is only at compile time (there was a `constexpr` missing on a `else
if` branch in v2). At runtime, it's a very simple check:
(__slice_begin[k] == exts.extent[k]) || ...
Even if the pre-check would make the condition simpler, for regular
(non-empty) cases we'd be forced to first check for empty, then
check for out-of-bounds. Unless I'm missing something, that's
strictly worse.
The main motivation here is to eliminate these checks when the slice
is a collapsing slice, because these checks adds a "lot" of code
compared to the hand-rolled version. (With the current
implementation: submdspan(m, i, j, k) and mdspan(&m[i, j, k], E{})
generate the same code, with E = std::extents<int>).
- As proposed during review, the code to figure out the layout mapping
type given a combination of slices has been rewritten. Essentially,
we have a `enum class _SliceKind` what's (forced to be) categorical.
From there one can use for-loops and immediate (consteval) functions
insteads of traditional recursive structs.
The twist here is that a __full slice is not a __unit_strided_slice
slice. This is because: I don't want to use weak/plain enums;
strong enums (enum class) don't do bitwise operations without
implementing a lot of boilerplate; bitset feels inappropriate.
Hence, this solution felt the one with the least boilerplate.
Again, I might be missing a trick.
- In __substrides_standardized I chose not use _M_strides, because
_M_strides has size `rank`; while __substrides_standardized only
returns an array of lenght `__subrank`. The difference can be seen,
e.g., for submdspan(m, 1, 2, 3, full) which (technically) performs 3
multiplications when zero are needed. Therefore, one would rely on
the optimizer to eliminate the extra operation. It might be
recognizable as dead code, but I've not checked.
- Let me know if you want me to move the __subextents code in a
separate commit.
- I started following the suggestion to always use qualified names for
functions. Let me know if you want to qualify everything (typenames,
concepts, etc.)
Luc Grosheintz (5):
libstdc++: Implement submdspan and submdspan_mapping for layout_left.
[PR110352]
libstdc++: Implement submdspan_mapping for layout_right. [PR110352]
libstdc++: Implement submdspan_mapping for layout_stride. [PR110352]
libstdc++: Implement submdspan_mapping for layout_left_padded.
[PR110352]
libstdc++: Implement submdspan_mapping for layout_right_padded.
[PR110352]
libstdc++-v3/include/std/mdspan | 713 ++++++++++++++++--
libstdc++-v3/src/c++23/std.cc.in | 2 +-
.../23_containers/mdspan/layout_traits.h | 4 +
.../mdspan/submdspan/submdspan.cc | 377 +++++++++
.../mdspan/submdspan/submdspan_mapping.cc | 301 ++++++++
.../mdspan/submdspan/submdspan_neg.cc | 179 +++++
6 files changed, 1514 insertions(+), 62 deletions(-)
create mode 100644
libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan.cc
create mode 100644
libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
create mode 100644
libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc
--
2.52.0
