[libcxx] Add mdspan/extents

This patch adds std::extents. extents is one of the core classes used by std::mdspan. It describes a multi-dimensional index space with a mix of compile time and runtime sizes. Furthermore, it is templated on the index type used to describe the multi-dimensional index space.

The class is designed to be highly optimizable in performance critical code sections, and is fully useable in constant expressions contexts.

Testing of this class tends to be somewhat combinatorical, due to the large number of possible corner cases involved in situations where we have both runtime and compile time extents. To add to this, the class is designed to be interoperable (in particular constructible) from arguments which only need to be convertible to the index_type, but are otherwise arbitrary user types. For a larger discussion on the design of this class refer to: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p0009r18.html

Co-authored-by: Damien L-G <[email protected]>

Reviewed By: ldionne, #libc

Spies: libcxx-commits, H-G-Hristov, tschuett, philnik, arichardson, Mordante, crtrott

Differential Revision: https://reviews.llvm.org/D148067

Author: crtrott

libcxx/CREDITS.TXT

@@ -92,6 +92,11 @@ E: [email protected]
 E: [email protected]
 D: Implemented floating-point to_chars.
 
+N: Damien Lebrun-Grandie
+E: [email protected]
+E: [email protected]
+D: Implementation of mdspan.
+
 N: Microsoft Corporation
 D: Contributed floating-point to_chars.
 
@@ -149,6 +154,10 @@ N: Stephan Tolksdorf
 E: [email protected]
 D: Minor <atomic> fix
 
+N: Christian Trott
+E: [email protected]
+D: Implementation of mdspan.
+
 N: Ruben Van Boxem
 E: vanboxem dot ruben at gmail dot com
 D: Initial Windows patches.

libcxx/docs/FeatureTestMacroTable.rst

@@ -332,6 +332,8 @@ Status
     ------------------------------------------------- -----------------
     ``__cpp_lib_is_scoped_enum``                      ``202011L``
     ------------------------------------------------- -----------------
+    ``__cpp_lib_mdspan``                              *unimplemented*
+    ------------------------------------------------- -----------------
     ``__cpp_lib_move_only_function``                  *unimplemented*
     ------------------------------------------------- -----------------
     ``__cpp_lib_optional``                            ``202110L``

libcxx/docs/Status/Cxx2bPapers.csv

@@ -51,7 +51,7 @@
 "`P2442R1 <https://wg21.link/P2442R1>`__","LWG","Windowing range adaptors: ``views::chunk`` and ``views::slide``","February 2022","","","|ranges|"
 "`P2443R1 <https://wg21.link/P2443R1>`__","LWG","``views::chunk_by``","February 2022","","","|ranges|"
 "","","","","","",""
-"`P0009R18 <https://wg21.link/P0009R18>`__","LWG","mdspan: A Non-Owning Multidimensional Array Reference","July 2022","",""
+"`P0009R18 <https://wg21.link/P0009R18>`__","LWG","mdspan: A Non-Owning Multidimensional Array Reference","July 2022","|In progress|",""
 "`P0429R9 <https://wg21.link/P0429R9>`__","LWG","A Standard ``flat_map``","July 2022","",""
 "`P1169R4 <https://wg21.link/P1169R4>`__","LWG","``static operator()``","July 2022","|Complete|","16.0"
 "`P1222R4 <https://wg21.link/P1222R4>`__","LWG","A Standard ``flat_set``","July 2022","",""

libcxx/include/CMakeLists.txt

@@ -463,6 +463,7 @@ set(files
   __locale_dir/locale_base_api/bsd_locale_fallbacks.h
   __locale_dir/locale_base_api/locale_guard.h
   __mbstate_t.h
+  __mdspan/extents.h
   __memory/addressof.h
   __memory/align.h
   __memory/aligned_alloc.h
@@ -913,6 +914,7 @@ set(files
   locale.h
   map
   math.h
+  mdspan
   memory
   memory_resource
   mutex