[gcc r11-4475] libstdc++: Make std::span layout-compatible with struct iovec [PR 95609]

[Jonathan Wakely]

https://gcc.gnu.org/g:0f7cd5e5735e5536bf7bc8ca2b998f7ce8b4ddee

commit r11-4475-g0f7cd5e5735e5536bf7bc8ca2b998f7ce8b4ddee
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Oct 28 12:07:40 2020 +0000

    libstdc++: Make std::span layout-compatible with struct iovec [PR 95609]
    
    This change reorders the data members of std::span so that span<byte> is
    layout-compatible with common implementations of struct iovec. This will
    allow span<byte> to be used directly in places that use a struct iovec
    to do scatter-gather I/O.
    
    It's important to note that POSIX doesn't specify the order of members
    in iovec. Also the equivalent type on Windows has members in the other
    order, and uses type ULONG (which is always 32-bit whereas size_t is
    64-bit for Win64). So this change will only help for certain targets and
    an indirection between std::span and I/O system calls will still be
    needed for the general case.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/95609
            * include/std/span (span): Reorder data members to match common
            implementations of struct iovec.
            * testsuite/23_containers/span/layout_compat.cc: New test.

Diff:
---
 libstdc++-v3/include/std/span                      | 12 +++---
 .../testsuite/23_containers/span/layout_compat.cc  | 48 ++++++++++++++++++++++
 2 files changed, 54 insertions(+), 6 deletions(-)

diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span
index fb349403c9e..24c61ba4172 100644
--- a/libstdc++-v3/include/std/span
+++ b/libstdc++-v3/include/std/span
@@ -38,8 +38,8 @@
 
 #if __cplusplus > 201703L
 
-#include <type_traits>
 #include <array>
+#include <cstddef>
 #include <bits/stl_iterator.h>
 #include <bits/ranges_base.h>
 
@@ -151,7 +151,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr
       span() noexcept
       requires ((_Extent + 1u) <= 1u)
-      : _M_extent(0), _M_ptr(nullptr)
+      : _M_ptr(nullptr), _M_extent(0)
       { }
 
       template<contiguous_iterator _It>
@@ -159,7 +159,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	constexpr explicit(extent != dynamic_extent)
 	span(_It __first, size_type __count)
 	noexcept
-	: _M_extent(__count), _M_ptr(std::to_address(__first))
+	: _M_ptr(std::to_address(__first)), _M_extent(__count)
 	{
 	  if constexpr (_Extent != dynamic_extent)
 	    {
@@ -173,8 +173,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	constexpr explicit(extent != dynamic_extent)
 	span(_It __first, _End __last)
 	noexcept(noexcept(__last - __first))
-	: _M_extent(static_cast<size_type>(__last - __first)),
-	  _M_ptr(std::to_address(__first))
+	: _M_ptr(std::to_address(__first)),
+	  _M_extent(static_cast<size_type>(__last - __first))
 	{
 	  if constexpr (_Extent != dynamic_extent)
 	    {
@@ -392,8 +392,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
 
     private:
-      [[no_unique_address]] __detail::__extent_storage<extent> _M_extent;
       pointer _M_ptr;
+      [[no_unique_address]] __detail::__extent_storage<extent> _M_extent;
     };
 
   // deduction guides
diff --git a/libstdc++-v3/testsuite/23_containers/span/layout_compat.cc b/libstdc++-v3/testsuite/23_containers/span/layout_compat.cc
new file mode 100644
index 00000000000..efc5b8e4706
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/layout_compat.cc
@@ -0,0 +1,48 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <span>
+#include <cstddef>
+
+#if __has_include(<sys/uio.h>)
+#include <sys/uio.h>
+#else
+struct iovec { void* iov_base; std::size_t iov_len; };
+#endif
+
+#if __cpp_lib_is_pointer_interconvertible
+using std::is_layout_compatible_v;
+#else
+// A poor substitute for is_layout_compatible_v
+template<typename T, typename U>
+  constexpr bool is_layout_compatible_v
+    = std::is_standard_layout_v<T> && std::is_standard_layout_v<U>
+      && sizeof(T) == sizeof(U) && alignof(T) == alignof(U);
+#endif
+
+void
+test_pr95609()
+{
+  using rbuf = std::span<const std::byte>;
+  using wbuf = std::span<std::byte>;
+
+  static_assert(is_layout_compatible_v<rbuf, struct iovec>);
+  static_assert(is_layout_compatible_v<wbuf, struct iovec>);
+}