[PATCH] Split std::align/assume_aligned to bits/align.h

[Jonathan Wakely]

On 09/09/20 17:38 -0700, Thomas Rodgers wrote:
>We would like to be able to use these things without having to pull in
>the whole of <memory>.
>
>        * include/Makefile.am (bits_headers): Add new header.
>	* include/Makefile.in: Regenerate.
>        * include/bits/align.h: New file.
>        * include/std/memory (align): Move definition to bits/align.h.
>        (assume_aligned): Likewise.
>---
> libstdc++-v3/include/Makefile.am  |   1 +
> libstdc++-v3/include/Makefile.in  |   1 +
> libstdc++-v3/include/bits/align.h | 104 ++++++++++++++++++++++++++++++
> libstdc++-v3/include/std/memory   |  60 +----------------
> 4 files changed, 107 insertions(+), 59 deletions(-)
> create mode 100644 libstdc++-v3/include/bits/align.h
>
>diff --git a/libstdc++-v3/include/Makefile.am b/libstdc++-v3/include/Makefile.am
>index bae97852348..eabec88219d 100644
>--- a/libstdc++-v3/include/Makefile.am
>+++ b/libstdc++-v3/include/Makefile.am
>@@ -99,6 +99,7 @@ bits_srcdir = ${glibcxx_srcdir}/include/bits
> bits_builddir = ./bits
> bits_headers = \
> 	${bits_srcdir}/algorithmfwd.h \
>+	${bits_srcdir}/align.h \
> 	${bits_srcdir}/alloc_traits.h \
> 	${bits_srcdir}/allocated_ptr.h \
> 	${bits_srcdir}/allocator.h \
>diff --git a/libstdc++-v3/include/bits/align.h b/libstdc++-v3/include/bits/align.h
>new file mode 100644
>index 00000000000..593b337f897
>--- /dev/null
>+++ b/libstdc++-v3/include/bits/align.h
>@@ -0,0 +1,104 @@
>+// align implementation -*- C++ -*-
>+
>+// Copyright (C) 2008-2020 Free Software Foundation, Inc.

std::align was added in 2014 by
2f6ca9d31799133f80e446bd1faad5d9a67bac00 so that should be the first
date in the new file.

>+//
>+// 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.
>+
>+// Under Section 7 of GPL version 3, you are granted additional
>+// permissions described in the GCC Runtime Library Exception, version
>+// 3.1, as published by the Free Software Foundation.
>+
>+// You should have received a copy of the GNU General Public License and
>+// a copy of the GCC Runtime Library Exception along with this program;
>+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
>+// <http://www.gnu.org/licenses/>.
>+
>+/** @file bits/align.h
>+ *  This is an internal header file, included by other library headers.
>+ *  Do not attempt to use it directly. @headername{memory}
>+ */
>+
>+#ifndef _ALIGN_H
>+#define _ALIGN_H 1

I'm a little worried this macro isn't unique enough and will clash
with something. Please use _GLIBCXX_ALIGN_H for the header guard
macro.

>+
>+#include <bits/c++config.h>
>+
>+#include <bit>          // std::has_single_bit
>+#include <stdint.h>     // uintptr_t
>+
>+namespace std _GLIBCXX_VISIBILITY(default)
>+{
>+_GLIBCXX_BEGIN_NAMESPACE_VERSION
>+
>+/**
>+ *  @brief Fit aligned storage in buffer.
>+ *  @ingroup memory
>+ *
>+ *  This function tries to fit @a __size bytes of storage with alignment
>+ *  @a __align into the buffer @a __ptr of size @a __space bytes.  If such
>+ *  a buffer fits then @a __ptr is changed to point to the first byte of the
>+ *  aligned storage and @a __space is reduced by the bytes used for alignment.
>+ *
>+ *  C++11 20.6.5 [ptr.align]
>+ *
>+ *  @param __align   A fundamental or extended alignment value.
>+ *  @param __size    Size of the aligned storage required.
>+ *  @param __ptr     Pointer to a buffer of @a __space bytes.
>+ *  @param __space   Size of the buffer pointed to by @a __ptr.
>+ *  @return the updated pointer if the aligned storage fits, otherwise nullptr.
>+ *
>+ */
>+inline void*
>+align(size_t __align, size_t __size, void*& __ptr, size_t& __space) noexcept
>+{
>+#ifdef _GLIBCXX_USE_C99_STDINT_TR1
>+  const auto __intptr = reinterpret_cast<uintptr_t>(__ptr);
>+#else
>+  // Cannot use std::uintptr_t so assume that std::size_t can be used instead.
>+  static_assert(sizeof(size_t) >= sizeof(void*),
>+      "std::size_t must be a suitable substitute for std::uintptr_t");
>+  const auto __intptr = reinterpret_cast<unsigned long long>(__ptr);

Why on earth did I check that we can use size_t ... and then not use
size_t?! Not your problem though, that can be fixed separately.

OK for trunk with the copyright year and header guard changes, thanks!