Re: [PATCH 1/2] libstdc++: Add std::align.

[Jonathan Wakely <jwakely at redhat dot com>]

On 13/10/14 13:10 +0100, Jonathan Wakely wrote:
> On 13/10/14 12:35 +0200, Rüdiger Sonderfeld wrote:
> > On Monday 13 October 2014 11:03:51 Jonathan Wakely wrote:
> > > Fixed (the new function should be in namespace std!) and applied to
> > > trunk. Thanks for the patch.
> >
> > Thanks for merging and improving my patches!
> >
> > If you have the time maybe you could also look at the put_time patch.  Unlike
> > get_time it does not require an ABI change.
>
> I was about to close https://gcc.gnu.org/PR57350 as fixed, but the
> test in the SO question it links to fails with your implementation
> that I've just committed:
>
> https://stackoverflow.com/questions/16305311/usage-issue-of-stdalign

I'm testing this fix.

commit 60aa345d8122053f7c8ba7c743b458e799eb1455
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Oct 13 14:02:39 2014 +0100

    	PR libstdc++/57350
    	* include/std/memory (align): Do not adjust correctly aligned address.
    	* testsuite/20_util/align/2.cc: New.

diff --git a/libstdc++-v3/include/std/memory b/libstdc++-v3/include/std/memory
index affc8b1..b5792ad 100644
--- a/libstdc++-v3/include/std/memory
+++ b/libstdc++-v3/include/std/memory
@@ -113,14 +113,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 inline void*
 align(size_t __align, size_t __size, void*& __ptr, size_t& __space) noexcept
 {
-  const size_t __diff = __align - reinterpret_cast<uintptr_t>(__ptr) % __align;
-  if (__diff + __size >= __space)
+  const auto __intptr = reinterpret_cast<uintptr_t>(__ptr);
+  const auto __aligned = (__intptr - 1u + __align) & -__align;
+  const auto __diff = __aligned - __intptr;
+  if ((__size + __diff) > __space)
     return nullptr;
   else
     {
       __space -= __diff;
-      __ptr = static_cast<char*>(__ptr) + __diff;
-      return __ptr;
+      return __ptr = reinterpret_cast<void*>(__aligned);
     }
 }
 
diff --git a/libstdc++-v3/testsuite/20_util/align/2.cc b/libstdc++-v3/testsuite/20_util/align/2.cc
new file mode 100644
index 0000000..efad56a
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/align/2.cc
@@ -0,0 +1,42 @@
+// { dg-options " -std=gnu++11 " }
+
+// Copyright (C) 2014 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/>.
+
+// C++11 [ptr.align] (20.6.5): std::align
+
+#include <memory>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  bool test __attribute__((unused)) = true;
+
+  int i = 0;
+  void* ptr = &i;
+  auto space = sizeof(i);
+  auto p2 = std::align(alignof(i), space, ptr, space);
+  VERIFY( ptr == &i );
+  VERIFY( p2 == &i );
+  VERIFY(space == sizeof(i));
+}
+
+int main()
+{
+  test01();
+}