Re: [libstdc++/65033] Give alignment info to libatomic

[Jonathan Wakely <jwakely@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 1205 bytes --]

On 25/03/15 12:04 -0700, Richard Henderson wrote:
>On 03/25/2015 11:49 AM, Jonathan Wakely wrote:
>> On 25/03/15 11:36 -0700, Richard Henderson wrote:
>>> On 03/25/2015 09:22 AM, Jonathan Wakely wrote:
>> On 25/03/15 11:39 -0700, Richard Henderson wrote:
>>> On 03/25/2015 09:22 AM, Jonathan Wakely wrote:
>>>> +static_assert( alignof(std::atomic<twoints>) > alignof(int),
>>>> +               "std::atomic not suitably aligned" );
>>>
>>> This is only true if int64_t has alignment larger than int32_t,
>>> which is unfortunately not always the case.
>>
>> Huh, didn't realise that. I could change the tests to check it's
>> alignof(std::int64_t) as the next assertion does, but is it safe to
>> assume that struct twoints { int a; int b; } is exactly 64 bits
>> everywhere?
>
>Certainly not.  But if you're going to explicitly use int64_t elsewhere, you
>might as well explicitly use int32_t as well.  Then I believe you can
>reasonably assert
>
>  alignof(twoint32) == alignof(int64_t)

Yes, that makes sense, thanks.  Here's what I plan to commit then.

This includes your fix to avoid decreasing alignment, but I didn't add
a test for that as I couldn't make it fail on any of the targets I
test on.

[-- Attachment #2: patch.txt --]
[-- Type: text/x-patch, Size: 3923 bytes --]

commit f796769ad20c0353490b9f1a7e019e2f0c1771fb
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Sep 3 15:39:53 2014 +0100

    	PR libstdc++/62259
    	PR libstdc++/65147
    	* include/std/atomic (atomic<T>): Increase alignment for types with
    	the same size as one of the integral types.
    	* testsuite/29_atomics/atomic/60695.cc: Adjust dg-error line number.
    	* testsuite/29_atomics/atomic/62259.cc: New.

diff --git a/libstdc++-v3/include/std/atomic b/libstdc++-v3/include/std/atomic
index 1a17427..42244bd 100644
--- a/libstdc++-v3/include/std/atomic
+++ b/libstdc++-v3/include/std/atomic
@@ -165,7 +165,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct atomic
     {
     private:
-      _Tp _M_i;
+      // Align 1/2/4/8/16-byte types the same as integer types of that size.
+      // This matches the alignment effects of the C11 _Atomic qualifier.
+      static constexpr int _S_min_alignment
+	= sizeof(_Tp) == sizeof(char)	   ? alignof(char)
+	: sizeof(_Tp) == sizeof(short)	   ? alignof(short)
+	: sizeof(_Tp) == sizeof(int)	   ? alignof(int)
+	: sizeof(_Tp) == sizeof(long)	   ? alignof(long)
+	: sizeof(_Tp) == sizeof(long long) ? alignof(long long)
+#ifdef _GLIBCXX_USE_INT128
+	: sizeof(_Tp) == sizeof(__int128)  ? alignof(__int128)
+#endif
+	: 0;
+
+      static constexpr int _S_alignment
+        = _S_min_alignment > alignof(_Tp) ? _S_min_alignment : alignof(_Tp);
+
+      alignas(_S_alignment) _Tp _M_i;
 
       static_assert(__is_trivially_copyable(_Tp),
 		    "std::atomic requires a trivially copyable type");
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/60695.cc b/libstdc++-v3/testsuite/29_atomics/atomic/60695.cc
index b59c6ba..6f618a0 100644
--- a/libstdc++-v3/testsuite/29_atomics/atomic/60695.cc
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/60695.cc
@@ -27,4 +27,4 @@ struct X {
   char stuff[0]; // GNU extension, type has zero size
 };
 
-std::atomic<X> a;  // { dg-error "not supported" "" { target *-*-* } 173 }
+std::atomic<X> a;  // { dg-error "not supported" "" { target *-*-* } 189 }
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/62259.cc b/libstdc++-v3/testsuite/29_atomics/atomic/62259.cc
new file mode 100644
index 0000000..cf5423a
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/62259.cc
@@ -0,0 +1,58 @@
+// Copyright (C) 2015 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-require-atomic-builtins "" }
+// { dg-require-cstdint "" }
+// { dg-options "-std=gnu++11" }
+// { dg-do compile }
+
+#include <atomic>
+#include <cstdint>
+
+using std::int32_t;
+using std::int64_t;
+
+// libstdc++/62259
+
+struct twoints {
+  int32_t a;
+  int32_t b;
+};
+
+static_assert( alignof(std::atomic<twoints>) == alignof(int64_t),
+               "std::atomic not suitably aligned" );
+
+// libstdc++/65147
+
+struct power_of_two_obj {
+    char c [8];
+};
+
+std::atomic<power_of_two_obj> obj1;
+
+static_assert( alignof(obj1) == alignof(int64_t),
+               "std::atomic not suitably aligned" );
+
+struct container_struct {
+   char c[1];
+   std::atomic<power_of_two_obj> ao;
+};
+
+container_struct obj2;
+
+static_assert( alignof(obj2.ao) == alignof(int64_t),
+               "std::atomic not suitably aligned" );