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

On 07/04/15 14:14 +0100, Jonathan Wakely wrote:

On 03/04/15 12:13 -0700, Richard Henderson wrote:

On 04/03/2015 07:13 AM, Jonathan Wakely wrote:

+++ b/libstdc++-v3/include/std/atomic
@@ -165,9 +165,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    struct atomic
    {
    private:
-      // Align 1/2/4/8/16-byte types the same as integer types of that size.
+      // Align 1/2/4/8/16-byte types the natural alignment of that size.
      // This matches the alignment effects of the C11 _Atomic qualifier.
-      static constexpr int _S_min_alignment
+      static constexpr int _S_int_alignment
	= sizeof(_Tp) == sizeof(char)	   ? alignof(char)
	: sizeof(_Tp) == sizeof(short)	   ? alignof(short)
	: sizeof(_Tp) == sizeof(int)	   ? alignof(int)
@@ -178,6 +178,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#endif
	: 0;

+      static constexpr int _S_min_alignment
+	= _S_int_alignment > sizeof(_Tp) ? _S_int_alignment : sizeof(_Tp);
+


This doesn't work for non-power-of-two sized _Tp.

Again, we don't have *any* target for which alignof(integer) > sizeof(integer).
So if you care about forcing natural alignment, don't bother with the alignof
on the integrals, as you're doing with _S_int_alignment at the moment.


OK, the attached patch uses the simpler version you proposed, so
integral types and non-integral types with size 1/2/4/8/16 are aligned
to at least their size.


Committed to trunk.



What about the __atomic_base<_PTp*> partial specialization for
pointers, do we need to use alignas on its data member, or are
pointers always aligned appropriately on all targets?

And what about these lines:

void *__a = reinterpret_cast<void *>(-__alignof(_M_i));
return __atomic_is_lock_free(sizeof(_M_i), __a);

Do we still need that if we use alignas on the data members?
If we do, do you agree with HP that it should use _S_alignment not
__alignof(_M_i)? That seems redundant to me once _M_i has been given a
fixed alignment with alignas.




commit 18fe6cb4dd74e5e5e4586ec10adba997e2e28c81
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Apr 3 15:14:40 2015 +0100

   2015-04-07  Jonathan Wakely  <jwakely@redhat.com>
   	    Richard Henderson  <rth@redhat.com>
PR libstdc++/65147 
   	* include/bits/atomic_base.h (__atomic_base<_ITp>): Increase
   	alignment.
   	* include/std/atomic (atomic): For types with a power of two size set
   	alignment to at least the size.
   	* testsuite/29_atomics/atomic/65147.cc: New.
   	* testsuite/29_atomics/atomic_integral/65147.cc: New.

diff --git a/libstdc++-v3/include/bits/atomic_base.h b/libstdc++-v3/include/bits/atomic_base.h
index 8104c98..79769cf 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -240,7 +240,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    private:
      typedef _ITp 	__int_type;

-      __int_type 	_M_i;
+      static constexpr int _S_alignment =
+	sizeof(_ITp) > alignof(_ITp) ? sizeof(_ITp) : alignof(_ITp);
+
+      alignas(_S_alignment) __int_type _M_i;

    public:
      __atomic_base() noexcept = default;
diff --git a/libstdc++-v3/include/std/atomic b/libstdc++-v3/include/std/atomic
index 88c8b17..125e37a 100644
--- a/libstdc++-v3/include/std/atomic
+++ b/libstdc++-v3/include/std/atomic
@@ -165,18 +165,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    struct atomic
    {
    private:
-      // 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.
+      // Align 1/2/4/8/16-byte types to at least their size.
      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;
+	= (sizeof(_Tp) & (sizeof(_Tp) - 1)) || sizeof(_Tp) > 16
+	? 0 : sizeof(_Tp);

      static constexpr int _S_alignment
        = _S_min_alignment > alignof(_Tp) ? _S_min_alignment : alignof(_Tp);
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/65147.cc b/libstdc++-v3/testsuite/29_atomics/atomic/65147.cc
new file mode 100644
index 0000000..e05ec17
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/65147.cc
@@ -0,0 +1,28 @@
+// 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-options "-std=gnu++11" }
+// { dg-do compile }
+
+#include <atomic>
+
+struct S16 {
+   char c[16];
+};
+
+static_assert( alignof(std::atomic<S16>) >= 16,
+    "atomic<S16> must be aligned to at least its size" );
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_integral/65147.cc b/libstdc++-v3/testsuite/29_atomics/atomic_integral/65147.cc
new file mode 100644
index 0000000..a5f5b74
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_integral/65147.cc
@@ -0,0 +1,32 @@
+// 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-options "-std=gnu++11" }
+// { dg-do compile }
+
+#include <atomic>
+
+static_assert( alignof(std::atomic<char>) >= sizeof(char),
+    "atomic<char> must be aligned to at least its size" );
+static_assert( alignof(std::atomic<short>) >= sizeof(short),
+    "atomic<short> must be aligned to at least its size" );
+static_assert( alignof(std::atomic<int>) >= sizeof(int),
+    "atomic<int> must be aligned to at least its size" );
+static_assert( alignof(std::atomic<long>) >= sizeof(long),
+    "atomic<long> must be aligned to at least its size" );
+static_assert( alignof(std::atomic<long long>) >= sizeof(long long),
+    "atomic<long long> must be aligned to at least its size" );
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