re PR libstdc++/65033 (C++11 atomics: is_lock_free result does not always match…

re PR libstdc++/65033 (C++11 atomics: is_lock_free result does not always match the real lock-free property)

PR libstdc++/65033

 * include/bits/atomic_base.h (__atomic_base<T>::is_lock_free): Build
 a fake pointer indicating type alignment.
 (__atomic_base<T *>::is_lock_free): Likewise.
 * include/std/atomic (atomic<T>::is_lock_free): Likewise.

From-SVN: r221701

libstdc++-v3/ChangeLog

+2015-03-26  Richard Henderson  <rth@redhat.com>
+
+	PR libstdc++/65033
+	* include/bits/atomic_base.h (__atomic_base<T>::is_lock_free): Build
+	a fake pointer indicating type alignment.
+	(__atomic_base<T *>::is_lock_free): Likewise.
+	* include/std/atomic (atomic<T>::is_lock_free): Likewise.
+
 2015-03-25  Alan Lawrence  <alan.lawrence@arm.com>
 
 	PR libstdc++/33394

libstdc++-v3/include/bits/atomic_base.h

@@ -346,11 +346,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       bool
       is_lock_free() const noexcept
-      { return __atomic_is_lock_free(sizeof(_M_i), nullptr); }
+      {
+	// Produce a fake, minimally aligned pointer.
+	void *__a = reinterpret_cast<void *>(-__alignof(_M_i));
+	return __atomic_is_lock_free(sizeof(_M_i), __a);
+      }
 
       bool
       is_lock_free() const volatile noexcept
-      { return __atomic_is_lock_free(sizeof(_M_i), nullptr); }
+      {
+	// Produce a fake, minimally aligned pointer.
+	void *__a = reinterpret_cast<void *>(-__alignof(_M_i));
+	return __atomic_is_lock_free(sizeof(_M_i), __a);
+      }
 
       _GLIBCXX_ALWAYS_INLINE void
       store(__int_type __i, memory_order __m = memory_order_seq_cst) noexcept
@@ -653,11 +661,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       bool
       is_lock_free() const noexcept
-      { return __atomic_is_lock_free(sizeof(__pointer_type), nullptr); }
+      {
+	// Produce a fake, minimally aligned pointer.
+	void *__a = reinterpret_cast<void *>(-__alignof(_M_p));
+	return __atomic_is_lock_free(sizeof(_M_p), __a);
+      }
 
       bool
       is_lock_free() const volatile noexcept
-      { return __atomic_is_lock_free(sizeof(__pointer_type), nullptr); }
+      {
+	// Produce a fake, minimally aligned pointer.
+	void *__a = reinterpret_cast<void *>(-__alignof(_M_p));
+	return __atomic_is_lock_free(sizeof(_M_p), __a);
+      }
 
       _GLIBCXX_ALWAYS_INLINE void
       store(__pointer_type __p,

libstdc++-v3/include/std/atomic

@@ -198,11 +198,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       bool
       is_lock_free() const noexcept
-      { return __atomic_is_lock_free(sizeof(_M_i), nullptr); }
+      {
+	// Produce a fake, minimally aligned pointer.
+	void *__a = reinterpret_cast<void *>(-__alignof(_M_i));
+	return __atomic_is_lock_free(sizeof(_M_i), __a);
+      }
 
       bool
       is_lock_free() const volatile noexcept
-      { return __atomic_is_lock_free(sizeof(_M_i), nullptr); }
+      {
+	// Produce a fake, minimally aligned pointer.
+	void *__a = reinterpret_cast<void *>(-__alignof(_M_i));
+	return __atomic_is_lock_free(sizeof(_M_i), __a);
+      }
 
       void
       store(_Tp __i, memory_order __m = memory_order_seq_cst) noexcept