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

[Richard Henderson]

When we fixed PR54005, making sure that atomic_is_lock_free returns the same
value for all objects of a given type, we probably should have changed the
interface so that we would pass size and alignment rather than size and object
pointer.

Instead, we decided that passing null for the object pointer would be
sufficient.  But as this PR shows, we really do need to take alignment into
account.

The following patch constructs a fake object pointer that is maximally
misaligned.  This allows the interface to both the builtin and to libatomic to
remain unchanged.  Which probably makes this back-portable to maintenance
releases as well.

I believe that for all of our current systems, size_t == uintptr_t, so the
reinterpret_cast ought not generate warnings.

The test case is problematic, as there's currently no good place to put it.
The libstdc++ testsuite doesn't have the libatomic library path configured, and
the libatomic testsuite doesn't have the libstdc++ include paths configured.
Yet another example where we really need an install tree for testing.  Thoughts?


Ok?


r~
-------------- next part --------------
	* 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.

diff --git a/libstdc++-v3/include/bits/atomic_base.h b/libstdc++-v3/include/bits/atomic_base.h
index fe6524f..8104c98 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/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,
diff --git a/libstdc++-v3/include/std/atomic b/libstdc++-v3/include/std/atomic
index 1a17427..cc4b5f1 100644
--- a/libstdc++-v3/include/std/atomic
+++ b/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
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 65033.cc
Type: text/x-c++src
Size: 1071 bytes
Desc: not available
URL: <http://gcc.gnu.org/pipermail/gcc-patches/attachments/20150212/210e7998/attachment.bin>