Tell the compiler that QArrayData returns aligned pointers

GCC 4.9 and later support the __attribute__((alloc_align)) attributes
that indicate the alignment of the data. To make it work on GCC since
4.7 and Clang as of 3.6, we instead use __builtin_assume_aligned(). I
don't know which version of ICC first implemented this, but ICC 15 does
and it also reports itself as GCC 4.9.

Change-Id: I58bd914b9bdd0ed3349ba56fa78220ab06114852
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 11 insertions, 4 deletions

diff --git a/src/corelib/tools/qarraydata.h b/src/corelib/tools/qarraydata.h
index 0063cf046f..94182a531c 100644
--- a/src/corelib/tools/qarraydata.h
+++ b/src/corelib/tools/qarraydata.h
@@ -218,16 +218,23 @@ struct QTypedArrayData
             AllocationOptions options = Default)
     {
         Q_STATIC_ASSERT(sizeof(QTypedArrayData) == sizeof(QArrayData));
-        return static_cast<QTypedArrayData *>(QArrayData::allocate(sizeof(T),
-                    alignof(AlignmentDummy), capacity, options));
+        void *result = QArrayData::allocate(sizeof(T),
+                    alignof(AlignmentDummy), capacity, options);
+#if (defined(Q_CC_GNU) && Q_CC_GNU >= 407) || QT_HAS_BUILTIN(__builtin_assume_aligned)
+        result = __builtin_assume_aligned(result, Q_ALIGNOF(AlignmentDummy));
+#endif
+        return static_cast<QTypedArrayData *>(result);
     }
 
     static QTypedArrayData *reallocateUnaligned(QTypedArrayData *data, size_t capacity,
             AllocationOptions options = Default)
     {
         Q_STATIC_ASSERT(sizeof(QTypedArrayData) == sizeof(QArrayData));
-        return static_cast<QTypedArrayData *>(QArrayData::reallocateUnaligned(data, sizeof(T),
-                    capacity, options));
+        void *result = QArrayData::reallocateUnaligned(data, sizeof(T), capacity, options));
+#if (defined(Q_CC_GNU) && Q_CC_GNU >= 407) || QT_HAS_BUILTIN(__builtin_assume_aligned)
+        result =__builtin_assume_aligned(result, Q_ALIGNOF(AlignmentDummy));
+#endif
+        return static_cast<QTypedArrayData *>(result);
     }
 
     static void deallocate(QArrayData *data)