BUG: fix string arrays not being aligned

If itemsize is a power of two use that as the required alignment up to
the maximum provided by the platform. Power of two sizes may be accessed
via larger moves than bytes.
Non-power of two sizes are accessed bytewise and can thus always be
considered aligned.
Closes gh-5293

2 files changed, 21 insertions, 1 deletions

diff --git a/numpy/core/src/multiarray/common.c b/numpy/core/src/multiarray/common.c
index 7015b3961..a8490f0e8 100644
--- a/numpy/core/src/multiarray/common.c
+++ b/numpy/core/src/multiarray/common.c
@@ -684,7 +684,16 @@ _IsAligned(PyArrayObject *ap)
 
     /* alignment 1 types should have a efficient alignment for copy loops */
     if (PyArray_ISFLEXIBLE(ap) || PyArray_ISSTRING(ap)) {
-        alignment = NPY_MAX_COPY_ALIGNMENT;
+        npy_intp itemsize = PyArray_ITEMSIZE(ap);
+        /* power of two sizes may be loaded in larger moves */
+        if (((itemsize & (itemsize - 1)) == 0)) {
+            alignment = itemsize > NPY_MAX_COPY_ALIGNMENT ?
+                NPY_MAX_COPY_ALIGNMENT : itemsize;
+        }
+        else {
+            /* if not power of two it will be accessed bytewise */
+            alignment = 1;
+        }
     }
 
     if (alignment == 1) {
diff --git a/numpy/core/tests/test_multiarray.py b/numpy/core/tests/test_multiarray.py
index d472a9569..a9762978b 100644
--- a/numpy/core/tests/test_multiarray.py
+++ b/numpy/core/tests/test_multiarray.py
@@ -68,6 +68,17 @@ class TestFlags(TestCase):
         assert_equal(self.a.flags.aligned, True)
         assert_equal(self.a.flags.updateifcopy, False)
 
+    def test_string_align(self):
+        a = np.zeros(4, dtype=np.dtype('|S4'))
+        assert_(a.flags.aligned)
+        # not power of two are accessed bytewise and thus considered aligned
+        a = np.zeros(5, dtype=np.dtype('|S4'))
+        assert_(a.flags.aligned)
+
+    def test_void_align(self):
+        a = np.zeros(4, dtype=np.dtype([("a", "i4"), ("b", "i4")]))
+        assert_(a.flags.aligned)
+
 class TestHash(TestCase):
     # see #3793
     def test_int(self):