ENH: Relax aligned flag for shape[i] <= 1

In this case, either the dimensions stride will never be used to
access an element, so that it does not matter to the data alignment,
or the array has a size of 0 and is thus never unaligned.

Relaxed align flag is only active if NPY_RELAXED_STRIDES_CHECKING
was set during compile time.

1 files changed, 12 insertions, 0 deletions

diff --git a/numpy/core/src/multiarray/common.c b/numpy/core/src/multiarray/common.c
index f27de491f..f0a6a761c 100644
--- a/numpy/core/src/multiarray/common.c
+++ b/numpy/core/src/multiarray/common.c
@@ -628,8 +628,20 @@ _IsAligned(PyArrayObject *ap)
     }
     ptr = (npy_intp) PyArray_DATA(ap);
     aligned = (ptr % alignment) == 0;
+
     for (i = 0; i < PyArray_NDIM(ap); i++) {
+#if NPY_RELAXED_STRIDES_CHECKING
+        if (PyArray_DIM(ap, i) > 1) {
+            /* if shape[i] == 1, the stride is never used */
+            aligned &= ((PyArray_STRIDES(ap)[i] % alignment) == 0);
+        }
+        else if (PyArray_DIM(ap, i) == 0) {
+            /* an array with zero elements is always aligned */
+            return 1;
+        }
+#else /* not NPY_RELAXED_STRIDES_CHECKING */
         aligned &= ((PyArray_STRIDES(ap)[i] % alignment) == 0);
+#endif /* not NPY_RELAXED_STRIDES_CHECKING */
     }
     return aligned != 0;
 }