BUG: nditer: Error in detecting double reduction loop (Fixes #1885)

This bug was in the double reduction loop optimization used during
buffering, and was triggered by the bug report example because it
reduced everything to a single value.

3 files changed, 21 insertions, 3 deletions

diff --git a/numpy/core/src/multiarray/nditer.c.src b/numpy/core/src/multiarray/nditer.c.src
index dfc0baa6f..ce16a53d0 100644
--- a/numpy/core/src/multiarray/nditer.c.src
+++ b/numpy/core/src/multiarray/nditer.c.src
@@ -6080,10 +6080,11 @@ npyiter_checkreducesize(NpyIter *iter, npy_intp count,
     }
 
     /*
-     * If there was any non-zero coordinate or the reduction inner
-     * loop doesn't fit in the buffersize, can't do the double loop
+     * If there was any non-zero coordinate, the reduction inner
+     * loop doesn't fit in the buffersize, or the reduction inner loop
+     * covered the entire iteration size, can't do the double loop.
      */
-    if (nonzerocoord || count < reducespace) {
+    if (nonzerocoord || count < reducespace || idim == ndim) {
         if (reducespace < count) {
             count = reducespace;
         }
diff --git a/numpy/core/tests/test_einsum.py b/numpy/core/tests/test_einsum.py
index 6a03a5913..d0161b366 100644
--- a/numpy/core/tests/test_einsum.py
+++ b/numpy/core/tests/test_einsum.py
@@ -421,6 +421,12 @@ class TestEinSum(TestCase):
         assert_equal(b, np.sum(a))
         assert_equal(b.dtype, np.dtype(dtype))
 
+        # A case which was failing (ticket #1885)
+        p = np.arange(2) + 1
+        q = np.arange(4).reshape(2,2) + 3
+        r = np.arange(4).reshape(2,2) + 7
+        assert_equal(np.einsum('z,mz,zm->', p, q, r), 253)
+
     def test_einsum_sums_int8(self):
         self.check_einsum_sums('i1');
 
diff --git a/numpy/core/tests/test_iterator.py b/numpy/core/tests/test_iterator.py
index fe6871cd3..f39d9584f 100644
--- a/numpy/core/tests/test_iterator.py
+++ b/numpy/core/tests/test_iterator.py
@@ -2284,5 +2284,16 @@ def test_iter_buffering_reduction():
         y[...] += x
     assert_equal(b, np.sum(a, axis=1))
 
+    # Iterator inner double loop was wrong on this one
+    p = np.arange(2) + 1
+    it = np.nditer([p,None],
+            ['delay_bufalloc','reduce_ok','buffered','external_loop'],
+            [['readonly'],['readwrite','allocate']],
+            op_axes=[[-1,0],[-1,-1]],
+            itershape=(2,2))
+    it.operands[1].fill(0)
+    it.reset()
+    assert_equal(it[0], [1,2,1,2])
+
 if __name__ == "__main__":
     run_module_suite()