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numba/np/npyimpl.py

@@ -384,10 +384,16 @@ def numpy_ufunc_kernel(context, builder, sig, args, ufunc, kernel_class):
 
     loopshape = outputs[0].shape
 
-    input_layouts = [inp.layout for inp in inputs if isinstance(inp, _ArrayHelper)]
+    # count the number of C and F layout arrays, respectively
+    input_layouts = [inp.layout for inp in inputs
+                     if isinstance(inp, _ArrayHelper)]
     num_c_layout = len([x for x in input_layouts if x == 'C'])
     num_f_layout = len([x for x in input_layouts if x == 'F'])
 
+    # Only choose F iteration order if more arrays are in F layout.
+    # Default to C order otherwise.
+    # This is a best effort for performance. NumPy has more fancy logic that
+    # uses array iterators in non-trivial cases.
     if num_f_layout > num_c_layout:
         order = 'F'
     else: