MAINT: Fill in correct buffer transferstride always

Also clarifies the comment, since it may be a bit confusing: The outer stride being zero means that there must be nonzero inner strides (otherwise they are all zero and we don't use reduce logic). But the inner stride being zero is not meaningful.
numpy · mattip · Dec 17, 2024 · Nov 28, 2024 · Nov 30, 2024 · Dec 2, 2024
commit 162a951e84fb150a13b561c49d25f136e7bdba03
diff --git a/numpy/_core/src/multiarray/nditer_api.c b/numpy/_core/src/multiarray/nditer_api.c
@@ -1863,8 +1863,9 @@ npyiter_fill_buffercopy_params(
 
     if ((opitflags & NPY_OP_ITFLAG_REDUCE) && (NAD_STRIDES(outer_axisdata)[iop] != 0)) {
         /*
-         * Reduce with inner stride ==0 (outer !=0), we buffer the outer stride
-         * which also means buffering only outersize items.
+         * Reduce with all inner strides ==0 (outer !=0).  We buffer the outer
+         * stride which also means buffering only outersize items.
+         * (If the outer stride is 0, some inner ones are guaranteed nonzero.)
          */
         assert(NAD_STRIDES(axisdata)[iop] == 0);
         *ndim_transfer = 1;

diff --git a/numpy/_core/src/multiarray/nditer_constr.c b/numpy/_core/src/multiarray/nditer_constr.c
@@ -3381,19 +3381,28 @@ npyiter_allocate_transfer_functions(NpyIter *iter)
     npy_intp *strides = NAD_STRIDES(axisdata), op_stride;
     NpyIter_TransferInfo *transferinfo = NBF_TRANSFERINFO(bufferdata);
 
+    npy_intp sizeof_axisdata = NIT_AXISDATA_SIZEOF(itflags, ndim, nop);
+    NpyIter_AxisData *reduce_axisdata = NIT_INDEX_AXISDATA(axisdata, bufferdata->outerdim);
+    npy_intp *reduce_strides = NAD_STRIDES(reduce_axisdata);
+
     /* combined cast flags, the new cast flags for each cast: */
     NPY_ARRAYMETHOD_FLAGS cflags = PyArrayMethod_MINIMAL_FLAGS;
     NPY_ARRAYMETHOD_FLAGS nc_flags;
 
     for (iop = 0; iop < nop; ++iop) {
         npyiter_opitflags flags = op_itflags[iop];
+
         /*
-         * Reduction operands may be buffered with a different stride,
-         * so we must pass NPY_MAX_INTP to the transfer function factory.
+         * Reduce operands buffer the outer stride if it is nonzero; compare
+         * `npyiter_fill_buffercopy_params`.
+         * (Inner strides cannot _all_ be zero if the outer is, but some can be.)
          */
-        // TODO: This should not be the case anymore!?  (was it ever?!?)
-        op_stride = (flags & NPY_OP_ITFLAG_REDUCE) ? NPY_MAX_INTP :
-                                                   strides[iop];
+        if ((op_itflags[iop] & NPY_OP_ITFLAG_REDUCE) && reduce_strides[iop] != 0) {
+            op_stride = reduce_strides[iop];
+        }
+        else {
+            op_stride = strides[iop];
+        }
 
         /*
          * If we have determined that a buffer may be needed,