Fixes boolean indexing for strided masks (#1370)

* Corrected boolean indexing cumsum
- The cumulative sum was being calculated incorrectly -- the offset from stride simplification was unused and the result was incorrect for some cases with non-C-contiguous strides
- To fix this, new functions ``compact_iteration_space`` and complementary function ``compact_iteration`` have been implemented

* Add a test for nonzero where dimension compacting occurs

* Added tests for the corrected behavior of boolean indexing and nonzero

* Removed dead branch in py_mask_positions

Compacting strides can reduce dimensionality of the array, but it can not
turn an input that is not already C-contiguous into a C-contiguous one.

Hence the branch checking if the input became C-contiguous after compacting
is effectively dead.

* Added docstring for compact_iteration

---------

Co-authored-by: Oleksandr Pavlyk <oleksandr.pavlyk@intel.com>

Author: ndgrigorian

dpctl/tensor/libtensor/include/kernels/boolean_advanced_indexing.hpp

@@ -424,7 +424,6 @@ typedef size_t (*mask_positions_strided_impl_fn_ptr_t)(
     size_t,
     const char *,
     int,
-    py::ssize_t,
     const py::ssize_t *,
     char *,
     std::vector<sycl::event> const &);
@@ -434,7 +433,6 @@ size_t mask_positions_strided_impl(sycl::queue q,
                                    size_t n_elems,
                                    const char *mask,
                                    int nd,
-                                   py::ssize_t input_offset,
                                    const py::ssize_t *shape_strides,
                                    char *cumsum,
                                    std::vector<sycl::event> const &depends = {})
@@ -444,7 +442,7 @@ size_t mask_positions_strided_impl(sycl::queue q,
     cumsumT *cumsum_data_ptr = reinterpret_cast<cumsumT *>(cumsum);
     size_t wg_size = 128;
 
-    StridedIndexer strided_indexer{nd, input_offset, shape_strides};
+    StridedIndexer strided_indexer{nd, 0, shape_strides};
     NonZeroIndicator<maskT, cumsumT> non_zero_indicator{};
 
     sycl::event comp_ev =

dpctl/tensor/libtensor/include/utils/strided_iters.hpp

@@ -909,6 +909,55 @@ contract_iter4(vecT shape,
                            out_strides3, disp3, out_strides4, disp4);
 }
 
+/*
+    For purposes of iterating over elements of an array with  `shape` and
+    strides `strides` given as pointers `compact_iteration(nd, shape, strides)`
+    may modify memory and returns the new length of the array.
+
+    The new shape and new strides `(new_shape, new_strides)` are such that
+    iterating over them will traverse the same elements in the same order,
+    possibly with reduced dimensionality.
+ */
+template <class ShapeTy, class StridesTy>
+int compact_iteration(const int nd, ShapeTy *shape, StridesTy *strides)
+{
+    if (nd < 2)
+        return nd;
+
+    bool contractable = true;
+    for (int i = 0; i < nd; ++i) {
+        if (strides[i] < 0) {
+            contractable = false;
+        }
+    }
+
+    int nd_ = nd;
+    while (contractable) {
+        bool changed = false;
+        for (int i = 0; i + 1 < nd_; ++i) {
+            StridesTy str = strides[i + 1];
+            StridesTy jump = strides[i] - (shape[i + 1] - 1) * str;
+
+            if (jump == str) {
+                changed = true;
+                shape[i] *= shape[i + 1];
+                for (int j = i; j < nd_; ++j) {
+                    strides[j] = strides[j + 1];
+                }
+                for (int j = i + 1; j + 1 < nd_; ++j) {
+                    shape[j] = shape[j + 1];
+                }
+                --nd_;
+                break;
+            }
+        }
+        if (!changed)
+            break;
+    }
+
+    return nd_;
+}
+
 } // namespace strides
 } // namespace tensor
 } // namespace dpctl

dpctl/tensor/libtensor/source/boolean_advanced_indexing.cpp

@@ -205,24 +205,14 @@ size_t py_mask_positions(dpctl::tensor::usm_ndarray mask,
     auto const &strides_vector = mask.get_strides_vector();
 
     using shT = std::vector<py::ssize_t>;
-    shT simplified_shape;
-    shT simplified_strides;
-    py::ssize_t offset(0);
+    shT compact_shape;
+    shT compact_strides;
 
     int mask_nd = mask.get_ndim();
     int nd = mask_nd;
 
-    dpctl::tensor::py_internal::simplify_iteration_space_1(
-        nd, shape, strides_vector, simplified_shape, simplified_strides,
-        offset);
-
-    if (nd == 1 && simplified_strides[0] == 1) {
-        auto fn = (use_i32)
-                      ? mask_positions_contig_i32_dispatch_vector[mask_typeid]
-                      : mask_positions_contig_i64_dispatch_vector[mask_typeid];
-
-        return fn(exec_q, mask_size, mask_data, cumsum_data, depends);
-    }
+    dpctl::tensor::py_internal::compact_iteration_space(
+        nd, shape, strides_vector, compact_shape, compact_strides);
 
     // Strided implementation
     auto strided_fn =
@@ -232,7 +222,7 @@ size_t py_mask_positions(dpctl::tensor::usm_ndarray mask,
 
     using dpctl::tensor::offset_utils::device_allocate_and_pack;
     const auto &ptr_size_event_tuple = device_allocate_and_pack<py::ssize_t>(
-        exec_q, host_task_events, simplified_shape, simplified_strides);
+        exec_q, host_task_events, compact_shape, compact_strides);
     py::ssize_t *shape_strides = std::get<0>(ptr_size_event_tuple);
     if (shape_strides == nullptr) {
         sycl::event::wait(host_task_events);
@@ -253,7 +243,7 @@ size_t py_mask_positions(dpctl::tensor::usm_ndarray mask,
     dependent_events.insert(dependent_events.end(), depends.begin(),
                             depends.end());
 
-    size_t total_set = strided_fn(exec_q, mask_size, mask_data, nd, offset,
+    size_t total_set = strided_fn(exec_q, mask_size, mask_data, nd,
                                   shape_strides, cumsum_data, dependent_events);
 
     sycl::event::wait(host_task_events);

dpctl/tensor/libtensor/source/simplify_iteration_space.cpp

@@ -369,6 +369,45 @@ void simplify_iteration_space_4(
     }
 }
 
+void compact_iteration_space(int &nd,
+                             const py::ssize_t *const &shape,
+                             std::vector<py::ssize_t> const &strides,
+                             // output
+                             std::vector<py::ssize_t> &compact_shape,
+                             std::vector<py::ssize_t> &compact_strides)
+{
+    using dpctl::tensor::strides::compact_iteration;
+    if (nd > 1) {
+        // Compact iteration space to reduce dimensionality
+        // and improve access pattern
+        compact_shape.reserve(nd);
+        compact_shape.insert(std::begin(compact_shape), shape, shape + nd);
+        assert(compact_shape.size() == static_cast<size_t>(nd));
+
+        compact_strides.reserve(nd);
+        compact_strides.insert(std::end(compact_strides), std::begin(strides),
+                               std::end(strides));
+        assert(compact_strides.size() == static_cast<size_t>(nd));
+
+        int contracted_nd =
+            compact_iteration(nd, compact_shape.data(), compact_strides.data());
+        compact_shape.resize(contracted_nd);
+        compact_strides.resize(contracted_nd);
+
+        nd = contracted_nd;
+    }
+    else if (nd == 1) {
+        // Populate vectors
+        compact_shape.reserve(nd);
+        compact_shape.push_back(shape[0]);
+        assert(compact_shape.size() == static_cast<size_t>(nd));
+
+        compact_strides.reserve(nd);
+        compact_strides.push_back(strides[0]);
+        assert(compact_strides.size() == static_cast<size_t>(nd));
+    }
+}
+
 py::ssize_t _ravel_multi_index_c(std::vector<py::ssize_t> const &mi,
                                  std::vector<py::ssize_t> const &shape)
 {

dpctl/tensor/libtensor/source/simplify_iteration_space.hpp

@@ -90,6 +90,13 @@ void simplify_iteration_space_4(int &,
                                 py::ssize_t &,
                                 py::ssize_t &);
 
+void compact_iteration_space(int &,
+                             const py::ssize_t *const &,
+                             std::vector<py::ssize_t> const &,
+                             // output
+                             std::vector<py::ssize_t> &,
+                             std::vector<py::ssize_t> &);
+
 py::ssize_t _ravel_multi_index_c(std::vector<py::ssize_t> const &,
                                  std::vector<py::ssize_t> const &);
 py::ssize_t _ravel_multi_index_f(std::vector<py::ssize_t> const &,

dpctl/tests/test_usm_ndarray_indexing.py

@@ -1044,6 +1044,19 @@ def test_extract_all_1d():
     res2 = dpt.extract(sel, x)
     assert (dpt.asnumpy(res2) == expected_res).all()
 
+    # test strided case
+    x = dpt.arange(15, dtype="i4")
+    sel_np = np.zeros(15, dtype="?")
+    np.put(sel_np, np.random.choice(sel_np.size, size=7), True)
+    sel = dpt.asarray(sel_np)
+
+    res = x[sel[::-1]]
+    expected_res = dpt.asnumpy(x)[sel_np[::-1]]
+    assert (dpt.asnumpy(res) == expected_res).all()
+
+    res2 = dpt.extract(sel[::-1], x)
+    assert (dpt.asnumpy(res2) == expected_res).all()
+
 
 def test_extract_all_2d():
     get_queue_or_skip()
@@ -1287,6 +1300,38 @@ def test_nonzero():
     assert (dpt.asnumpy(i) == np.array([3, 4, 5, 6])).all()
 
 
+def test_nonzero_f_contig():
+    "See gh-1370"
+    get_queue_or_skip
+
+    mask = dpt.zeros((5, 5), dtype="?", order="F")
+    mask[2, 3] = True
+
+    expected_res = (2, 3)
+    res = dpt.nonzero(mask)
+
+    assert expected_res == res
+    assert mask[res]
+
+
+def test_nonzero_compacting():
+    """See gh-1370.
+    Test with input where dimensionality
+    of iteration space is compacted from 3d to 2d
+    """
+    get_queue_or_skip
+
+    mask = dpt.zeros((5, 5, 5), dtype="?", order="F")
+    mask[3, 2, 1] = True
+    mask_view = mask[..., :3]
+
+    expected_res = (3, 2, 1)
+    res = dpt.nonzero(mask_view)
+
+    assert expected_res == res
+    assert mask_view[res]
+
+
 def test_assign_scalar():
     get_queue_or_skip()
     x = dpt.arange(-5, 5, dtype="i8")