Fix create view usm offset

CHANGELOG.md

@@ -85,6 +85,7 @@ This release is compatible with NumPy 2.5.
 * Fixed a crash in boolean-mask advanced indexing (`dpnp.ndarray` get/set item) when the selection is empty (e.g. a scalar `False` index that injects a length-0 axis) [#3019](https://github.com/IntelPython/dpnp/pull/3019)
 * Released the GIL before the remaining blocking OneMKL BLAS and LAPACK calls to prevent host tasks contention, completing the work started in [#2850](https://github.com/IntelPython/dpnp/pull/2850) [#3027](https://github.com/IntelPython/dpnp/pull/3027)
 * Fixed `dpnp.repeat` raising an unclear `TypeError` for a nested sequence of `repeats` [#3024](https://github.com/IntelPython/dpnp/pull/3024)
+* Fixed `dpnp.ndarray.view` ignoring the USM element offset of a sliced array, which also caused `dpnp.einsum` to silently return wrong results for a single sliced operand with no summed index [#3037](https://github.com/IntelPython/dpnp/pull/3037)
 
 ### Security
 

dpnp/dpnp_array.py

@@ -701,12 +701,27 @@ def _create_view(self, array_class, shape, dtype, strides):
         if dtype is None:
             dtype = self.dtype
 
+        new_itemsize = dpnp.dtype(dtype).itemsize
+
+        # `buffer=self._array_obj` views the whole USM allocation, so `self`'s
+        # element offset within it must be forwarded explicitly
+
+        byte_offset = self._array_obj._element_offset * self.itemsize
+        offset, rem = divmod(byte_offset, new_itemsize)
+        if rem:
+            raise ValueError(
+                "The offset of the array data in memory is not a multiple "
+                "of the new data type size and so the requested view is "
+                "not possible"
+            )
+
         # create the underlying usm_ndarray view
         usm_view = dpt.usm_ndarray(
             shape,
             dtype=dtype,
             buffer=self._array_obj,
-            strides=tuple(s // dpnp.dtype(dtype).itemsize for s in strides),
+            strides=tuple(s // new_itemsize for s in strides),
+            offset=offset,
         )
 
         # wrap the view into the appropriate class

dpnp/tests/test_linalg.py

@@ -593,6 +593,17 @@ def test_trivial_cases(self):
         expected = numpy.einsum("i,i,i", b_np, b_np, b_np, optimize="greedy")
         assert_dtype_allclose(result, expected)
 
+    def test_sliced_operand_view_path(self):
+        # a single-operand einsum with no summed index returns a view of the
+        # operand; the view must respect the USM offset of a sliced operand
+        a = numpy.arange(24.0, dtype=numpy.float32).reshape(2, 3, 4)
+        ia = dpnp.array(a)
+
+        for subscripts in ["abc->abc", "abc->cab"]:
+            result = dpnp.einsum(subscripts, ia[:, 1:, :])
+            expected = numpy.einsum(subscripts, a[:, 1:, :])
+            assert_dtype_allclose(result, expected)
+
     def test_out(self):
         a = dpnp.ones((5, 5))
         out = dpnp.empty((5,))

dpnp/tests/test_ndarray.py

@@ -328,6 +328,218 @@ def test_python_types(self, dt):
         expected = a.view(dt)
         assert_allclose(result, expected)
 
+    def test_nonzero_offset(self):
+        # the view must preserve the USM element offset of a sliced array
+        # instead of rebasing onto the start of the allocation
+        a = numpy.arange(10, dtype=numpy.int32)
+        ia = dpnp.array(a)
+
+        expected = a[3:].view()
+        result = ia[3:].view()
+        assert_array_equal(result, expected)
+
+        expected = a[3:].view(numpy.uint32)
+        result = ia[3:].view(dpnp.uint32)
+        assert_array_equal(result, expected)
+
+    def test_nonzero_offset_2d(self):
+        a = numpy.arange(12, dtype=numpy.int32).reshape(3, 4)
+        ia = dpnp.array(a)
+
+        expected = a[1:].view()
+        result = ia[1:].view()
+        assert_array_equal(result, expected)
+
+        # itemsize-decreasing view on an array with non-zero offset
+        expected = a[1:].view(numpy.int16)
+        result = ia[1:].view(dpnp.int16)
+        assert_array_equal(result, expected)
+
+        # itemsize-increasing view on an array with non-zero offset
+        expected = a[1:].view(numpy.int64)
+        result = ia[1:].view(dpnp.int64)
+        assert_array_equal(result, expected)
+
+    def test_nonzero_offset_negative_strides(self):
+        a = numpy.arange(10, dtype=numpy.int32)
+        ia = dpnp.array(a)
+
+        expected = a[::-1].view()
+        result = ia[::-1].view()
+        assert_array_equal(result, expected)
+
+        expected = a[8:2:-2].view()
+        result = ia[8:2:-2].view()
+        assert_array_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "dt", get_all_dtypes(no_none=True, no_float16=False)
+    )
+    @pytest.mark.parametrize(
+        "sl",
+        [
+            slice(3, None),
+            slice(1, 8),
+            slice(2, None, 3),
+            slice(None, None, -1),
+            slice(8, 2, -2),
+            slice(10, None),  # empty result with non-zero offset
+        ],
+    )
+    def test_nonzero_offset_all_dtypes(self, dt, sl):
+        a = numpy.arange(10).astype(dt)
+        ia = dpnp.array(a)
+
+        expected = a[sl].view()
+        result = ia[sl].view()
+        assert_array_equal(result, expected)
+
+    def test_nonzero_offset_0d(self):
+        a = numpy.arange(10, dtype=numpy.int32)
+        ia = dpnp.array(a)
+
+        expected = a[5, ...].view()
+        result = ia[5, ...].view()
+        assert_array_equal(result, expected)
+
+    def test_nonzero_offset_chained_views(self):
+        a = numpy.arange(20, dtype=numpy.int32)
+        ia = dpnp.array(a)
+
+        # offsets accumulated over several slicing steps
+        expected = a[2:][3:].view()
+        result = ia[2:][3:].view()
+        assert_array_equal(result, expected)
+
+        # a view of a view keeps the offset as well
+        expected = a[4:].view().view(numpy.uint32)
+        result = ia[4:].view().view(dpnp.uint32)
+        assert_array_equal(result, expected)
+
+    def test_nonzero_offset_shares_memory(self):
+        a = numpy.arange(10, dtype=numpy.int32)
+        ia = dpnp.array(a)
+
+        iv = ia[3:].view()
+        assert iv.data.ptr == ia[3:].data.ptr
+
+        # writing through the view must modify the parent array
+        iv[0] = -7
+        a[3] = -7
+        assert_array_equal(ia, a)
+
+    def test_nonzero_offset_complex_real(self):
+        a = numpy.arange(8).astype(numpy.complex64)
+        ia = dpnp.array(a)
+
+        expected = a[2:].view(numpy.float32)
+        result = ia[2:].view(dpnp.float32)
+        assert_array_equal(result, expected)
+
+        b = numpy.arange(8).astype(numpy.float32)
+        ib = dpnp.array(b)
+
+        expected = b[2:].view(numpy.complex64)
+        result = ib[2:].view(dpnp.complex64)
+        assert_array_equal(result, expected)
+
+    def test_nonzero_offset_3d(self):
+        a = numpy.arange(24, dtype=numpy.int32).reshape(2, 3, 4)
+        ia = dpnp.array(a)
+
+        for sl in [
+            numpy.s_[1:],
+            numpy.s_[:, 1:, :],
+            numpy.s_[:, :, 2:],
+            numpy.s_[1:, 1:, 1:],
+        ]:
+            expected = a[sl].view()
+            result = ia[sl].view()
+            assert_array_equal(result, expected)
+
+    def test_nonzero_offset_f_order(self):
+        a = numpy.asfortranarray(
+            numpy.arange(12, dtype=numpy.int32).reshape(3, 4)
+        )
+        ia = dpnp.asarray(a, order="F")
+
+        expected = a[1:].view()
+        result = ia[1:].view()
+        assert_array_equal(result, expected)
+        assert result.strides == ia[1:].strides
+
+    def test_nonzero_offset_non_contiguous(self):
+        a = numpy.arange(12, dtype=numpy.int32).reshape(3, 4)
+        ia = dpnp.array(a)
+
+        # a transposed slice and a column slice both have a non-zero offset
+        # and a non-contiguous last axis
+        for expected, result in [
+            (a[1:].T.view(), ia[1:].T.view()),
+            (a[:, 1:].view(), ia[:, 1:].view()),
+        ]:
+            assert_array_equal(result, expected)
+
+        # changing the itemsize is rejected for a non-contiguous last axis,
+        # exactly as numpy does
+        for xp, x in [(numpy, a[1:].T), (dpnp, ia[1:].T)]:
+            with pytest.raises(
+                ValueError, match="last axis must be contiguous"
+            ):
+                x.view(xp.int16)
+
+    @pytest.mark.parametrize("usm_type", ["device", "shared", "host"])
+    def test_nonzero_offset_keeps_usm_type_and_queue(self, usm_type):
+        ia = dpnp.arange(10, dtype=dpnp.int32, usm_type=usm_type)
+        iv = ia[3:].view()
+
+        assert iv.usm_type == ia.usm_type
+        assert iv.sycl_queue == ia.sycl_queue
+        assert_array_equal(iv, numpy.arange(3, 10, dtype=numpy.int32))
+
+    def test_nonzero_offset_write_through_dtype_change(self):
+        a = numpy.arange(6, dtype=numpy.int32)
+        ia = dpnp.array(a)
+
+        av, iav = a[2:].view(numpy.int16), ia[2:].view(dpnp.int16)
+        av[0] = 99
+        iav[0] = 99
+
+        # the write must land in the parent array, at the right offset
+        assert_array_equal(ia, a)
+
+    def test_nonzero_offset_compute(self):
+        # the kernels must read the view through its own offset, not from the
+        # base of the parent allocation
+        a = numpy.arange(24, dtype=numpy.float32).reshape(2, 3, 4)
+        ia = dpnp.array(a)
+
+        av, iav = a[:, 1:, :].view(), ia[:, 1:, :].view()
+        assert_allclose(dpnp.sum(iav), numpy.sum(av))
+        assert_allclose(iav * 2, av * 2)
+
+    def test_nonzero_offset_buffer_ctor(self):
+        # combines the offset of the `buffer=` array with the offset passed
+        # to the ndarray constructor, and then views the result
+        base = dpnp.arange(12, dtype=dpnp.int32)
+        ia = dpnp.ndarray((4,), dtype=dpnp.int32, buffer=base[2:], offset=1)
+
+        expected = numpy.arange(3, 7, dtype=numpy.int32)
+        assert_array_equal(ia, expected)
+        assert_array_equal(ia.view(), expected)
+        assert_array_equal(ia.view(dpnp.uint32), expected.view(numpy.uint32))
+
+    def test_misaligned_offset_error(self):
+        ia = dpnp.arange(10, dtype=dpnp.int16)
+        # numpy supports such a view, but usm_ndarray cannot address memory
+        # at a fraction of its element size
+        with pytest.raises(ValueError, match="not a multiple"):
+            ia[1:9].view(dpnp.int32)
+
+        ia = dpnp.arange(8, dtype=dpnp.float32)
+        with pytest.raises(ValueError, match="not a multiple"):
+            ia[1:7].view(dpnp.complex64)
+
     def test_subclass_basic(self):
         class MyArray(dpnp.ndarray):
             pass
@@ -339,6 +551,16 @@ class MyArray(dpnp.ndarray):
         assert type(view) is MyArray
         assert (view == x).all()
 
+    def test_subclass_nonzero_offset(self):
+        class MyArray(dpnp.ndarray):
+            pass
+
+        x = dpnp.arange(10, dtype=dpnp.int32)
+        view = x[3:].view(type=MyArray)
+
+        assert type(view) is MyArray
+        assert_array_equal(view, numpy.arange(3, 10, dtype=numpy.int32))
+
     def test_dtype_type_subclass(self):
         class MyArray(dpnp.ndarray):
             pass