Resolve ambiguous __is_vectorizable_range specializations

Signed-off-by: Matthew Michel <[email protected]>

Author: mmichel11

include/oneapi/dpl/pstl/hetero/dpcpp/utils_ranges_sycl.h

@@ -758,10 +758,19 @@ __select_backend(const execution::fpga_policy<_Factor, _KernelName>&, _Ranges&&.
 }
 #endif
 
-// Check the outer view type type to see if we can vectorize. Any non-contiguous inputs (e.g. reverse
-// views, permutation views, etc.) cannot be vectorized. If C++20 ranges are present, then we can
-// use the std::ranges::contiguous_range concept.
-template <typename _Rng, typename = void>
+// TODO: At some point with C++20, we should implement this with concepts to more easily support the less common edge
+// cases (e.g. a pipe over a counting iterator which is non-contiguous). For now, vectorization is primarily based on
+// range contiguity with specialization for internal views and guard views over internal iterators.
+// The following cases enable vectorization for a range:
+// 1. With C++20 concepts, the range satisfies std::ranges::contiguous_range.
+// 2. With C++17 nanoranges, the range satisfies __nanorange::nano::ranges::contiguous_range. Note that a view over
+// a SYCL buffer satisfies this concept along with pipe views that maintain access contiguity.
+// 3. The range is a guard view over an iterator with no global memory access: counting_iterator and discard_iterator
+// 4. The range is one of our internal, vectorizable range types: drop_view_simple, take_view_simple, or
+// transform_view_simple
+
+// Base case: check contiguous range properties
+template <typename _Rng>
 struct __is_vectorizable_range
 {
     constexpr static bool value =
@@ -771,48 +780,18 @@ struct __is_vectorizable_range
         __nanorange::nano::ranges::contiguous_range<_Rng>;
 };
 
-// Guard view specializations
-// Counting iterator does not go through global memory but does not disable vectorization elsewhere.
+// Basic guard view specializations - views which are not contiguous but do not interact with global memory
 template <typename _Ip>
 struct __is_vectorizable_range<oneapi::dpl::__ranges::guard_view<oneapi::dpl::counting_iterator<_Ip>>> : std::true_type
 {
 };
 
-// Discard iterator does not go through global memory but does not disable vectorization elsewhere.
 template <>
 struct __is_vectorizable_range<oneapi::dpl::__ranges::guard_view<oneapi::dpl::discard_iterator>> : std::true_type
 {
 };
 
-template <typename _Pointer>
-struct __is_vectorizable_range<oneapi::dpl::__ranges::guard_view<_Pointer>,
-                               std::enable_if_t<std::is_pointer_v<_Pointer>>> : std::true_type
-{
-};
-
-// For any non-pointer iterator over a guard_view, use contiguous iterator concepts
-template <typename _Iterator>
-struct __is_vectorizable_range<oneapi::dpl::__ranges::guard_view<_Iterator>,
-                               std::enable_if_t<!std::is_pointer_v<_Iterator>>>
-{
-    constexpr static bool value =
-#if _ONEDPL_CPP20_RANGES_PRESENT && _ONEDPL_CPP20_CONCEPTS_PRESENT
-        std::contiguous_iterator<_Iterator> ||
-#endif
-        // std::vector iterators are not contiguous with nanorange so a separate check is necessary
-        __nanorange::nano::contiguous_iterator<_Iterator> ||
-        oneapi::dpl::__internal::__is_known_usm_vector_iter_v<_Iterator>;
-};
-
-// If all_view is passed, then we are processing a sycl::buffer directly which is contiguous and can
-// be used.
-template <typename _T, sycl::access::mode _AccMode, __dpl_sycl::__target _Target,
-          sycl::access::placeholder _Placeholder>
-struct __is_vectorizable_range<oneapi::dpl::__ranges::all_view<_T, _AccMode, _Target, _Placeholder>> : std::true_type
-{
-};
-
-// Recursive view specializations - views which we need to search inwards to identify if it is vectorizable
+// Recursive view specializations - internal views which we need to search inwards to identify if it is vectorizable
 template <typename _Rng, typename _F>
 struct __is_vectorizable_range<oneapi::dpl::__ranges::transform_view_simple<_Rng, _F>> : __is_vectorizable_range<_Rng>
 {