Address several PR comments

* Move __stride_recommender into __parallel_for_large_submitter
* Use {} to invoke constructor
* Simplify if-else statements in for dispatch

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

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

@@ -275,6 +275,39 @@ struct __parallel_for_large_submitter<__internal::__optional_kernel_name<_Name..
     // Limit the work-group size to 512 which has empirically yielded the best results across different architectures.
     static constexpr std::uint16_t __max_work_group_size = 512;
 
+    // SPIR-V compilation targets show best performance with a stride of the sub-group size.
+    // Other compilation targets perform best with a work-group size stride. This utility can only be called from the
+    // device.
+    static inline std::tuple<std::size_t, std::size_t, bool>
+    __stride_recommender(const sycl::nd_item<1>& __item, std::size_t __count, std::size_t __iters_per_work_item,
+                         std::size_t __work_group_size)
+    {
+        if constexpr (oneapi::dpl::__internal::__is_spirv_target_v)
+        {
+            const __dpl_sycl::__sub_group __sub_group = __item.get_sub_group();
+            const std::uint32_t __sub_group_size = __sub_group.get_local_linear_range();
+            const std::uint32_t __sub_group_id = __sub_group.get_group_linear_id();
+            const std::uint32_t __sub_group_local_id = __sub_group.get_local_linear_id();
+            const std::size_t __work_group_id = __item.get_group().get_group_linear_id();
+
+            const std::size_t __sub_group_start_idx =
+                __iters_per_work_item * (__work_group_id * __work_group_size + __sub_group_size * __sub_group_id);
+            const bool __is_full_sub_group =
+                __sub_group_start_idx + __iters_per_work_item * __sub_group_size <= __count;
+            const std::size_t __work_item_idx = __sub_group_start_idx + __sub_group_local_id;
+            return std::make_tuple(__work_item_idx, __sub_group_size, __is_full_sub_group);
+        }
+        else
+        {
+            const std::size_t __work_group_start_idx =
+                __item.get_group().get_group_linear_id() * __work_group_size * __iters_per_work_item;
+            const std::size_t __work_item_idx = __work_group_start_idx + __item.get_local_linear_id();
+            const bool __is_full_work_group =
+                __work_group_start_idx + __iters_per_work_item * __work_group_size <= __count;
+            return std::make_tuple(__work_item_idx, __work_group_size, __is_full_work_group);
+        }
+    }
+
     // Once there is enough work to launch a group on each compute unit with our chosen __iters_per_item,
     // then we should start using this code path.
     template <typename _ExecutionPolicy>
@@ -357,22 +390,14 @@ __parallel_for(oneapi::dpl::__internal::__device_backend_tag, _ExecutionPolicy&&
     // then only compile the basic kernel as the two versions are effectively the same.
     if constexpr (__large_submitter::__iters_per_work_item > 1)
     {
-        if (__count < __large_submitter::__estimate_best_start_size(__exec))
+        if (__count >= __large_submitter::__estimate_best_start_size(__exec))
         {
-            return __small_submitter()(std::forward<_ExecutionPolicy>(__exec), __brick, __count,
+            return __large_submitter{}(std::forward<_ExecutionPolicy>(__exec), __brick, __count,
                                        std::forward<_Ranges>(__rngs)...);
         }
-        else
-        {
-            return __large_submitter()(std::forward<_ExecutionPolicy>(__exec), __brick, __count,
-                                       std::forward<_Ranges>(__rngs)...);
-        }
-    }
-    else
-    {
-        return __small_submitter()(std::forward<_ExecutionPolicy>(__exec), __brick, __count,
-                                   std::forward<_Ranges>(__rngs)...);
     }
+    return __small_submitter{}(std::forward<_ExecutionPolicy>(__exec), __brick, __count,
+                               std::forward<_Ranges>(__rngs)...);
 }
 
 //------------------------------------------------------------------------