[oneDPL][ranges][merge] + fix for __serial_merge_out_lim call; +comments

Author: MikeDvorskiy

include/oneapi/dpl/pstl/algorithm_impl.h

@@ -3020,11 +3020,16 @@ std::pair<_It1, _It2>
 ___merge_path_out_lim(_Tag, _ExecutionPolicy&& __exec, _It1 __it_1, _Index1 __n_1, _It2 __it_2, _Index2 __n_2,
                       _OutIt __it_out, _Index3 __n_out, _Comp __comp)
 {
-    return __serial_merge_out_lim(__it_1, __it_1 + __n_1, __it_2, __it_2 + __n_2, __it_out, __it_out + __n_out, __comp);
+    //___merge_path_out_lim is called with reverse order: (2nd sequience, 1st sequience)
+    //__serial_merge_out_lim does merging in direct order: (1st sequience and 2nd sequience).
+    // So, the following call passes 1st sequience 2nd sequience in "a revert maner".
+    return __serial_merge_out_lim(__it_2, __it_2 + __n_2, __it_1, __it_1 + __n_1, __it_out, __it_out + __n_out, __comp);
 }
 
 inline constexpr std::size_t __merge_path_cut_off = 2000;
 
+//Parallel version of ___merge_path_out_lim merges 1st sequience and 2nd sequience in "revert maner":
+//the identical elements from 2nd sequience are being merged first.
 template <typename _IsVector, typename _ExecutionPolicy, typename _It1, typename _Index1, typename _It2,
           typename _Index2, typename _OutIt, typename _Index3, typename _Comp>
 std::pair<_It1, _It2>

include/oneapi/dpl/pstl/algorithm_ranges_impl.h

@@ -471,6 +471,9 @@ __pattern_merge(_Tag __tag, _ExecutionPolicy&& __exec, _R1&& __r1, _R2&& __r2, _
     if (__n_out == 0)
         return __return_type{__it_1, __it_2, __it_out};
 
+    //Parallel version of ___merge_path_out_lim merges 1st sequience and 2nd sequience in "revert maner":
+    //the identical elements from 2nd sequience are being merged first.
+    //So, the following call passes 1st sequience 2nd sequience in "a revert maner".
     std::pair __res = ___merge_path_out_lim(__tag, std::forward<_ExecutionPolicy>(__exec), __it_2, __n_2, __it_1, __n_1,
                                             __it_out, __n_out, __comp_2);