Update documentation for oneTBB 2021.13.0 (#1422)

Author: omalyshe

doc/main/reference/reference.rst

@@ -19,6 +19,7 @@ It also describes features that are not included in the oneTBB specification.
     parallel_for_each_semantics
     parallel_sort_ranges_extension
     scalable_memory_pools/malloc_replacement_log
+    rvalue_reduce
 
 Preview features
 ****************

doc/main/reference/rvalue_reduce.rst

@@ -0,0 +1,89 @@
+.. _rvalue_reduce:
+
+Parallel Reduction for rvalues
+==============================
+
+.. contents::
+    :local:
+    :depth: 1
+
+Description
+***********
+
+|full_name| implementation extends the `ParallelReduceFunc <https://spec.oneapi.io/versions/latest/elements/oneTBB/source/named_requirements/algorithms/par_reduce_func.html>`_ and
+`ParallelReduceReduction <https://spec.oneapi.io/versions/latest/elements/oneTBB/source/named_requirements/algorithms/par_reduce_reduction.html>`_
+to optimize operating with ``rvalues`` using functional form of ``tbb::parallel_reduce`` and ``tbb::parallel_deterministic_reduce`` algorithms.
+
+API
+***
+
+Header
+------
+
+.. code:: cpp
+
+    #include <oneapi/tbb/parallel_reduce.h>
+
+ParallelReduceFunc Requirements: Pseudo-Signature, Semantics
+------------------------------------------------------------
+
+.. cpp:function:: Value Func::operator()(const Range& range, Value&& x) const
+
+or
+
+.. cpp:function:: Value Func::operator()(const Range& range, const Value& x) const
+
+    Accumulates the result for a subrange, starting with initial value ``x``. The ``Range`` type must meet the `Range requirements <https://spec.oneapi.io/versions/latest/elements/oneTBB/source/named_requirements/algorithms/range.html>_`.
+    The ``Value`` type must be the same as a corresponding template parameter for the `parallel_reduce algorithm <https://spec.oneapi.io/versions/latest/elements/oneTBB/source/algorithms/functions/parallel_reduce_func.html>`_.
+
+    If both ``rvalue`` and ``lvalue`` forms are provided, the ``rvalue`` is preferred.
+
+ParallelReduceReduction Requirements: Pseudo-Signature, Semantics
+-----------------------------------------------------------------
+
+.. cpp:function:: Value Reduction::operator()(Value&& x, Value&& y) const
+
+or
+
+.. cpp:function:: Value Reduction::operator()(const Value& x, const Value& y) const
+
+    Combines the ``x`` and ``y`` results. The ``Value`` type must be the same as a corresponding template parameter for the `parallel_reduce algorithm <https://spec.oneapi.io/versions/latest/elements/oneTBB/source/algorithms/functions/parallel_reduce_func.html>`_.
+
+    If both ``rvalue`` and ``lvalue`` forms are provided, the ``rvalue`` is preferred.
+
+Example
+*******
+
+.. code:: cpp
+    // C++17
+    #include <oneapi/tbb/parallel_reduce.h>
+    #include <oneapi/tbb/blocked_range.h>
+    #include <vector>
+    #include <set>
+
+    int main() {
+        std::vector<std::set<int>> sets = ...;
+
+        oneapi::tbb::parallel_reduce(oneapi::tbb::blocked_range<size_t>(0, sets.size()),
+                                     std::set<int>{}, // identity element - empty set
+                                     [&](const oneapi::tbb::blocked_range<size_t>& range, std::set<int>&& value) {
+                                         for (size_t i = range.begin(); i < range.end(); ++i) {
+                                             // Having value as a non-const rvalue reference allows to efficiently
+                                             // transfer nodes from sets[i] without copying/moving the data
+                                             value.merge(std::move(sets[i]));
+                                         }
+                                         return value;
+                                     },
+                                     [&](std::set<int>&& x, std::set<int>&& y) {
+                                         x.merge(std::move(y));
+                                         return x;
+                                     }
+                                     );
+    }
+
+.. rubric:: See also
+
+* `oneapi::tbb::parallel_reduce specification <https://spec.oneapi.io/versions/latest/elements/oneTBB/source/algorithms/functions/parallel_reduce_func.html>`_
+* `oneapi::tbb::parallel_deterministic_reduce specification <https://spec.oneapi.io/versions/latest/elements/oneTBB/source/algorithms/functions/parallel_deterministic_reduce_func.html>`_
+* `ParallelReduceFunc specification <https://spec.oneapi.io/versions/latest/elements/oneTBB/source/named_requirements/algorithms/par_reduce_func.html>`_
+* `ParallelReduceReduction specification <https://spec.oneapi.io/versions/latest/elements/oneTBB/source/named_requirements/algorithms/par_reduce_reduction.html>`_