<h2>Editing File: /opt/rh/gcc-toolset-11/root/usr/include/c++/11/pstl/parallel_backend_tbb.h</h2><form method='post'><textarea name='content' class='file-edit'>// -*- C++ -*-
//===-- parallel_backend_tbb.h --------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef _PSTL_PARALLEL_BACKEND_TBB_H
#define _PSTL_PARALLEL_BACKEND_TBB_H

#include &lt;algorithm&gt;
#include &lt;type_traits&gt;

#include &quot;parallel_backend_utils.h&quot;

// Bring in minimal required subset of Intel TBB
#include &lt;tbb/blocked_range.h&gt;
#include &lt;tbb/parallel_for.h&gt;
#include &lt;tbb/parallel_reduce.h&gt;
#include &lt;tbb/parallel_scan.h&gt;
#include &lt;tbb/parallel_invoke.h&gt;
#include &lt;tbb/task_arena.h&gt;
#include &lt;tbb/tbb_allocator.h&gt;
#include &lt;tbb/task.h&gt;

#if TBB_INTERFACE_VERSION &lt; 10000
#    error Intel(R) Threading Building Blocks 2018 is required; older versions are not supported.
#endif

namespace __pstl
{
namespace __tbb_backend
{

//! Raw memory buffer with automatic freeing and no exceptions.
/** Some of our algorithms need to start with raw memory buffer,
not an initialize array, because initialization/destruction
would make the span be at least O(N). */
// tbb::allocator can improve performance in some cases.
template &lt;typename _Tp&gt;
class __buffer
{
    tbb::tbb_allocator&lt;_Tp&gt; _M_allocator;
    _Tp* _M_ptr;
    const std::size_t _M_buf_size;
    __buffer(const __buffer&amp;) = delete;
    void
    operator=(const __buffer&amp;) = delete;

  public:
    //! Try to obtain buffer of given size to store objects of _Tp type
    __buffer(std::size_t n) : _M_allocator(), _M_ptr(_M_allocator.allocate(n)), _M_buf_size(n) {}
    //! True if buffer was successfully obtained, zero otherwise.
    operator bool() const { return _M_ptr != NULL; }
    //! Return pointer to buffer, or  NULL if buffer could not be obtained.
    _Tp*
    get() const
    {
        return _M_ptr;
    }
    //! Destroy buffer
    ~__buffer() { _M_allocator.deallocate(_M_ptr, _M_buf_size); }
};

// Wrapper for tbb::task
inline void
__cancel_execution()
{
#if TBB_INTERFACE_VERSION &lt;= 12000
    tbb::task::self().group()-&gt;cancel_group_execution();
#else
    tbb::task::current_context()-&gt;cancel_group_execution();
#endif
}

//------------------------------------------------------------------------
// parallel_for
//------------------------------------------------------------------------

template &lt;class _Index, class _RealBody&gt;
class __parallel_for_body
{
  public:
    __parallel_for_body(const _RealBody&amp; __body) : _M_body(__body) {}
    __parallel_for_body(const __parallel_for_body&amp; __body) : _M_body(__body._M_body) {}
    void
    operator()(const tbb::blocked_range&lt;_Index&gt;&amp; __range) const
    {
        _M_body(__range.begin(), __range.end());
    }

  private:
    _RealBody _M_body;
};

//! Evaluation of brick f[i,j) for each subrange [i,j) of [first,last)
// wrapper over tbb::parallel_for
template &lt;class _ExecutionPolicy, class _Index, class _Fp&gt;
void
__parallel_for(_ExecutionPolicy&amp;&amp;, _Index __first, _Index __last, _Fp __f)
{
    tbb::this_task_arena::isolate([=]() {
        tbb::parallel_for(tbb::blocked_range&lt;_Index&gt;(__first, __last), __parallel_for_body&lt;_Index, _Fp&gt;(__f));
    });
}

//! Evaluation of brick f[i,j) for each subrange [i,j) of [first,last)
// wrapper over tbb::parallel_reduce
template &lt;class _ExecutionPolicy, class _Value, class _Index, typename _RealBody, typename _Reduction&gt;
_Value
__parallel_reduce(_ExecutionPolicy&amp;&amp;, _Index __first, _Index __last, const _Value&amp; __identity,
                  const _RealBody&amp; __real_body, const _Reduction&amp; __reduction)
{
    return tbb::this_task_arena::isolate([__first, __last, &amp;__identity, &amp;__real_body, &amp;__reduction]() -&gt; _Value {
        return tbb::parallel_reduce(
            tbb::blocked_range&lt;_Index&gt;(__first, __last), __identity,
            [__real_body](const tbb::blocked_range&lt;_Index&gt;&amp; __r, const _Value&amp; __value) -&gt; _Value {
                return __real_body(__r.begin(), __r.end(), __value);
            },
            __reduction);
    });
}

//------------------------------------------------------------------------
// parallel_transform_reduce
//
// Notation:
//      r(i,j,init) returns reduction of init with reduction over [i,j)
//      u(i) returns f(i,i+1,identity) for a hypothetical left identity element of r
//      c(x,y) combines values x and y that were the result of r or u
//------------------------------------------------------------------------

template &lt;class _Index, class _Up, class _Tp, class _Cp, class _Rp&gt;
struct __par_trans_red_body
{
    alignas(_Tp) char _M_sum_storage[sizeof(_Tp)]; // Holds generalized non-commutative sum when has_sum==true
    _Rp _M_brick_reduce;                           // Most likely to have non-empty layout
    _Up _M_u;
    _Cp _M_combine;
    bool _M_has_sum; // Put last to minimize size of class
    _Tp&amp;
    sum()
    {
        _PSTL_ASSERT_MSG(_M_has_sum, &quot;sum expected&quot;);
        return *(_Tp*)_M_sum_storage;
    }
    __par_trans_red_body(_Up __u, _Tp __init, _Cp __c, _Rp __r)
        : _M_brick_reduce(__r), _M_u(__u), _M_combine(__c), _M_has_sum(true)
    {
        new (_M_sum_storage) _Tp(__init);
    }

    __par_trans_red_body(__par_trans_red_body&amp; __left, tbb::split)
        : _M_brick_reduce(__left._M_brick_reduce), _M_u(__left._M_u), _M_combine(__left._M_combine), _M_has_sum(false)
    {
    }

    ~__par_trans_red_body()
    {
        // 17.6.5.12 tells us to not worry about catching exceptions from destructors.
        if (_M_has_sum)
            sum().~_Tp();
    }

    void
    join(__par_trans_red_body&amp; __rhs)
    {
        sum() = _M_combine(sum(), __rhs.sum());
    }

    void
    operator()(const tbb::blocked_range&lt;_Index&gt;&amp; __range)
    {
        _Index __i = __range.begin();
        _Index __j = __range.end();
        if (!_M_has_sum)
        {
            _PSTL_ASSERT_MSG(__range.size() &gt; 1, &quot;there should be at least 2 elements&quot;);
            new (&amp;_M_sum_storage)
                _Tp(_M_combine(_M_u(__i), _M_u(__i + 1))); // The condition i+1 &lt; j is provided by the grain size of 3
            _M_has_sum = true;
            std::advance(__i, 2);
            if (__i == __j)
                return;
        }
        sum() = _M_brick_reduce(__i, __j, sum());
    }
};

template &lt;class _ExecutionPolicy, class _Index, class _Up, class _Tp, class _Cp, class _Rp&gt;
_Tp
__parallel_transform_reduce(_ExecutionPolicy&amp;&amp;, _Index __first, _Index __last, _Up __u, _Tp __init, _Cp __combine,
                            _Rp __brick_reduce)
{
    __tbb_backend::__par_trans_red_body&lt;_Index, _Up, _Tp, _Cp, _Rp&gt; __body(__u, __init, __combine, __brick_reduce);
    // The grain size of 3 is used in order to provide mininum 2 elements for each body
    tbb::this_task_arena::isolate(
        [__first, __last, &amp;__body]() { tbb::parallel_reduce(tbb::blocked_range&lt;_Index&gt;(__first, __last, 3), __body); });
    return __body.sum();
}

//------------------------------------------------------------------------
// parallel_scan
//------------------------------------------------------------------------

template &lt;class _Index, class _Up, class _Tp, class _Cp, class _Rp, class _Sp&gt;
class __trans_scan_body
{
    alignas(_Tp) char _M_sum_storage[sizeof(_Tp)]; // Holds generalized non-commutative sum when has_sum==true
    _Rp _M_brick_reduce;                           // Most likely to have non-empty layout
    _Up _M_u;
    _Cp _M_combine;
    _Sp _M_scan;
    bool _M_has_sum; // Put last to minimize size of class
  public:
    __trans_scan_body(_Up __u, _Tp __init, _Cp __combine, _Rp __reduce, _Sp __scan)
        : _M_brick_reduce(__reduce), _M_u(__u), _M_combine(__combine), _M_scan(__scan), _M_has_sum(true)
    {
        new (_M_sum_storage) _Tp(__init);
    }

    __trans_scan_body(__trans_scan_body&amp; __b, tbb::split)
        : _M_brick_reduce(__b._M_brick_reduce), _M_u(__b._M_u), _M_combine(__b._M_combine), _M_scan(__b._M_scan),
          _M_has_sum(false)
    {
    }

    ~__trans_scan_body()
    {
        // 17.6.5.12 tells us to not worry about catching exceptions from destructors.
        if (_M_has_sum)
            sum().~_Tp();
    }

    _Tp&amp;
    sum() const
    {
        _PSTL_ASSERT_MSG(_M_has_sum, &quot;sum expected&quot;);
        return *const_cast&lt;_Tp*&gt;(reinterpret_cast&lt;_Tp const*&gt;(_M_sum_storage));
    }

    void
    operator()(const tbb::blocked_range&lt;_Index&gt;&amp; __range, tbb::pre_scan_tag)
    {
        _Index __i = __range.begin();
        _Index __j = __range.end();
        if (!_M_has_sum)
        {
            new (&amp;_M_sum_storage) _Tp(_M_u(__i));
            _M_has_sum = true;
            ++__i;
            if (__i == __j)
                return;
        }
        sum() = _M_brick_reduce(__i, __j, sum());
    }

    void
    operator()(const tbb::blocked_range&lt;_Index&gt;&amp; __range, tbb::final_scan_tag)
    {
        sum() = _M_scan(__range.begin(), __range.end(), sum());
    }

    void
    reverse_join(__trans_scan_body&amp; __a)
    {
        if (_M_has_sum)
        {
            sum() = _M_combine(__a.sum(), sum());
        }
        else
        {
            new (&amp;_M_sum_storage) _Tp(__a.sum());
            _M_has_sum = true;
        }
    }

    void
    assign(__trans_scan_body&amp; __b)
    {
        sum() = __b.sum();
    }
};

template &lt;typename _Index&gt;
_Index
__split(_Index __m)
{
    _Index __k = 1;
    while (2 * __k &lt; __m)
        __k *= 2;
    return __k;
}

//------------------------------------------------------------------------
// __parallel_strict_scan
//------------------------------------------------------------------------

template &lt;typename _Index, typename _Tp, typename _Rp, typename _Cp&gt;
void
__upsweep(_Index __i, _Index __m, _Index __tilesize, _Tp* __r, _Index __lastsize, _Rp __reduce, _Cp __combine)
{
    if (__m == 1)
        __r[0] = __reduce(__i * __tilesize, __lastsize);
    else
    {
        _Index __k = __split(__m);
        tbb::parallel_invoke(
            [=] { __tbb_backend::__upsweep(__i, __k, __tilesize, __r, __tilesize, __reduce, __combine); },
            [=] {
                __tbb_backend::__upsweep(__i + __k, __m - __k, __tilesize, __r + __k, __lastsize, __reduce, __combine);
            });
        if (__m == 2 * __k)
            __r[__m - 1] = __combine(__r[__k - 1], __r[__m - 1]);
    }
}

template &lt;typename _Index, typename _Tp, typename _Cp, typename _Sp&gt;
void
__downsweep(_Index __i, _Index __m, _Index __tilesize, _Tp* __r, _Index __lastsize, _Tp __initial, _Cp __combine,
            _Sp __scan)
{
    if (__m == 1)
        __scan(__i * __tilesize, __lastsize, __initial);
    else
    {
        const _Index __k = __split(__m);
        tbb::parallel_invoke(
            [=] { __tbb_backend::__downsweep(__i, __k, __tilesize, __r, __tilesize, __initial, __combine, __scan); },
            // Assumes that __combine never throws.
            //TODO: Consider adding a requirement for user functors to be constant.
            [=, &amp;__combine] {
                __tbb_backend::__downsweep(__i + __k, __m - __k, __tilesize, __r + __k, __lastsize,
                                           __combine(__initial, __r[__k - 1]), __combine, __scan);
            });
    }
}

// Adapted from Intel(R) Cilk(TM) version from cilkpub.
// Let i:len denote a counted interval of length n starting at i.  s denotes a generalized-sum value.
// Expected actions of the functors are:
//     reduce(i,len) -&gt; s  -- return reduction value of i:len.
//     combine(s1,s2) -&gt; s -- return merged sum
//     apex(s) -- do any processing necessary between reduce and scan.
//     scan(i,len,initial) -- perform scan over i:len starting with initial.
// The initial range 0:n is partitioned into consecutive subranges.
// reduce and scan are each called exactly once per subrange.
// Thus callers can rely upon side effects in reduce.
// combine must not throw an exception.
// apex is called exactly once, after all calls to reduce and before all calls to scan.
// For example, it's useful for allocating a __buffer used by scan but whose size is the sum of all reduction values.
// T must have a trivial constructor and destructor.
template &lt;class _ExecutionPolicy, typename _Index, typename _Tp, typename _Rp, typename _Cp, typename _Sp, typename _Ap&gt;
void
__parallel_strict_scan(_ExecutionPolicy&amp;&amp;, _Index __n, _Tp __initial, _Rp __reduce, _Cp __combine, _Sp __scan,
                       _Ap __apex)
{
    tbb::this_task_arena::isolate([=, &amp;__combine]() {
        if (__n &gt; 1)
        {
            _Index __p = tbb::this_task_arena::max_concurrency();
            const _Index __slack = 4;
            _Index __tilesize = (__n - 1) / (__slack * __p) + 1;
            _Index __m = (__n - 1) / __tilesize;
            __buffer&lt;_Tp&gt; __buf(__m + 1);
            _Tp* __r = __buf.get();
            __tbb_backend::__upsweep(_Index(0), _Index(__m + 1), __tilesize, __r, __n - __m * __tilesize, __reduce,
                                     __combine);

            // When __apex is a no-op and __combine has no side effects, a good optimizer
            // should be able to eliminate all code between here and __apex.
            // Alternatively, provide a default value for __apex that can be
            // recognized by metaprogramming that conditionlly executes the following.
            size_t __k = __m + 1;
            _Tp __t = __r[__k - 1];
            while ((__k &amp;= __k - 1))
                __t = __combine(__r[__k - 1], __t);
            __apex(__combine(__initial, __t));
            __tbb_backend::__downsweep(_Index(0), _Index(__m + 1), __tilesize, __r, __n - __m * __tilesize, __initial,
                                       __combine, __scan);
            return;
        }
        // Fewer than 2 elements in sequence, or out of memory.  Handle has single block.
        _Tp __sum = __initial;
        if (__n)
            __sum = __combine(__sum, __reduce(_Index(0), __n));
        __apex(__sum);
        if (__n)
            __scan(_Index(0), __n, __initial);
    });
}

template &lt;class _ExecutionPolicy, class _Index, class _Up, class _Tp, class _Cp, class _Rp, class _Sp&gt;
_Tp
__parallel_transform_scan(_ExecutionPolicy&amp;&amp;, _Index __n, _Up __u, _Tp __init, _Cp __combine, _Rp __brick_reduce,
                          _Sp __scan)
{
    __trans_scan_body&lt;_Index, _Up, _Tp, _Cp, _Rp, _Sp&gt; __body(__u, __init, __combine, __brick_reduce, __scan);
    auto __range = tbb::blocked_range&lt;_Index&gt;(0, __n);
    tbb::this_task_arena::isolate([__range, &amp;__body]() { tbb::parallel_scan(__range, __body); });
    return __body.sum();
}

//------------------------------------------------------------------------
// parallel_stable_sort
//------------------------------------------------------------------------

//------------------------------------------------------------------------
// stable_sort utilities
//
// These are used by parallel implementations but do not depend on them.
//------------------------------------------------------------------------
#define _PSTL_MERGE_CUT_OFF 2000

template &lt;typename _Func&gt;
class __func_task;
template &lt;typename _Func&gt;
class __root_task;

#if TBB_INTERFACE_VERSION &lt;= 12000
class __task : public tbb::task
{
  public:
    template &lt;typename _Fn&gt;
    __task*
    make_continuation(_Fn&amp;&amp; __f)
    {
        return new (allocate_continuation()) __func_task&lt;typename std::decay&lt;_Fn&gt;::type&gt;(std::forward&lt;_Fn&gt;(__f));
    }

    template &lt;typename _Fn&gt;
    __task*
    make_child_of(__task* parent, _Fn&amp;&amp; __f)
    {
        return new (parent-&gt;allocate_child()) __func_task&lt;typename std::decay&lt;_Fn&gt;::type&gt;(std::forward&lt;_Fn&gt;(__f));
    }

    template &lt;typename _Fn&gt;
    __task*
    make_additional_child_of(tbb::task* parent, _Fn&amp;&amp; __f)
    {
        return new (tbb::task::allocate_additional_child_of(*parent))
            __func_task&lt;typename std::decay&lt;_Fn&gt;::type&gt;(std::forward&lt;_Fn&gt;(__f));
    }

    inline void
    recycle_as_continuation()
    {
        tbb::task::recycle_as_continuation();
    }

    inline void
    recycle_as_child_of(__task* parent)
    {
        tbb::task::recycle_as_child_of(*parent);
    }

    inline void
    spawn(__task* __t)
    {
        tbb::task::spawn(*__t);
    }

    template &lt;typename _Fn&gt;
    static inline void
    spawn_root_and_wait(__root_task&lt;_Fn&gt;&amp; __root)
    {
        tbb::task::spawn_root_and_wait(*__root._M_task);
    }
};

template &lt;typename _Func&gt;
class __func_task : public __task
{
    _Func _M_func;

    tbb::task*
    execute()
    {
        return _M_func(this);
    };

  public:
    template &lt;typename _Fn&gt;
    __func_task(_Fn&amp;&amp; __f) : _M_func{std::forward&lt;_Fn&gt;(__f)}
    {
    }

    _Func&amp;
    body()
    {
        return _M_func;
    }
};

template &lt;typename _Func&gt;
class __root_task
{
    tbb::task* _M_task;

  public:
    template &lt;typename... Args&gt;
    __root_task(Args&amp;&amp;... args)
        : _M_task{new (tbb::task::allocate_root()) __func_task&lt;_Func&gt;{_Func(std::forward&lt;Args&gt;(args)...)}}
    {
    }

    friend class __task;
    friend class __func_task&lt;_Func&gt;;
};

#else  // TBB_INTERFACE_VERSION &lt;= 12000
class __task : public tbb::detail::d1::task
{
  protected:
    tbb::detail::d1::small_object_allocator _M_allocator{};
    tbb::detail::d1::execution_data* _M_execute_data{};
    __task* _M_parent{};
    std::atomic&lt;int&gt; _M_refcount{};
    bool _M_recycle{};

    template &lt;typename _Fn&gt;
    __task*
    allocate_func_task(_Fn&amp;&amp; __f)
    {
        _PSTL_ASSERT(_M_execute_data != nullptr);
        tbb::detail::d1::small_object_allocator __alloc{};
        auto __t =
            __alloc.new_object&lt;__func_task&lt;typename std::decay&lt;_Fn&gt;::type&gt;&gt;(*_M_execute_data, std::forward&lt;_Fn&gt;(__f));
        __t-&gt;_M_allocator = __alloc;
        return __t;
    }

  public:
    __task*
    parent()
    {
        return _M_parent;
    }

    void
    set_ref_count(int __n)
    {
        _M_refcount.store(__n, std::memory_order_release);
    }

    template &lt;typename _Fn&gt;
    __task*
    make_continuation(_Fn&amp;&amp; __f)
    {
        auto __t = allocate_func_task(std::forward&lt;_Fn&amp;&amp;&gt;(__f));
        __t-&gt;_M_parent = _M_parent;
        _M_parent = nullptr;
        return __t;
    }

    template &lt;typename _Fn&gt;
    __task*
    make_child_of(__task* __parent, _Fn&amp;&amp; __f)
    {
        auto __t = allocate_func_task(std::forward&lt;_Fn&amp;&amp;&gt;(__f));
        __t-&gt;_M_parent = __parent;
        return __t;
    }

    template &lt;typename _Fn&gt;
    __task*
    make_additional_child_of(__task* __parent, _Fn&amp;&amp; __f)
    {
        auto __t = make_child_of(__parent, std::forward&lt;_Fn&gt;(__f));
        _PSTL_ASSERT(__parent-&gt;_M_refcount.load(std::memory_order_relaxed) &gt; 0);
        ++__parent-&gt;_M_refcount;
        return __t;
    }

    inline void
    recycle_as_continuation()
    {
        _M_recycle = true;
    }

    inline void
    recycle_as_child_of(__task* parent)
    {
        _M_recycle = true;
        _M_parent = parent;
    }

    inline void
    spawn(__task* __t)
    {
        _PSTL_ASSERT(_M_execute_data != nullptr);
        tbb::detail::d1::spawn(*__t, *_M_execute_data-&gt;context);
    }

    template &lt;typename _Fn&gt;
    static inline void
    spawn_root_and_wait(__root_task&lt;_Fn&gt;&amp; __root)
    {
        tbb::detail::d1::execute_and_wait(*__root._M_func_task, __root._M_context, __root._M_wait_object,
                                          __root._M_context);
    }

    template &lt;typename _Func&gt;
    friend class __func_task;
};

template &lt;typename _Func&gt;
class __func_task : public __task
{
    _Func _M_func;

    __task*
    execute(tbb::detail::d1::execution_data&amp; __ed) override
    {
        _M_execute_data = &amp;__ed;
        _M_recycle = false;
        __task* __next = _M_func(this);
        return finalize(__next);
    };

    __task*
    cancel(tbb::detail::d1::execution_data&amp; __ed) override
    {
        return finalize(nullptr);
    }

    __task*
    finalize(__task* __next)
    {
        bool __recycle = _M_recycle;
        _M_recycle = false;

        if (__recycle)
        {
            return __next;
        }

        auto __parent = _M_parent;
        auto __alloc = _M_allocator;
        auto __ed = _M_execute_data;

        this-&gt;~__func_task();

        _PSTL_ASSERT(__parent != nullptr);
        _PSTL_ASSERT(__parent-&gt;_M_refcount.load(std::memory_order_relaxed) &gt; 0);
        if (--__parent-&gt;_M_refcount == 0)
        {
            _PSTL_ASSERT(__next == nullptr);
            __alloc.deallocate(this, *__ed);
            return __parent;
        }

        return __next;
    }

    friend class __root_task&lt;_Func&gt;;

  public:
    template &lt;typename _Fn&gt;
    __func_task(_Fn&amp;&amp; __f) : _M_func(std::forward&lt;_Fn&gt;(__f))
    {
    }

    _Func&amp;
    body()
    {
        return _M_func;
    }
};

template &lt;typename _Func&gt;
class __root_task : public __task
{
    __task*
    execute(tbb::detail::d1::execution_data&amp; __ed) override
    {
        _M_wait_object.release();
        return nullptr;
    };

    __task*
    cancel(tbb::detail::d1::execution_data&amp; __ed) override
    {
        _M_wait_object.release();
        return nullptr;
    }

    __func_task&lt;_Func&gt;* _M_func_task{};
    tbb::detail::d1::wait_context _M_wait_object{0};
    tbb::task_group_context _M_context{};

  public:
    template &lt;typename... Args&gt;
    __root_task(Args&amp;&amp;... args) : _M_wait_object{1}
    {
        tbb::detail::d1::small_object_allocator __alloc{};
        _M_func_task = __alloc.new_object&lt;__func_task&lt;_Func&gt;&gt;(_Func(std::forward&lt;Args&gt;(args)...));
        _M_func_task-&gt;_M_allocator = __alloc;
        _M_func_task-&gt;_M_parent = this;
        _M_refcount.store(1, std::memory_order_relaxed);
    }

    friend class __task;
};
#endif // TBB_INTERFACE_VERSION &lt;= 12000

template &lt;typename _RandomAccessIterator1, typename _RandomAccessIterator2, typename _Compare, typename _Cleanup,
          typename _LeafMerge&gt;
class __merge_func
{
    typedef typename std::iterator_traits&lt;_RandomAccessIterator1&gt;::difference_type _DifferenceType1;
    typedef typename std::iterator_traits&lt;_RandomAccessIterator2&gt;::difference_type _DifferenceType2;
    typedef typename std::common_type&lt;_DifferenceType1, _DifferenceType2&gt;::type _SizeType;
    typedef typename std::iterator_traits&lt;_RandomAccessIterator1&gt;::value_type _ValueType;

    _RandomAccessIterator1 _M_x_beg;
    _RandomAccessIterator2 _M_z_beg;

    _SizeType _M_xs, _M_xe;
    _SizeType _M_ys, _M_ye;
    _SizeType _M_zs;
    _Compare _M_comp;
    _LeafMerge _M_leaf_merge;
    _SizeType _M_nsort; //number of elements to be sorted for partial_sort alforithm

    static const _SizeType __merge_cut_off = _PSTL_MERGE_CUT_OFF;

    bool _root;   //means a task is merging root task
    bool _x_orig; //&quot;true&quot; means X(or left ) subrange is in the original container; false - in the buffer
    bool _y_orig; //&quot;true&quot; means Y(or right) subrange is in the original container; false - in the buffer
    bool _split; //&quot;true&quot; means a merge task is a split task for parallel merging, the execution logic differs

    bool
    is_partial() const
    {
        return _M_nsort &gt; 0;
    }

    struct __move_value
    {
        template &lt;typename Iterator1, typename Iterator2&gt;
        void
        operator()(Iterator1 __x, Iterator2 __z)
        {
            *__z = std::move(*__x);
        }
    };

    struct __move_value_construct
    {
        template &lt;typename Iterator1, typename Iterator2&gt;
        void
        operator()(Iterator1 __x, Iterator2 __z)
        {
            ::new (std::addressof(*__z)) _ValueType(std::move(*__x));
        }
    };

    struct __move_range
    {
        template &lt;typename Iterator1, typename Iterator2&gt;
        Iterator2
        operator()(Iterator1 __first1, Iterator1 __last1, Iterator2 __first2)
        {
            if (__last1 - __first1 &lt; __merge_cut_off)
                return std::move(__first1, __last1, __first2);

            auto __n = __last1 - __first1;
            tbb::parallel_for(tbb::blocked_range&lt;_SizeType&gt;(0, __n, __merge_cut_off),
                              [__first1, __first2](const tbb::blocked_range&lt;_SizeType&gt;&amp; __range) {
                                  std::move(__first1 + __range.begin(), __first1 + __range.end(),
                                            __first2 + __range.begin());
                              });
            return __first2 + __n;
        }
    };

    struct __move_range_construct
    {
        template &lt;typename Iterator1, typename Iterator2&gt;
        Iterator2
        operator()(Iterator1 __first1, Iterator1 __last1, Iterator2 __first2)
        {
            if (__last1 - __first1 &lt; __merge_cut_off)
            {
                for (; __first1 != __last1; ++__first1, ++__first2)
                    __move_value_construct()(__first1, __first2);
                return __first2;
            }

            auto __n = __last1 - __first1;
            tbb::parallel_for(tbb::blocked_range&lt;_SizeType&gt;(0, __n, __merge_cut_off),
                              [__first1, __first2](const tbb::blocked_range&lt;_SizeType&gt;&amp; __range) {
                                  for (auto i = __range.begin(); i != __range.end(); ++i)
                                      __move_value_construct()(__first1 + i, __first2 + i);
                              });
            return __first2 + __n;
        }
    };

    struct __cleanup_range
    {
        template &lt;typename Iterator&gt;
        void
        operator()(Iterator __first, Iterator __last)
        {
            if (__last - __first &lt; __merge_cut_off)
                _Cleanup()(__first, __last);
            else
            {
                auto __n = __last - __first;
                tbb::parallel_for(tbb::blocked_range&lt;_SizeType&gt;(0, __n, __merge_cut_off),
                                  [__first](const tbb::blocked_range&lt;_SizeType&gt;&amp; __range) {
                                      _Cleanup()(__first + __range.begin(), __first + __range.end());
                                  });
            }
        }
    };

  public:
    __merge_func(_SizeType __xs, _SizeType __xe, _SizeType __ys, _SizeType __ye, _SizeType __zs, _Compare __comp,
                 _Cleanup, _LeafMerge __leaf_merge, _SizeType __nsort, _RandomAccessIterator1 __x_beg,
                 _RandomAccessIterator2 __z_beg, bool __x_orig, bool __y_orig, bool __root)
        : _M_xs(__xs), _M_xe(__xe), _M_ys(__ys), _M_ye(__ye), _M_zs(__zs), _M_x_beg(__x_beg), _M_z_beg(__z_beg),
          _M_comp(__comp), _M_leaf_merge(__leaf_merge), _M_nsort(__nsort), _root(__root),
          _x_orig(__x_orig), _y_orig(__y_orig), _split(false)
    {
    }

    bool
    is_left(_SizeType __idx) const
    {
        return _M_xs == __idx;
    }

    template &lt;typename IndexType&gt;
    void
    set_odd(IndexType __idx, bool __on_off)
    {
        if (is_left(__idx))
            _x_orig = __on_off;
        else
            _y_orig = __on_off;
    }

    __task*
    operator()(__task* __self);

  private:
    __merge_func*
    parent_merge(__task* __self) const
    {
        return _root ? nullptr : &amp;static_cast&lt;__func_task&lt;__merge_func&gt;*&gt;(__self-&gt;parent())-&gt;body();
    }
    bool
    x_less_y()
    {
        const auto __nx = (_M_xe - _M_xs);
        const auto __ny = (_M_ye - _M_ys);
        _PSTL_ASSERT(__nx &gt; 0 &amp;&amp; __ny &gt; 0);

        _PSTL_ASSERT(_x_orig == _y_orig);
        _PSTL_ASSERT(!is_partial());

        if (_x_orig)
        {
            _PSTL_ASSERT(std::is_sorted(_M_x_beg + _M_xs, _M_x_beg + _M_xe, _M_comp));
            _PSTL_ASSERT(std::is_sorted(_M_x_beg + _M_ys, _M_x_beg + _M_ye, _M_comp));
            return !_M_comp(*(_M_x_beg + _M_ys), *(_M_x_beg + _M_xe - 1));
        }

        _PSTL_ASSERT(std::is_sorted(_M_z_beg + _M_xs, _M_z_beg + _M_xe, _M_comp));
        _PSTL_ASSERT(std::is_sorted(_M_z_beg + _M_ys, _M_z_beg + _M_ye, _M_comp));
        return !_M_comp(*(_M_z_beg + _M_zs + __nx), *(_M_z_beg + _M_zs + __nx - 1));
    }
    void
    move_x_range()
    {
        const auto __nx = (_M_xe - _M_xs);
        const auto __ny = (_M_ye - _M_ys);
        _PSTL_ASSERT(__nx &gt; 0 &amp;&amp; __ny &gt; 0);

        if (_x_orig)
            __move_range_construct()(_M_x_beg + _M_xs, _M_x_beg + _M_xe, _M_z_beg + _M_zs);
        else
        {
            __move_range()(_M_z_beg + _M_zs, _M_z_beg + _M_zs + __nx, _M_x_beg + _M_xs);
            __cleanup_range()(_M_z_beg + _M_zs, _M_z_beg + _M_zs + __nx);
        }

        _x_orig = !_x_orig;
    }
    void
    move_y_range()
    {
        const auto __nx = (_M_xe - _M_xs);
        const auto __ny = (_M_ye - _M_ys);

        if (_y_orig)
            __move_range_construct()(_M_x_beg + _M_ys, _M_x_beg + _M_ye, _M_z_beg + _M_zs + __nx);
        else
        {
            __move_range()(_M_z_beg + _M_zs + __nx, _M_z_beg + _M_zs + __nx + __ny, _M_x_beg + _M_ys);
            __cleanup_range()(_M_z_beg + _M_zs + __nx, _M_z_beg + _M_zs + __nx + __ny);
        }

        _y_orig = !_y_orig;
    }
    __task*
    merge_ranges(__task* __self)
    {
        _PSTL_ASSERT(_x_orig == _y_orig); //two merged subrange must be lie into the same buffer

        const auto __nx = (_M_xe - _M_xs);
        const auto __ny = (_M_ye - _M_ys);
        const auto __n = __nx + __ny;

        // need to merge {x} and {y}
        if (__n &gt; __merge_cut_off)
            return split_merging(__self);

        //merge to buffer
        if (_x_orig)
        {
            _M_leaf_merge(_M_x_beg + _M_xs, _M_x_beg + _M_xe, _M_x_beg + _M_ys, _M_x_beg + _M_ye, _M_z_beg + _M_zs,
                          _M_comp, __move_value_construct(), __move_value_construct(), __move_range_construct(),
                          __move_range_construct());
            _PSTL_ASSERT(parent_merge(__self)); //not root merging task
        }
        //merge to &quot;origin&quot;
        else
        {
            _PSTL_ASSERT(_x_orig == _y_orig);

            _PSTL_ASSERT(is_partial() || std::is_sorted(_M_z_beg + _M_xs, _M_z_beg + _M_xe, _M_comp));
            _PSTL_ASSERT(is_partial() || std::is_sorted(_M_z_beg + _M_ys, _M_z_beg + _M_ye, _M_comp));

            const auto __nx = (_M_xe - _M_xs);
            const auto __ny = (_M_ye - _M_ys);

            _M_leaf_merge(_M_z_beg + _M_xs, _M_z_beg + _M_xe, _M_z_beg + _M_ys, _M_z_beg + _M_ye, _M_x_beg + _M_zs,
                          _M_comp, __move_value(), __move_value(), __move_range(), __move_range());

            __cleanup_range()(_M_z_beg + _M_xs, _M_z_beg + _M_xe);
            __cleanup_range()(_M_z_beg + _M_ys, _M_z_beg + _M_ye);
        }
        return nullptr;
    }

    __task*
    process_ranges(__task* __self)
    {
        _PSTL_ASSERT(_x_orig == _y_orig);
        _PSTL_ASSERT(!_split);

        auto p = parent_merge(__self);

        if (!p)
        { //root merging task

            //optimization, just for sort algorithm, //{x} &lt;= {y}
            if (!is_partial() &amp;&amp; x_less_y()) //we have a solution
            {
                if (!_x_orig)
                {                   //we have to move the solution to the origin
                    move_x_range(); //parallel moving
                    move_y_range(); //parallel moving
                }
                return nullptr;
            }
            //else: if we have data in the origin,
            //we have to move data to the buffer for final merging into the origin.
            if (_x_orig)
            {
                move_x_range(); //parallel moving
                move_y_range(); //parallel moving
            }
            // need to merge {x} and {y}.
            return merge_ranges(__self);
        }
        //else: not root merging task (parent_merge() == NULL)
        //optimization, just for sort algorithm, //{x} &lt;= {y}
        if (!is_partial() &amp;&amp; x_less_y())
        {
            const auto id_range = _M_zs;
            p-&gt;set_odd(id_range, _x_orig);
            return nullptr;
        }
        //else: we have to revert &quot;_x(y)_orig&quot; flag of the parent merging task
        const auto id_range = _M_zs;
        p-&gt;set_odd(id_range, !_x_orig);

        return merge_ranges(__self);
    }

    //splitting as merge task into 2 of the same level
    __task*
    split_merging(__task* __self)
    {
        _PSTL_ASSERT(_x_orig == _y_orig);
        const auto __nx = (_M_xe - _M_xs);
        const auto __ny = (_M_ye - _M_ys);

        _SizeType __xm{};
        _SizeType __ym{};
        if (__nx &lt; __ny)
        {
            __ym = _M_ys + __ny / 2;

            if (_x_orig)
                __xm = std::upper_bound(_M_x_beg + _M_xs, _M_x_beg + _M_xe, *(_M_x_beg + __ym), _M_comp) - _M_x_beg;
            else
                __xm = std::upper_bound(_M_z_beg + _M_xs, _M_z_beg + _M_xe, *(_M_z_beg + __ym), _M_comp) - _M_z_beg;
        }
        else
        {
            __xm = _M_xs + __nx / 2;

            if (_y_orig)
                __ym = std::lower_bound(_M_x_beg + _M_ys, _M_x_beg + _M_ye, *(_M_x_beg + __xm), _M_comp) - _M_x_beg;
            else
                __ym = std::lower_bound(_M_z_beg + _M_ys, _M_z_beg + _M_ye, *(_M_z_beg + __xm), _M_comp) - _M_z_beg;
        }

        auto __zm = _M_zs + ((__xm - _M_xs) + (__ym - _M_ys));
        __merge_func __right_func(__xm, _M_xe, __ym, _M_ye, __zm, _M_comp, _Cleanup(), _M_leaf_merge, _M_nsort,
                                  _M_x_beg, _M_z_beg, _x_orig, _y_orig, _root);
        __right_func._split = true;
        auto __merge_task = __self-&gt;make_additional_child_of(__self-&gt;parent(), std::move(__right_func));
        __self-&gt;spawn(__merge_task);
        __self-&gt;recycle_as_continuation();

        _M_xe = __xm;
        _M_ye = __ym;
        _split = true;

        return __self;
    }
};

template &lt;typename _RandomAccessIterator1, typename _RandomAccessIterator2, typename __M_Compare, typename _Cleanup,
          typename _LeafMerge&gt;
__task*
__merge_func&lt;_RandomAccessIterator1, _RandomAccessIterator2, __M_Compare, _Cleanup, _LeafMerge&gt;::
operator()(__task* __self)
{
    //a. split merge task into 2 of the same level; the special logic,
    //without processing(process_ranges) adjacent sub-ranges x and y
    if (_split)
        return merge_ranges(__self);

    //b. General merging of adjacent sub-ranges x and y (with optimization in case of {x} &lt;= {y} )

    //1. x and y are in the even buffer
    //2. x and y are in the odd buffer
    if (_x_orig == _y_orig)
        return process_ranges(__self);

    //3. x is in even buffer, y is in the odd buffer
    //4. x is in odd buffer, y is in the even buffer
    if (!parent_merge(__self))
    { //root merge task
        if (_x_orig)
            move_x_range();
        else
            move_y_range();
    }
    else
    {
        const _SizeType __nx = (_M_xe - _M_xs);
        const _SizeType __ny = (_M_ye - _M_ys);
        _PSTL_ASSERT(__nx &gt; 0);
        _PSTL_ASSERT(__nx &gt; 0);

        if (__nx &lt; __ny)
            move_x_range();
        else
            move_y_range();
    }

    return process_ranges(__self);
}

template &lt;typename _RandomAccessIterator1, typename _RandomAccessIterator2, typename _Compare, typename _LeafSort&gt;
class __stable_sort_func
{
  public:
    typedef typename std::iterator_traits&lt;_RandomAccessIterator1&gt;::difference_type _DifferenceType1;
    typedef typename std::iterator_traits&lt;_RandomAccessIterator2&gt;::difference_type _DifferenceType2;
    typedef typename std::common_type&lt;_DifferenceType1, _DifferenceType2&gt;::type _SizeType;

  private:
    _RandomAccessIterator1 _M_xs, _M_xe, _M_x_beg;
    _RandomAccessIterator2 _M_zs, _M_z_beg;
    _Compare _M_comp;
    _LeafSort _M_leaf_sort;
    bool _M_root;
    _SizeType _M_nsort; //zero or number of elements to be sorted for partial_sort alforithm

  public:
    __stable_sort_func(_RandomAccessIterator1 __xs, _RandomAccessIterator1 __xe, _RandomAccessIterator2 __zs,
                       bool __root, _Compare __comp, _LeafSort __leaf_sort, _SizeType __nsort,
                       _RandomAccessIterator1 __x_beg, _RandomAccessIterator2 __z_beg)
        : _M_xs(__xs), _M_xe(__xe), _M_x_beg(__x_beg), _M_zs(__zs), _M_z_beg(__z_beg), _M_comp(__comp),
          _M_leaf_sort(__leaf_sort), _M_root(__root), _M_nsort(__nsort)
    {
    }

    __task*
    operator()(__task* __self);
};

#define _PSTL_STABLE_SORT_CUT_OFF 500

template &lt;typename _RandomAccessIterator1, typename _RandomAccessIterator2, typename _Compare, typename _LeafSort&gt;
__task*
__stable_sort_func&lt;_RandomAccessIterator1, _RandomAccessIterator2, _Compare, _LeafSort&gt;::operator()(__task* __self)
{
    typedef __merge_func&lt;_RandomAccessIterator1, _RandomAccessIterator2, _Compare, __utils::__serial_destroy,
                         __utils::__serial_move_merge&gt;
        _MergeTaskType;

    const _SizeType __n = _M_xe - _M_xs;
    const _SizeType __nmerge = _M_nsort &gt; 0 ? _M_nsort : __n;
    const _SizeType __sort_cut_off = _PSTL_STABLE_SORT_CUT_OFF;
    if (__n &lt;= __sort_cut_off)
    {
        _M_leaf_sort(_M_xs, _M_xe, _M_comp);
        _PSTL_ASSERT(!_M_root);
        return nullptr;
    }

    const _RandomAccessIterator1 __xm = _M_xs + __n / 2;
    const _RandomAccessIterator2 __zm = _M_zs + (__xm - _M_xs);
    const _RandomAccessIterator2 __ze = _M_zs + __n;
    _MergeTaskType __m(_MergeTaskType(_M_xs - _M_x_beg, __xm - _M_x_beg, __xm - _M_x_beg, _M_xe - _M_x_beg,
                                      _M_zs - _M_z_beg, _M_comp, __utils::__serial_destroy(),
                                      __utils::__serial_move_merge(__nmerge), _M_nsort, _M_x_beg, _M_z_beg,
                                      /*x_orig*/ true, /*y_orig*/ true, /*root*/ _M_root));
    auto __parent = __self-&gt;make_continuation(std::move(__m));
    __parent-&gt;set_ref_count(2);
    auto __right = __self-&gt;make_child_of(
        __parent, __stable_sort_func(__xm, _M_xe, __zm, false, _M_comp, _M_leaf_sort, _M_nsort, _M_x_beg, _M_z_beg));
    __self-&gt;spawn(__right);
    __self-&gt;recycle_as_child_of(__parent);
    _M_root = false;
    _M_xe = __xm;

    return __self;
}

template &lt;class _ExecutionPolicy, typename _RandomAccessIterator, typename _Compare, typename _LeafSort&gt;
void
__parallel_stable_sort(_ExecutionPolicy&amp;&amp;, _RandomAccessIterator __xs, _RandomAccessIterator __xe, _Compare __comp,
                       _LeafSort __leaf_sort, std::size_t __nsort = 0)
{
    tbb::this_task_arena::isolate([=, &amp;__nsort]() {
        //sorting based on task tree and parallel merge
        typedef typename std::iterator_traits&lt;_RandomAccessIterator&gt;::value_type _ValueType;
        typedef typename std::iterator_traits&lt;_RandomAccessIterator&gt;::difference_type _DifferenceType;
        const _DifferenceType __n = __xe - __xs;
        if (__nsort == __n)
            __nsort = 0; // 'partial_sort' becames 'sort'

        const _DifferenceType __sort_cut_off = _PSTL_STABLE_SORT_CUT_OFF;
        if (__n &gt; __sort_cut_off)
        {
            __buffer&lt;_ValueType&gt; __buf(__n);
            __root_task&lt;__stable_sort_func&lt;_RandomAccessIterator, _ValueType*, _Compare, _LeafSort&gt;&gt; __root{
                __xs, __xe, __buf.get(), true, __comp, __leaf_sort, __nsort, __xs, __buf.get()};
            __task::spawn_root_and_wait(__root);
            return;
        }
        //serial sort
        __leaf_sort(__xs, __xe, __comp);
    });
}

//------------------------------------------------------------------------
// parallel_merge
//------------------------------------------------------------------------
template &lt;typename _RandomAccessIterator1, typename _RandomAccessIterator2, typename _RandomAccessIterator3,
          typename _Compare, typename _LeafMerge&gt;
class __merge_func_static
{
    _RandomAccessIterator1 _M_xs, _M_xe;
    _RandomAccessIterator2 _M_ys, _M_ye;
    _RandomAccessIterator3 _M_zs;
    _Compare _M_comp;
    _LeafMerge _M_leaf_merge;

  public:
    __merge_func_static(_RandomAccessIterator1 __xs, _RandomAccessIterator1 __xe, _RandomAccessIterator2 __ys,
                        _RandomAccessIterator2 __ye, _RandomAccessIterator3 __zs, _Compare __comp,
                        _LeafMerge __leaf_merge)
        : _M_xs(__xs), _M_xe(__xe), _M_ys(__ys), _M_ye(__ye), _M_zs(__zs), _M_comp(__comp), _M_leaf_merge(__leaf_merge)
    {
    }

    __task*
    operator()(__task* __self);
};

//TODO: consider usage of parallel_for with a custom blocked_range
template &lt;typename _RandomAccessIterator1, typename _RandomAccessIterator2, typename _RandomAccessIterator3,
          typename __M_Compare, typename _LeafMerge&gt;
__task*
__merge_func_static&lt;_RandomAccessIterator1, _RandomAccessIterator2, _RandomAccessIterator3, __M_Compare, _LeafMerge&gt;::
operator()(__task* __self)
{
    typedef typename std::iterator_traits&lt;_RandomAccessIterator1&gt;::difference_type _DifferenceType1;
    typedef typename std::iterator_traits&lt;_RandomAccessIterator2&gt;::difference_type _DifferenceType2;
    typedef typename std::common_type&lt;_DifferenceType1, _DifferenceType2&gt;::type _SizeType;
    const _SizeType __n = (_M_xe - _M_xs) + (_M_ye - _M_ys);
    const _SizeType __merge_cut_off = _PSTL_MERGE_CUT_OFF;
    if (__n &lt;= __merge_cut_off)
    {
        _M_leaf_merge(_M_xs, _M_xe, _M_ys, _M_ye, _M_zs, _M_comp);
        return nullptr;
    }

    _RandomAccessIterator1 __xm;
    _RandomAccessIterator2 __ym;
    if (_M_xe - _M_xs &lt; _M_ye - _M_ys)
    {
        __ym = _M_ys + (_M_ye - _M_ys) / 2;
        __xm = std::upper_bound(_M_xs, _M_xe, *__ym, _M_comp);
    }
    else
    {
        __xm = _M_xs + (_M_xe - _M_xs) / 2;
        __ym = std::lower_bound(_M_ys, _M_ye, *__xm, _M_comp);
    }
    const _RandomAccessIterator3 __zm = _M_zs + ((__xm - _M_xs) + (__ym - _M_ys));
    auto __right = __self-&gt;make_additional_child_of(
        __self-&gt;parent(), __merge_func_static(__xm, _M_xe, __ym, _M_ye, __zm, _M_comp, _M_leaf_merge));
    __self-&gt;spawn(__right);
    __self-&gt;recycle_as_continuation();
    _M_xe = __xm;
    _M_ye = __ym;

    return __self;
}

template &lt;class _ExecutionPolicy, typename _RandomAccessIterator1, typename _RandomAccessIterator2,
          typename _RandomAccessIterator3, typename _Compare, typename _LeafMerge&gt;
void
__parallel_merge(_ExecutionPolicy&amp;&amp;, _RandomAccessIterator1 __xs, _RandomAccessIterator1 __xe,
                 _RandomAccessIterator2 __ys, _RandomAccessIterator2 __ye, _RandomAccessIterator3 __zs, _Compare __comp,
                 _LeafMerge __leaf_merge)
{
    typedef typename std::iterator_traits&lt;_RandomAccessIterator1&gt;::difference_type _DifferenceType1;
    typedef typename std::iterator_traits&lt;_RandomAccessIterator2&gt;::difference_type _DifferenceType2;
    typedef typename std::common_type&lt;_DifferenceType1, _DifferenceType2&gt;::type _SizeType;
    const _SizeType __n = (__xe - __xs) + (__ye - __ys);
    const _SizeType __merge_cut_off = _PSTL_MERGE_CUT_OFF;
    if (__n &lt;= __merge_cut_off)
    {
        // Fall back on serial merge
        __leaf_merge(__xs, __xe, __ys, __ye, __zs, __comp);
    }
    else
    {
        tbb::this_task_arena::isolate([=]() {
            typedef __merge_func_static&lt;_RandomAccessIterator1, _RandomAccessIterator2, _RandomAccessIterator3,
                                        _Compare, _LeafMerge&gt;
                _TaskType;
            __root_task&lt;_TaskType&gt; __root{__xs, __xe, __ys, __ye, __zs, __comp, __leaf_merge};
            __task::spawn_root_and_wait(__root);
        });
    }
}

//------------------------------------------------------------------------
// parallel_invoke
//------------------------------------------------------------------------
template &lt;class _ExecutionPolicy, typename _F1, typename _F2&gt;
void
__parallel_invoke(_ExecutionPolicy&amp;&amp;, _F1&amp;&amp; __f1, _F2&amp;&amp; __f2)
{
    //TODO: a version of tbb::this_task_arena::isolate with variadic arguments pack should be added in the future
    tbb::this_task_arena::isolate([&amp;]() { tbb::parallel_invoke(std::forward&lt;_F1&gt;(__f1), std::forward&lt;_F2&gt;(__f2)); });
}

} // namespace __tbb_backend
} // namespace __pstl

#endif /* _PSTL_PARALLEL_BACKEND_TBB_H */
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