reduce.h

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//
// Copyright 2018 Pixar
//
// Licensed under the Apache License, Version 2.0 (the "Apache License")
// with the following modification; you may not use this file except in
// compliance with the Apache License and the following modification to it:
// Section 6. Trademarks. is deleted and replaced with:
//
// 6. Trademarks. This License does not grant permission to use the trade
//    names, trademarks, service marks, or product names of the Licensor
//    and its affiliates, except as required to comply with Section 4(c) of
//    the License and to reproduce the content of the NOTICE file.
//
// You may obtain a copy of the Apache License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the Apache License with the above modification is
// distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied. See the Apache License for the specific
// language governing permissions and limitations under the Apache License.
//
#ifndef PXR_BASE_WORK_REDUCE_H
#define PXR_BASE_WORK_REDUCE_H
 
#include "pxr/pxr.h"
#include "pxr/base/work/threadLimits.h"
#include "pxr/base/work/api.h"
 
#include <tbb/blocked_range.h>
#include <tbb/parallel_reduce.h>
#include <tbb/task_group.h>
 
PXR_NAMESPACE_OPEN_SCOPE
 
 
template <typename Fn, typename Rn, typename V>
V
WorkParallelReduceN(
    const V &identity,
    size_t n,
    Fn &&loopCallback,
    Rn &&reductionCallback,
    size_t grainSize)
{
    if (n == 0)
        return identity;
 
    // Don't bother with parallel_reduce, if concurrency is limited to 1.
    if (WorkHasConcurrency()) {
 
        class Work_Body_TBB
        {
        public:
            Work_Body_TBB(Fn &fn) : _fn(fn) { }
 
            V operator()(
                const tbb::blocked_range<size_t> &r,
                const V &value) const {
                // Note that we std::forward _fn using Fn in order get the
                // right operator().
                // We maintain the right type in this way:
                //  If Fn is T&, then reference collapsing gives us T& for _fn
                //  If Fn is T, then std::forward correctly gives us T&& for _fn
                return std::forward<Fn>(_fn)(r.begin(), r.end(), value);
            }
        private:
            Fn &_fn;
        };
 
        // In most cases we do not want to inherit cancellation state from the
        // parent context, so we create an isolated task group context.
        tbb::task_group_context ctx(tbb::task_group_context::isolated);
        return tbb::parallel_reduce(tbb::blocked_range<size_t>(0,n,grainSize),
            identity,
            Work_Body_TBB(loopCallback),
            std::forward<Rn>(reductionCallback),
            tbb::auto_partitioner(),
            ctx);
    }
        
    // If concurrency is limited to 1, execute serially.
    return std::forward<Fn>(loopCallback)(0, n, identity);
}
 
template <typename Fn, typename Rn, typename V>
V
WorkParallelReduceN(
    const V &identity,
    size_t n,
    Fn &&loopCallback,
    Rn &&reductionCallback)
{
    return WorkParallelReduceN(identity, n, loopCallback, reductionCallback, 1);
}
 
PXR_NAMESPACE_CLOSE_SCOPE
 
#endif // PXR_BASE_WORK_REDUCE_H