Intel® oneAPI Threading Building Blocks Developer Guide and API Reference
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Parallel Reduction for rvalues
Description
oneAPI Threading Building Blocks (oneTBB) implementation extends the ParallelReduceFunc and ParallelReduceReduction to optimize operating with rvalues using functional form of tbb::parallel_reduce and tbb::parallel_deterministic_reduce algorithms.
API
Header
#include <oneapi/tbb/parallel_reduce.h>
ParallelReduceFunc Requirements: Pseudo-Signature, Semantics
ValueFunc::operator()(constRange&range, Value&&x)const
or
ValueFunc::operator()(constRange&range, constValue&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.
If both rvalue and lvalue forms are provided, the rvalue is preferred.
ParallelReduceReduction Requirements: Pseudo-Signature, Semantics
ValueReduction::operator()(Value&&x, Value&&y)const
or
ValueReduction::operator()(constValue&x, constValue&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.
If both rvalue and lvalue forms are provided, the rvalue is preferred.
Example
// 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;
}
);
}
See also