[BUG]: Suboptimal swap performance on universal vectors

[gevtushenko]

Is this a duplicate?

• I confirmed there appear to be no duplicate issues for this bug and that I agree to the Code of Conduct

Type of Bug

Performance

Component

Thrust

Describe the bug

Swap of universal vectors is unnecessary slow and occupies more memory than it should.
Device and host vectors have swap overloads in thrust:: namespace.
These overloads call efficient .swap member function:

cccl/thrust/thrust/device_vector.h

Lines 539 to 543 in 80031e2

	template <typename T, typename Alloc>
	void swap(device_vector<T, Alloc>& a, device_vector<T, Alloc>& b)
	{
	a.swap(b);
	}

Universal and universal host pinned vectors are missing such an overload and the vector_base they alias to has swap overload in thrust::details, so it’s unreachable.

How to Reproduce

template <template <typename> class Vector>
void swap_time(const char* str) {
    Vector<char> a(1 << 30, 'a');
    Vector<char> b(1 << 30, 'b');

    auto begin = std::chrono::high_resolution_clock::now();
    thrust::swap(a, b);
    // a.swap(b);
    auto end = std::chrono::high_resolution_clock::now();
    std::cout << str << " swap time: " << std::chrono::duration<double>(end - begin).count() << " s" << std::endl;
    std::cout << "a: " << static_cast<char>(a[0]) << "; "
              << "b: " << static_cast<char>(b[0]) << std::endl;
}

int main() {
    swap_time<thrust::device_vector>("device");
    swap_time<thrust::host_vector>("host");
    swap_time<thrust::universal_vector>("universal");
    swap_time<thrust::universal_host_pinned_vector>("universal_host_pinned");
}

Expected behavior

• Swap of universal host pinned vector is 30 million times faster
• Swap of universal vector is 4 million times faster

Reproduction link

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Operating System

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nvidia-smi output

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NVCC version

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