CUDA Thrust sort_by_key when the key is a tuple dealt with by zip_iterator's with custom comparison predicate - iterator

Question

I've looked through a lot of questions here for something similar and there are quite a few, albeit with one minor change. I'm trying to sort values with a zip_iterator as a compound key.
Specifically, I have the following function:
void thrustSort(
unsigned int * primaryKey,
float * secondaryKey,
unsigned int * values,
unsigned int numberOfPoints)
{
thrust::device_ptr dev_ptr_pkey = thrust::device_pointer_cast(primaryKey);
thrust::device_ptr dev_ptr_skey = thrust::device_pointer_cast(secondaryKey);
thrust::device_ptr dev_ptr_values = thrust::device_pointer_cast(values);
thrust::tuple,thrust::device_ptr> keytup_begin =
thrust::make_tuple,thrust::device_ptr>(dev_ptr_pkey, dev_ptr_skey);
thrust::zip_iterator, thrust::device_ptr > > first =
thrust::make_zip_iterator, thrust::device_ptr > >(keytup_begin);
thrust::sort_by_key(first, first + numberOfPoints, dev_ptr_values, ZipComparator());
}
and this custom predicate:
typedef thrust::device_ptr<unsigned int> tdp_uint ;
typedef thrust::device_ptr<float> tdp_float ;
typedef thrust::tuple<tdp_uint, tdp_float> tdp_uif_tuple ;
struct ZipComparator
{
__host__ __device__
inline bool operator() (const tdp_uif_tuple &a, const tdp_uif_tuple &b)
{
if(a.head < b.head) return true;
if(a.head == b.head) return a.tail < b.tail;
return false;
}
};
The errors I'm getting are:
Error 1 error : no instance of constructor "thrust::device_ptr::device_ptr [with T=unsigned int]" matches the argument list C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v4.0\include\thrust\detail\tuple.inl 309 1 ---
Error 2 error : no instance of constructor "thrust::device_ptr::device_ptr [with T=float]" matches the argument list C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v4.0\include\thrust\detail\tuple.inl 401 1 ---
Any ideas what might cause this / how do I write a predicate that indeed works?
Thanks in Advance,
Nathan

Answer 1

The comparator takes arguments of type const thrust::tuple<unsigned int, float>&. The const tdp_uif_tuple& type you defined expands to const thrust::tuple<thrust::device_ptr<unsigned int>, thrust:device_ptr<float> >&
The code below compiles for me:
struct ZipComparator
{
__host__ __device__
inline bool operator() (const thrust::tuple<unsigned int, float> &a, const thrust::tuple<unsigned int, float> &b)
{
if(a.head < b.head) return true;
if(a.head == b.head) return a.tail < b.tail;
return false;
}
};
Hope it does for you as well :)
http://code.google.com/p/thrust/wiki/QuickStartGuide#zip_iterator has more details on the zip iterator.
Not required, but if you're looking to clean up the length of those templates, you can do this:
void thrustSort(
unsigned int * primaryKey,
float * secondaryKey,
unsigned int * values,
unsigned int numberOfPoints)
{
tdp_uint dev_ptr_pkey(primaryKey);
tdp_float dev_ptr_skey(secondaryKey);
tdp_uint dev_ptr_values(values);
thrust::tuple<tdp_uint, tdp_float> keytup_begin = thrust::make_tuple(dev_ptr_pkey, dev_ptr_skey);
thrust::zip_iterator<thrust::tuple<tdp_uint, tdp_float> > first =
thrust::make_zip_iterator(keytup_begin);
thrust::sort_by_key(first, first + numberOfPoints, dev_ptr_values, ZipComparator());
}
A lot of the template arguments can be inferred from the arguments.

Answer 2

This is a fully worked example on how using sort_by_key when the key is a tuple dealt with by zip_iterator's and a customized comparison operator.
#include <thrust/device_vector.h>
#include <thrust/sort.h>
#include "Utilities.cuh"
// --- Defining tuple type
typedef thrust::tuple<int, int> Tuple;
/**************************/
/* TUPLE ORDERING FUNCTOR */
/**************************/
struct TupleComp
{
__host__ __device__ bool operator()(const Tuple& t1, const Tuple& t2)
{
if (t1.get<0>() < t2.get<0>())
return true;
if (t1.get<0>() > t2.get<0>())
return false;
return t1.get<1>() < t2.get<1>();
}
};
/********/
/* MAIN */
/********/
int main()
{
const int N = 8;
// --- Keys and values on the host: allocation and definition
int h_keys1[N] = { 1, 3, 3, 3, 2, 3, 2, 1 };
int h_keys2[N] = { 1, 5, 3, 8, 2, 8, 1, 1 };
float h_values[N] = { 0.3, 5.1, 3.2, -0.08, 2.1, 5.2, 1.1, 0.01};
printf("\n\n");
printf("Original\n");
for (int i = 0; i < N; i++) {
printf("%i %i %f\n", h_keys1[i], h_keys2[i], h_values[i]);
}
// --- Keys and values on the device: allocation
int *d_keys1; gpuErrchk(cudaMalloc(&d_keys1, N * sizeof(int)));
int *d_keys2; gpuErrchk(cudaMalloc(&d_keys2, N * sizeof(int)));
float *d_values; gpuErrchk(cudaMalloc(&d_values, N * sizeof(float)));
// --- Keys and values: host -> device
gpuErrchk(cudaMemcpy(d_keys1, h_keys1, N * sizeof(int), cudaMemcpyHostToDevice));
gpuErrchk(cudaMemcpy(d_keys2, h_keys2, N * sizeof(int), cudaMemcpyHostToDevice));
gpuErrchk(cudaMemcpy(d_values, h_values, N * sizeof(float), cudaMemcpyHostToDevice));
// --- From raw pointers to device_ptr
thrust::device_ptr<int> dev_ptr_keys1 = thrust::device_pointer_cast(d_keys1);
thrust::device_ptr<int> dev_ptr_keys2 = thrust::device_pointer_cast(d_keys2);
thrust::device_ptr<float> dev_ptr_values = thrust::device_pointer_cast(d_values);
// --- Declare outputs
thrust::device_vector<float> d_values_output(N);
thrust::device_vector<Tuple> d_keys_output(N);
auto begin_keys = thrust::make_zip_iterator(thrust::make_tuple(dev_ptr_keys1, dev_ptr_keys2));
auto end_keys = thrust::make_zip_iterator(thrust::make_tuple(dev_ptr_keys1 + N, dev_ptr_keys2 + N));
thrust::sort_by_key(begin_keys, end_keys, dev_ptr_values, TupleComp());
int *h_keys1_output = (int *)malloc(N * sizeof(int));
int *h_keys2_output = (int *)malloc(N * sizeof(int));
float *h_values_output = (float *)malloc(N * sizeof(float));
gpuErrchk(cudaMemcpy(h_keys1_output, d_keys1, N * sizeof(int), cudaMemcpyDeviceToHost));
gpuErrchk(cudaMemcpy(h_keys2_output, d_keys2, N * sizeof(int), cudaMemcpyDeviceToHost));
gpuErrchk(cudaMemcpy(h_values_output, d_values, N * sizeof(float), cudaMemcpyDeviceToHost));
printf("\n\n");
printf("Ordered\n");
for (int i = 0; i < N; i++) {
printf("%i %i %f\n", h_keys1_output[i], h_keys2_output[i], h_values_output[i]);
}
}