Illegal memory access on trying to use `DeviceReduce::Sum()` to count number of non-zeros

[Aleksandar Samardžić]

Below is slightly modified example_device_reduce.cu example from CUB distribution, demonstrating an illegal memory access issue when trying to combine TransformInputIterator with DeviceReduce::Sum() to calculate number of non-zero elements in given array.

Reproducible example

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/******************************************************************************
 * Simple example of DeviceReduce::Sum().
 *
 * Sums an array of int keys.
 *
 * To compile using the command line:
 *   nvcc -arch=sm_XX example_device_reduce.cu -I../.. -lcudart -O3
 *
 ******************************************************************************/

// Ensure printing of CUDA runtime errors to console
#define CUB_STDERR

#include <stdio.h>

#include <cub/util_allocator.cuh>
#include <cub/device/device_reduce.cuh>
#include <cub/iterator/transform_input_iterator.cuh>

#include "../../test/test_util.h"

using namespace cub;


//---------------------------------------------------------------------
// Globals, constants and typedefs
//---------------------------------------------------------------------

CachingDeviceAllocator  g_allocator(true);  // Caching allocator for device memory


//---------------------------------------------------------------------
// Test generation
//---------------------------------------------------------------------

/**
 * Initialize problem
 */
void Initialize(
    int   *h_in,
    int     num_items)
{
    for (int i = 0; i < num_items; ++i)
        h_in[i] = i;
}

/**
 * Compute solution
 */
void Solve(
    int           *h_in,
    int           &h_reference,
    int             num_items)
{
    for (int i = 0; i < num_items; ++i)
    {
        if (i == 0)
            h_reference = h_in[0] != 0;
        else
            h_reference += h_in[i] != 0;
    }
}

template<typename T>
struct NonZeroOp
{
    __host__ __device__ __forceinline__ bool operator()(const T& a) const {
      return (a!=T(0));
    }
};

//---------------------------------------------------------------------
// Main
//---------------------------------------------------------------------

/**
 * Main
 */
int main(int argc, char** argv)
{
    int num_items = 46000 * 46000;

    // Initialize command line
    CommandLineArgs args(argc, argv);

    // Print usage
    if (args.CheckCmdLineFlag("help"))
    {
        printf("%s "
            "[--device=<device-id>] "
            "\n", argv[0]);
        exit(0);
    }

    // Initialize device
    CubDebugExit(args.DeviceInit());

    printf("cub::DeviceReduce::Sum() %d items (%d-byte elements)\n",
        num_items, (int) sizeof(int));
    fflush(stdout);

    // Allocate host arrays
    int* h_in = new int[num_items];
    int  h_reference;

    // Initialize problem and solution
    Initialize(h_in, num_items);
    Solve(h_in, h_reference, num_items);

    // Allocate problem device arrays
    int *d_in = NULL;
    CubDebugExit(g_allocator.DeviceAllocate((void**)&d_in, sizeof(int) * num_items));

    // Initialize device input
    CubDebugExit(cudaMemcpy(d_in, h_in, sizeof(int) * num_items, cudaMemcpyHostToDevice));

    // Allocate device output array
    int *d_out = NULL;
    CubDebugExit(g_allocator.DeviceAllocate((void**)&d_out, sizeof(int) * 1));

    cub::TransformInputIterator<bool, NonZeroOp<int>, const int*> iter(d_in, NonZeroOp<int>());

    // Request and allocate temporary storage
    void            *d_temp_storage = NULL;
    size_t          temp_storage_bytes = 0;
    CubDebugExit(DeviceReduce::Sum(nullptr, temp_storage_bytes, iter, d_out, num_items));
    CubDebugExit(g_allocator.DeviceAllocate(&d_temp_storage, temp_storage_bytes));
    CubDebugExit(cudaDeviceSynchronize());
    printf("CUDA error (before DeviceReduce::Sum) = %d\n", cudaGetLastError());

    // Run
    CubDebugExit(DeviceReduce::Sum(d_temp_storage, temp_storage_bytes, iter, d_out, num_items));

    CubDebugExit(cudaDeviceSynchronize());
    printf("CUDA error (after DeviceReduce::Sum) = %d\n", cudaGetLastError());

    // Check for correctness (and display results, if specified)
    int compare = CompareDeviceResults(&h_reference, d_out, 1, false, false);
    printf("\t%s", compare ? "FAIL" : "PASS");
    AssertEquals(0, compare);

    // Cleanup
    if (h_in) delete[] h_in;
    if (d_in) CubDebugExit(g_allocator.DeviceFree(d_in));
    if (d_out) CubDebugExit(g_allocator.DeviceFree(d_out));
    if (d_temp_storage) CubDebugExit(g_allocator.DeviceFree(d_temp_storage));

    printf("\n\n");

    return 0;
}

Testted with CUB 1.16.0, on an A100 GPU. The problem is not there in case d_in used instead of iter in DeviceReduce::Sum calls (this is what original code in the example is doing). The problem is also not there in case number of elements in the input array decreased (note that here this number is close to INT_MAX).

[Georgii Evtushenko]

Hello @alexsamardzic and thank you for reporting the issue!

I can reproduce it with CUB 1.16.0 on A100. The issue is not there for recent versions of CUB. There's a chance that it was already addressed by #592 or #589. Could you please verify the latest release of CUB?

[Aleksandar Samardžić]

Thanks, I confirm that with the CUB latest main the example above works fine.