`cublasIsamin` returns an incorrect value

Question

Arrays are stored as xyzxyz..., I want to get the maximum and minimum for some direction(x or y or z), and here is the test program:

#include <cuda_runtime.h>
#include <cuda_runtime_api.h> // cudaMalloc, cudaMemcpy, etc.
#include <cublas_v2.h>
#include <helper_functions.h> // shared functions common to CUDA Samples
#include <helper_cuda.h>      // CUDA error checking

#include <stdio.h> // printf
#include <iostream>

template <typename T>
void print_arr(T *arr, int L)
{
    for (int i = 0; i < L; i++)
    {
        std::cout << arr[i] << " ";
    }
    std::cout << std::endl;
}

int main()
{
    float hV[10] = {3, 0, 7, 1, 2, 8, 6, 7, 6, 4};
    print_arr(hV, 10);

    float *dV;
    cudaMalloc(&dV, sizeof(float) * 10);
    cudaMemcpy(dV, hV, sizeof(float) * 10, cudaMemcpyHostToDevice);

    cublasHandle_t cublasHandle = NULL;
    checkCudaErrors(cublasCreate(&cublasHandle));

    int hResult[2] = {0};
    checkCudaErrors(cublasIsamax(cublasHandle, 10, dV, 3, hResult + 0));
    checkCudaErrors(cublasIsamin(cublasHandle, 10, dV, 3, hResult + 1));
    print_arr(hResult, 2);

    return 0;
}

expected result:

3 0 7 1 2 8 6 7 6 4 
3 2

result:

3 0 7 1 2 8 6 7 6 4 
3 5

Is there a problem with this result? Or I misunderstood?

link to cublasIsamin.

Answer 1

cublasIsamin finds the index of the minimum value. This index is not computed over the original array, but also takes the incx parameter into account. Furthermore, it will search over n elements (the first parameter) regardless of other parameters such as incx.

You have an array like this:

index:    0 1 2 3 4 5 6 7 8 9
x/y/z:    x y z x y z x y z x
value:    3 0 7 1 2 8 6 7 6 4
x index:  1     2     3     4

Therefore the minimum x value is at index 3, searching over a total of n=4 (not 10) elements. With respect to the x values, we must begin searching dV at offset 0 with an increment of 3, for a maximum of n=4 elements.

Taking all this into account, the correct calls are:

cublasIsamax(cublasHandle, 4, dV, 3, hResult + 0));
cublasIsamin(cublasHandle, 4, dV, 3, hResult + 1));

And the expected result is:

3 2