Subtracting two integers causes integer-underflow in device code

Question

In my cuda device code I am doing a check where I subtracting the thread's id and the blockDim to see weather or not the data I might want to use is in range. But when this number goes bellow 0 it seems to wrap back around to be max instead.

#include <iostream>
#include <cuda_runtime.h>
#include <device_launch_parameters.h>

float input[] =
{
1.5f, 2.5f, 3.5f,
4.5f, 5.5f, 6.5f,
7.5f, 8.5f, 9.5f,
};

__global__ void underflowCausingFunction(float* in, float* out)
{
    int id = (blockDim.x * blockIdx.x) + threadIdx.x;
    out[id] = id - blockDim.x;
}

int main()
{
    float* in;
    float* out;

    cudaMalloc(&in, sizeof(float) * 9);
    cudaMemcpy(in, input, sizeof(float) * 9, cudaMemcpyHostToDevice);
    cudaMalloc(&out, sizeof(float) * 9);

    underflowCausingFunction<<<3, 3>>>(in, out);

    float recivedOut[9];
    cudaMemcpy(recivedOut, out, sizeof(float) * 9, cudaMemcpyDeviceToHost);

    cudaDeviceSynchronize();

    std::cout << recivedOut[0] << " " << recivedOut[1] << " " << recivedOut[2] << "\n"
    << recivedOut[3] << " " << recivedOut[4] << " "  << recivedOut[5] << "\n"
    << recivedOut[6] << " " << recivedOut[7] <<  " " << recivedOut[8] << "\n";

     cudaFree(in);
     cudaFree(out);

     std::cin.get();
}

The output of this is:

4.29497e+09 4.29497e+09 4.29497e+09
0 1 2
3 4 5

I'm not sure why it's acting like an unsigned int. If it is relevant I am using GTX 970 and the NVCC compiler that comes with the visual studio plugin. If somebody could explain what's happening or what I'm doing on wrong that would be great.

Answer 1

The built-in variables like threadIdx and blockIdx are composed of unsigned quantities.

In C++, when you subtract an unsigned quantity from a signed integer quantity:

out[id] = id - blockDim.x;

the arithmetic that gets performed is unsigned arithmetic.

Since you want signed arithmetic (apparently) the correct thing to do is to make sure both quantities being subtracted are of signed type (let's use int in this case):

out[id] = id - (int)blockDim.x;