Finding the prev of a node in SSSP implementation in CUDA? [duplicate] - cuda

Question

I'd like to implement this atomic function in CUDA:
__device__ float lowest; // global var
__device__ int lowIdx; // global var
float realNum; // thread reg var
int index; // thread reg var
if(realNum < lowest) {
lowest= realNum; // the new lowest
lowIdx= index; // update the 'low' index
}
I don't believe I can do this with any of the atomic functions. I need to lock down a couple global memory loc's for a couple instructions.
Might I be able to implement this with PTXAS (assembly) code?

Answer 1

As I stated in my second comment above, it's possible to combine your two 32-bit quantities into a single 64-bit atomically managed quantity, and deal with the problem that way. We then manage the 64-bit quantity atomically using the arbitrary atomic example as a rough guide. Obviously you can't extend this idea beyond two 32-bit quantities. Here's an example:
#include <stdio.h>
#define DSIZE 5000
#define nTPB 256
#define cudaCheckErrors(msg) \
do { \
cudaError_t __err = cudaGetLastError(); \
if (__err != cudaSuccess) { \
fprintf(stderr, "Fatal error: %s (%s at %s:%d)\n", \
msg, cudaGetErrorString(__err), \
__FILE__, __LINE__); \
fprintf(stderr, "*** FAILED - ABORTING\n"); \
exit(1); \
} \
} while (0)
typedef union {
float floats[2]; // floats[0] = lowest
int ints[2]; // ints[1] = lowIdx
unsigned long long int ulong; // for atomic update
} my_atomics;
__device__ my_atomics test;
__device__ unsigned long long int my_atomicMin(unsigned long long int* address, float val1, int val2)
{
my_atomics loc, loctest;
loc.floats[0] = val1;
loc.ints[1] = val2;
loctest.ulong = *address;
while (loctest.floats[0] > val1)
loctest.ulong = atomicCAS(address, loctest.ulong, loc.ulong);
return loctest.ulong;
}
__global__ void min_test(const float* data)
{
int idx = (blockDim.x * blockIdx.x) + threadIdx.x;
if (idx < DSIZE)
my_atomicMin(&(test.ulong), data[idx],idx);
}
int main() {
float *d_data, *h_data;
my_atomics my_init;
my_init.floats[0] = 10.0f;
my_init.ints[1] = DSIZE;
h_data = (float *)malloc(DSIZE * sizeof(float));
if (h_data == 0) {printf("malloc fail\n"); return 1;}
cudaMalloc((void **)&d_data, DSIZE * sizeof(float));
cudaCheckErrors("cm1 fail");
// create random floats between 0 and 1
for (int i = 0; i < DSIZE; i++) h_data[i] = rand()/(float)RAND_MAX;
cudaMemcpy(d_data, h_data, DSIZE*sizeof(float), cudaMemcpyHostToDevice);
cudaCheckErrors("cmcp1 fail");
cudaMemcpyToSymbol(test, &(my_init.ulong), sizeof(unsigned long long int));
cudaCheckErrors("cmcp2 fail");
min_test<<<(DSIZE+nTPB-1)/nTPB, nTPB>>>(d_data);
cudaDeviceSynchronize();
cudaCheckErrors("kernel fail");
cudaMemcpyFromSymbol(&(my_init.ulong), test, sizeof(unsigned long long int));
cudaCheckErrors("cmcp3 fail");
printf("device min result = %f\n", my_init.floats[0]);
printf("device idx result = %d\n", my_init.ints[1]);
float host_val = 10.0f;
int host_idx = DSIZE;
for (int i=0; i<DSIZE; i++)
if (h_data[i] < host_val){
host_val = h_data[i];
host_idx = i;
}
printf("host min result = %f\n", host_val);
printf("host idx result = %d\n", host_idx);
return 0;
}
Here is a similar example that does atomic update of 2 float quantities.
Here is another custom atomic example.

Answer 2

#Robert Crovella: Excellent idea, but I think the function should be modified a little bit as follows:
__device__ unsigned long long int my_atomicMin(unsigned long long int* address, float val1, int val2)
{
my_atomics loc, loctest, old;
loc.floats[0] = val1;
loc.ints[1] = val2;
loctest.ulong = *address;
old.ulong = loctest.ulong;
while (loctest.floats[0] > val1){
old.ulong = loctest.ulong;
loctest.ulong = atomicCAS(address, loctest.ulong, loc.ulong);
}
return old.ulong;
}