Memory issues and unknown behaviour

I am a beginner to CUDA programming and took a big leap on code but now I’m stuck on a really weird thing.

global void calc_hist(float *arr_b, float *arr_g, float *arr_r, float *histo_b, float *histo_g, float *histo_r, int window_size)
int i = blockIdx.x;
int x;
shared int temp_b[256];
shared int temp_g[256];
shared int temp_r[256];

if(i == 0)
	for( x = 0; x < 256; x++)
		temp_b[x] = 0;
		temp_g[x] = 0;
		temp_r[x] = 0;

printf("%d %d\n", (int)arr_b[i], temp_b[(int)arr_b[i]]);
atomicAdd( &(temp_b[(int)arr_b[i]]), 1);
atomicAdd( &(temp_g[(int)arr_g[i]]), 1);
atomicAdd( &(temp_r[(int)arr_r[i]]), 1);

// if(i == 0){
// for(int a = 0; a<256; a++)
// 	printf("%d ",temp_b[a]);
// printf("\n");
// }

atomicFloatAdd( &(histo_b[(int)arr_b[i]]), temp_b[(int)arr_b[i]] );
atomicFloatAdd( &(histo_g[(int)arr_g[i]]), temp_g[(int)arr_g[i]] );
atomicFloatAdd( &(histo_r[(int)arr_r[i]]), temp_r[(int)arr_r[i]] );

if(i == 0)
for(int a = 0; a<256; a++)


This is my custom function for histogram calculation.

device inline float atomicFloatAdd(float* address, float value)
// printf("%f\n",value);

float old = value;

float ret=atomicExch(address, 0.0f);

float new_old=ret+old;

while ((old = atomicExch(address, new_old))!=0.0f)


new_old = atomicExch(address, 0.0f);

new_old += old;

return ret;


And this is a custom atomic add for float, copied from somewhere.

The problem is, when I pass an array img_b to histogram function with values 1,4,7,10,13,16,19,22,25
It doesn’t increment histo_b at 16 and 19. Weird thing is, when I print values of histo_b to check this, I find out the above problem. But, if I don’t print them, but only count them, I get 9 increments. Also, If I just print incremented addresses and no. of increments, I get 8.

Please help :(

That is an odd attempt at atomic addition. No idea how that is supposed to work. Maybe “somewhere” isn’t the preferrential source for critical code?
You can derive better code from the double-precision example in Appendix 12 of the CUDA Programming Guide.

But why bother at all and not just use the built-in atomicAdd() function, which compiles straight to a single dedicated machine instruction?