Hello everybody! I just started learning opencl and was reading some sample codes. In a matrix multiplication example illustrating the use of local device memory, the code is as follows:

//optimized matrix multiplication using work-group and local memory

__kernel void multMatrix(__global float *mO,

```
__global float *mA,
__global float *mB,
uint widthA, uint widthB)
```

{

```
uint lx = get_local_id(0);
uint ly = get_local_id(1);
int gx = get_group_id(0);
int gy = get_group_id(1);
// calculate the starting index of the global array for the each sub matrix
uint iSubA = BLOCKSIZE * gy * widthA;
uint iSubB = BLOCKSIZE * gx;
// get the number of groups in
int n = get_num_groups(0);
// varaiable to hold the running total
float sum = 0;
// for each block
for(int i=0; i< n;i++)
{
// declare local memory for each sub matrix
__local float tA[BLOCKSIZE][BLOCKSIZE];
__local float tB[BLOCKSIZE][BLOCKSIZE];
// copy a portion of the input matrices into the sub matrices
tA[ly][lx] = mA[ly*widthA + lx + (iSubA + i* BLOCKSIZE)];
tB[ly][lx] = mB[ly*widthB + lx + (iSubB + i* BLOCKSIZE * widthB)];
// wait for all work-items int the group to finish copying
barrier(CLK_LOCAL_MEM_FENCE);
// multiply the two sub matrices together.
for(int k=0; k<BLOCKSIZE; k++)
{
sum += tA[ly][k] * tB[k][lx];
}
}
// copy the final result to the output buffer
int globalIdx=get_global_id(0);
int globalIdy=get_global_id(1);
mO[globalIdy * widthA + globalIdx] = sum;
```

}

I got confused by the two lines that are highlighted with red: Since every thread will run the kernel, does it mean every thread in one work-group will allocate the local memory respectfully?

I ran the code above, the result is correct.

I thought it should go with like this:

if(i==0)

{

__local float tA[BLOCKSIZE][BLOCKSIZE];

__local float tB[BLOCKSIZE][BLOCKSIZE];

}

so that the memory shall be allocated only by one thread in the same work group.

Could anyone help me figure this out?Thanks.