openCL Kernel to calculate Pi is not correct value - c

Question

Good day,
I have an openCL kernel that is using the Leibniz formula to calculate pi. Currently my issue is that the value I get back isn't pi, but instead just 4.
__kernel void calculatePi(int numIterations, __global float *outputPi,
__local float* local_result, int numWorkers)
{
__private const uint gid = get_global_id(0);
__private const uint lid = get_local_id(0);
__private const uint offset = numIterations*gid*2;
__private float sum = 0.0f;
// Have the first worker initialize local_result
if (gid == 0)
{
for (int i = 0; i < numWorkers; i++)
{
local_result[i] = 0.0f;
}
}
// Have all workers wait until this is completed
barrier(CLK_GLOBAL_MEM_FENCE);
// Have each worker calculate their portion of pi
// This is a private value
for (int i=0; i<numIterations; i++)
{
if (i % 2 == 0)
{
sum += 1 / (1 + 2*i + offset);
}
else
{
sum -= 1 / (1 + 2*i + offset);
}
}
// Have each worker move their value to the appropriate
// local_result slot so that the first worker can see it
// when reducing next
local_result[gid] = sum;
// Make sure all workers complete this task before continuing
barrier(CLK_LOCAL_MEM_FENCE);
// Have the first worker add up all of the other worker's values
// to get the final value
if (lid == 0)
{
outputPi[0] = 0;
for (int i = 0; i < numWorkers; i++)
{
outputPi[0] += local_result[i];
}
outputPi[0] *= 4;
}
}
I've steered all of my inputs to my output to verify that they are what I expect. numIterations is 16 and numWorkers is also 16.
When sum is calculated then for the first worker, I would expect the sum to be
1 - 1/3 + 1/5 - 1/7 + 1/9 - 1/11 + 1/13 - 1/15 + 1/17 - 1/19 + 1/21 - 1/23 + 1/25 - 1/27 + 1/29 - 1/31
Using this calculator for the first 16 times, I expect the result to be around 3.2 : https://scratch.mit.edu/projects/19546118/
If I modify my last bit of code to be this so that I can look at a worker's calculated value of "sum":
// Have the first worker add up all of the other worker's values
// to get the final value
if (lid == 0)
{
outputPi[0] = sum * 4;
}
Then the value returned for the first worker is 4 instead of the expected 3.2
Modifying to any other number except lid == 0, all other workers are reporting their sum as 0. So my question is why is that the calculated value? Am I doing something wrong with my sum variable? This should be a private variable and the for loop should be sequential from my understanding for each worker but numerous loops are executed in parallel based on the number of workers.
Here's a link to my github that has the kernel and main code uploaded.
https://github.com/TreverWagenhals/TreverWagenhals/tree/master/School/Heterogeneous%20Computing/Lab2
Thanks

Answer 1

you are performing integral divisions in your code, should be floats:
if (i % 2 == 0)
{
sum += 1. / (1 + 2*i + offset); // notice the 1.
}
else
{
sum -= 1. / (1 + 2*i + offset);
}