Why is this code slow?

[Salih Dincer]

On Thursday, 28 March 2024 at 11:50:38 UTC, rkompass wrote:
> 
> Turning back to this: Are there similarly simple libraries for 
> C, that allow for
> parallel computation?

You can achieve parallelism in C using libraries such as OpenMP, 
which provides a set of compiler directives and runtime library 
routines for parallel programming.

Here’s an example of how you might modify the code to use OpenMP 
for parallel processing:

```c
#include <stdio.h>
#include <time.h>
#include <omp.h>

#define ITERS 1000000000
#define STEPS 31

double leibniz(int i) {
   double r = (i == ITERS) ? 0.5 * ((i % 2) ? -1.0 : 1.0) / (i * 
2.0 + 1.0) : 0.0;
   for (--i; i >= 0; i -= STEPS)
     r += ((i % 2) ? -1.0 : 1.0) / (i * 2.0 + 1.0);
   return r * 4.0;
}

int main() {
   double start_time = omp_get_wtime();

   double result = 0.0;

   #pragma omp parallel for reduction(+:result)
   for (int s = ITERS; s >= 0; s -= STEPS) {
     result += leibniz(s);
   }

   // Calculate the time taken
   double time_taken = omp_get_wtime() - start_time;

   printf("%.16f\n", result);
   printf("%f (seconds)\n", time_taken);

   return 0;
}
```
To compile this code with OpenMP support, you would use a command 
like gcc -fopenmp your_program.c. This tells the GCC compiler to 
enable OpenMP directives. The #pragma omp parallel for directive 
tells the compiler to parallelize the loop, and the reduction 
clause is used to safely accumulate the result variable across 
multiple threads.

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