Let's say I want to create an array of random numbers and do some operations on them:

void main() {

    import std.random;

    //Generate array of random numbers
    int arrSize = 100000000;
    double[] arr = new double[](arrSize);
    foreach (i; 0..arrSize)
        arr[i] = uniform01();

    //Call funcA on array elements
    foreach (i; 1..arr.length-1)
        funcA(arr,i);
}

void funcA(double[] arr, size_t i) {
    arr[i+1] = arr[i-1]+arr[i];
    funcB(arr,i);
}

void funcB(double[] arr, size_t i) {
    arr[i-1]= arr[i] + arr[i+1];
    arr[i] = arr[i-1] + arr[i+1];
    arr[i+1]= arr[i-1] + arr[i];
}

Now I dmd -profile it and look at the performance of funcA with d-profile-viewer. Inside funcA, only 20% of time is spend in funcB, but the rest 80% is self-time of funcA. How is it possible, when funcB has three times the calculations of funcA? It appears that the call to funcB itself is very expensive.

On Saturday, 21 January 2017 at 12:33:57 UTC, albert-j wrote:
> Now I dmd -profile it and look at the performance of funcA with d-profile-viewer. Inside funcA, only 20% of time is spend in funcB, but the rest 80% is self-time of funcA. How is it possible, when funcB has three times the calculations of funcA? It appears that the call to funcB itself is very expensive.

I'm not sure if it's what happening in this case but, in code as simple as this, function calls can sometimes be the bottleneck. You should see how compiling with/without -O affects performance, and adding `pragma(inline)` to funcB.

> I'm not sure if it's what happening in this case but, in code as simple as this, function calls can sometimes be the bottleneck. You should see how compiling with/without -O affects performance, and adding `pragma(inline)` to funcB.

When compiled with -inline, the profiler does not report the performance of funcA and funcB individually, and this is what I want to measure.

> I'm not sure if it's what happening in this case but, in code as simple as this, function calls can sometimes be the bottleneck. You should see how compiling with/without -O affects performance, and adding `pragma(inline)` to funcB.

I guess my question is whether it is possible to have meaningful profiling results for this case, given a large cost of calling funcB? In release builds funcA and funcB are inlined, so profiler cannot report on them individually (is it correct, or am I misusing the profiler?). Profiling without inlining will show a large cost of calling funcB, but this cost will not be there in a release build, so the profiling results are irrelevant.