static float ComputeSumSimd(float *arr, int count)
{
// We're just going to assume that the length of the data is a multiple of 4, otherwise we'd have to handle the
// other cases. It's not hard, but tedious.
Assert.IsTrue(count % 4 == 0);
if (Ssse3.IsSsse3Supported)
{
// To sum up all values in the array, we split the array into 4 subarrays and store their sums in the variable
// `sum` below.
v128 sum = new v128(0f);
for (int i = 0; i < count; i += 4)
{
// Load 4 floats from memory.
v128 reg = loadu_ps(arr + i);
sum = add_ps(sum, reg);
}
// At this point, we have the sums of 4 subarrays in `sum` and we still need to merge them. SSE3 has a helpful
// instruction for this:
sum = Sse3.hadd_ps(sum, sum);
// Now the first and third lane hold the sum of the first two subarrays and the second and fourth lane contain
// the sum of the last two subarrays.
sum = Sse3.hadd_ps(sum, sum);
// Finally, all four lanes hold the same value (the sum of all subarrays) and we can return the first value
// as a float.
return(cvtss_f32(sum));
// or alternatively, simply write:
// return sum.Float0 + sum.Float1 + sum.Float2 + sum.Float3;
}
else if (IsNeonSupported)
{
// Same as above: 4 subarrays to accumulate the sum
v128 sum = new v128(0f);
for (int i = 0; i < count; i += 4)
{
// Load 4 floats from memory.
v128 reg = vld1q_f32(arr + i);
sum = vaddq_f32(sum, reg);
}
return(vaddvq_f32(sum));
}
else
{
// Managed fallback, equivalent to ComputeSum()
float sum = 0;
for (int i = 0; i < count; i++)
{
sum += arr[i];
}
return(sum);
}
}