On Mon, 25 Oct 2021 13:08:38 -0700
Kalesh Singh <kaleshsingh@xxxxxxxxxx> wrote:
> == Results ==
>
> Divisor is a power of 2 (divisor == 32):
>
> test_hist_field_div_not_optimized | 8,717,091 cpu-cycles
> test_hist_field_div_optimized | 1,643,137 cpu-cycles
>
> If the divisor is a power of 2, the optimized version is ~5.3x faster.
>
> Divisor is not a power of 2 (divisor == 33):
>
> test_hist_field_div_not_optimized | 4,444,324 cpu-cycles
> test_hist_field_div_optimized | 5,497,958 cpu-cycles
To optimize this even more, if the divisor is constant, we could make a
separate function to not do the branch, and just shift or divide.
And even if it is not a power of 2, for constants, we could implement a
multiplication and shift, and guarantee an accuracy up to a defined max.
If div is a constant, then we can calculate the mult and shift, and max
dividend. Let's use 20 for shift.
// This works best for small divisors
if (div > max_div) {
// only do a real division
return;
}
shift = 20;
mult = ((1 << shift) + div - 1) / div;
delta = mult * div - (1 << shift);
if (!delta) {
/* div is a power of 2 */
max = -1;
return;
}
max = (1 << shift) / delta;
We would of course need to use 64 bit operations (maybe only do this for 64
bit machines). And perhaps even use bigger shift values to get a bigger max.
Then we could do:
if (val1 < max)
return (val1 * mult) >> shift;
-- Steve
