performance cost of sample conversion

[Psychological Cleanup via Digitalmars-d-learn]

On Thursday, 7 September 2017 at 05:45:58 UTC, Ali Çehreli wrote:
> On 09/06/2017 07:06 PM, Psychological Cleanup wrote:
>> if I have a non-double buffer and temporarily convert to 
>> double then
>> convert back, do I save many cycles rather than just using a 
>> double
>> buffer? I know it will bea lot more memory, but I'm 
>> specifically talking
>> about the cycles in converting to and from vs no conversion.
>>
>> Using a double for everything gives the highest precision and 
>> makes
>> things much easier but is that the way to go or does it costs 
>> quite a
>> bit in performance?
>
> You have to measure. Here's a start:
>
> import std.conv;
> import std.range;
> import std.datetime;
> import std.stdio;
>
> double workWithDouble(double d) {
>     return d * d / 7;
> }
>
> void workWithFloats(float[] floats) {
>     foreach (ref f; floats) {
>         f = workWithDouble(f).to!float;
>     }
> }
>
> void workWithDoubles(double[] doubles) {
>     foreach (ref d; doubles) {
>         d = workWithDouble(d);
>     }
> }
>
> void main() {
>     foreach (n; [ 1_000, 1_000_000, 10_000_000 ]) {
>         const beg = -1f;
>         const end = 1f;
>         const step = (end - beg) / n;
>         auto floats = iota(beg, end, step).array;
>         auto doubles = iota(double(beg), end, step).array;
>         {
>             auto sw = StopWatch(AutoStart.yes);
>             workWithDoubles(doubles);
>             writefln("%10s no   conversion: %10s usecs", n, 
> sw.peek().usecs);
>         }
>         {
>             auto sw = StopWatch(AutoStart.yes);
>             workWithFloats(floats);
>             writefln("%10s with conversion: %10s usecs", n, 
> sw.peek().usecs);
>         }
>     }
> }
>
> Conversion seems to be more costly:
>
>       1000 no   conversion:         27 usecs
>       1000 with conversion:         40 usecs
>    1000000 no   conversion:       1715 usecs
>    1000000 with conversion:       5412 usecs
>   10000000 no   conversion:      16280 usecs
>   10000000 with conversion:      47190 usecs
>
> Ali

Thanks. my results

dmd x86 debug
     asserts on the line `auto floats = iota(beg, end, 
step).array;`


dmd x64 debug
       1000 no   conversion:         15 usecs
       1000 with conversion:          5 usecs
    1000000 no   conversion:       2824 usecs
    1000000 with conversion:       5689 usecs
   10000000 no   conversion:      24148 usecs
   10000000 with conversion:      56335 usecs

dmd release x86
       1000 no   conversion:          1 usecs
       1000 with conversion:          1 usecs
    1000000 no   conversion:       1903 usecs
    1000000 with conversion:       1262 usecs
   10000000 no   conversion:      19156 usecs
   10000000 with conversion:      12831 usecs

dmd release x64
       1000 no   conversion:          4 usecs
       1000 with conversion:         17 usecs
    1000000 no   conversion:       4531 usecs
    1000000 with conversion:       4516 usecs
   10000000 no   conversion:      45928 usecs
   10000000 with conversion:      46080 usecs

ldc x86 debug
       1000 no   conversion:          3 usecs
       1000 with conversion:         32 usecs
    1000000 no   conversion:       3563 usecs
    1000000 with conversion:      19240 usecs
   10000000 no   conversion:      35986 usecs
   10000000 with conversion:     192025 usecs

ldc x64 debug
       1000 no   conversion:          2 usecs
       1000 with conversion:         10 usecs
    1000000 no   conversion:       2855 usecs
    1000000 with conversion:      10309 usecs
   10000000 no   conversion:      28254 usecs
   10000000 with conversion:     101380 usecs

ldc x86 release
       1000 no   conversion:          0 usecs
       1000 with conversion:          0 usecs
    1000000 no   conversion:       1280 usecs
    1000000 with conversion:        532 usecs
   10000000 no   conversion:      10403 usecs
   10000000 with conversion:       5752 usecs

ldc x64 release
       1000 no   conversion:          0 usecs
       1000 with conversion:          1 usecs
    1000000 no   conversion:        887 usecs
    1000000 with conversion:        550 usecs
   10000000 no   conversion:      10730 usecs
   10000000 with conversion:       5482 usecs

The results are strange, sometimes the conversion wins.