D Mir: standard deviation speed

[9il]

On Wednesday, 15 July 2020 at 11:23:00 UTC, jmh530 wrote:
> On Wednesday, 15 July 2020 at 05:57:56 UTC, tastyminerals wrote:
>> [snip]
>>
>> Here is a (WIP) project as of now.
>> Line 160 in 
>> https://github.com/tastyminerals/mir_benchmarks_2/blob/master/source/basic_ops.d
>>
>> std of [60, 60] matrix 0.0389492 (> 0.001727)
>> std of [300, 300] matrix 1.03592 (> 0.043452)
>> std of [600, 600] matrix 4.2875 (> 0.182177)
>> std of [800, 800] matrix 7.9415 (> 0.345367)
>
> I changed the dflags-ldc to "-mcpu-native -O" and compiled with 
> `dub run --compiler=ldc2`. I got similar results as yours for 
> both in the initial run.
>
> I changed sd to
>
> @fmamath private double sd(T)(Slice!(T*, 1) flatMatrix)

@fastmath violates all summation algorithms except `"fast"`.
The same bug is in the original author's post.