Why is std.math slower than the C baseline?

[data pulverizer]

On Thursday, 4 June 2020 at 14:14:22 UTC, Andrei Alexandrescu 
wrote:
> D should just use the C functions when they offer better speed.
>
> https://www.reddit.com/r/programming/comments/gvuy59/a_look_at_chapel_d_and_julia_using_kernel_matrix/fsr4w5o/

If it's any help or informative I explicitly benchmark a 
selection of mathematics functions from `std.math`, 
`core.stdc.math`, and `LDC_intrinsic` here: 
https://github.com/dataPulverizer/DMathBench/blob/master/report.md