Why is std.math slower than the C baseline?

[Andrei Alexandrescu]

On 6/4/20 10:39 AM, jmh530 wrote:
> 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/ 
>>
> 
> Below is a typical example of a std.math implementation for a trig 
> function. It casts everything to real to improve accuracy. This doesn't 
> explain everything, but it's a general strategy in std.math to prefer 
> accuracy over speed.
> 
> real cosh(real x) @safe pure nothrow @nogc
> {
>      //  cosh = (exp(x)+exp(-x))/2.
>      // The naive implementation works correctly.
>      const real y = exp(x);
>      return (y + 1.0/y) * 0.5;
> }
> 
> /// ditto
> double cosh(double x) @safe pure nothrow @nogc { return cosh(cast(real) 
> x); }
> 
> /// ditto
> float cosh(float x) @safe pure nothrow @nogc  { return cosh(cast(real) 
> x); }

This needs to change. It's one thing to offer more precision to the user 
who consciously uses 80-bit reals, and it's an entirely different thing 
to force that bias on the user who's okay with double precision.