Re: [avr-libc-dev] tanh in libm

[E. Weddington]

René Liebscher wrote:

> Hi,
>
> I have question about the tanh function in libm.
>
> As I can see in
> http://savannah.nongnu.org/cgi-bin/viewcvs/avr-libc/avr-libc/libm/fplib/tanh.S?rev=1.2&content-type=text/vnd.viewcvs-markup 
>
> there are two ways to calculate the result.
>
> The questions is why was sinh/cosh choosen. (The other way is 
> commented out.)
> Comparing which functions are called to calculate tanh I find 
> sinh/cosh needs one call to exp, two calls to inverse and one call to 
> ldexp more than the other way. So I think the current solution is 
> slower than the other.
> (And it takes more memory if you don't use sinh or cosh in your program.)

AFAIK, the whole math library in avr-libc was a port of Michael Stumpf's 
FPLib library to the AVR. See the readme.fplib file in CVS:
<http://savannah.nongnu.org/cgi-bin/viewcvs/avr-libc/avr-libc/libm/fplib/readme.fplib?rev=1.1&content-type=text/vnd.viewcvs-markup>
So a lot of the decisions on how to do things were very likely his.


> If the other solution brings also correct results and is faster, is 
> there a simple way to integrate it in my program without to recompile 
> the math library.

Probably only if you're making your own tanh function then you wouldn't 
have to recompile the library. Obviously if you wanted it to be a part 
of the library then it would have to be rebuilt.

HTH
Eric