Re: [ft-devel] FT_MulDiv optimization

[Werner LEMBERG]

> FT_MulFix optimized like this on line 543:
>     if ( ua <= 2048 && ub <= 1048576L )
>       ua = ( ua * ub + 0x8000UL ) >> 16;
> 
> Of course, when we anchor 2048 and use unsigned multiplication, it
> should safely have been
>      if ( ua <= 2048 && ub <= 2097135L )
> 
> but that's not my point. I do not know why 2048 is chosen, but we
> can relax the condition similarly to the recent FT_MulDiv change. We
> can use this to cover the old range *completely* and grab some more:
> 
>     if ( ua + ( ub >> 8 ) <= 8191UL )
> 
> So, what was so special about 2048?

I can't remember, sorry.  The git archive doesn't show a result
either, so I guess it was an ad-hoc assumption.  If you can provide a
better version: This is OK with me :-) To verify the function, it
might be prudent to write a small test program, comparing the old and
the new results...


    Werner