[libre-riscv-dev] [isa-dev] Re: FP transcendentals (trigonometry, root/exp/log) proposal

[Jacob Lifshay]

On Thu, Aug 8, 2019, 05:37 lkcl <luke.leighton at gmail.com> wrote:

> On Thursday, August 8, 2019 at 7:56:15 PM UTC+8, Jacob Lifshay wrote:
>
> >
> > One more part: hw can implement a less accurate mode as if a more
> accurate mode was selected, so, for example, hw can implement all modes
> using hw that produces correctly-rounded results (which will be the most
> accurate mode defined) and just ignore the mode field since
> correct-rounding is not less accurate than any of the defined modes.
>
> Hmm don't know. Hendrik pointed out that Ahmdahl / IBM370 mainframe
> problem that extra accuracy caused.
>
if portable results are desired, correct rounding produces the same results
on all (even non-risc-v) hw, for all implementation algorithms.

less accurate modes produce results that depend on the exact algorithm
chosen, which is a choice that should be left for implementers.