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

[Jacob Lifshay]

On Wed, Jan 15, 2020, 17:10 'MitchAlsup' via RISC-V ISA Dev <
isa-dev at groups.riscv.org> wrote:

>
>
> On Sunday, September 15, 2019 at 7:35:56 PM UTC-5, lkcl wrote:
>>
>> The problem with changing the accuracy on a per op basis (which would be
>> fine on a GPU) is as Jacob points out: it would not meet IEEE754.
>>
>
> There is one way around this that I did invent::
> Consider a set of instructions that for a "very vast majority" of input
> operands deliver results completely compatible with IEEE 754-2008.
> For those results that are not, we are at most off by a rounding
> (otherwise the very vast majority could not have been correct.)
> Since these are only different by a rounding (conforming to the word
> "Faithful" rounding), we can detect those pre-rounded results that could
> potentially be misrounded, and instead of delivering an incorrect IEEE
> 754-2008 but a faithfully rounded result, instead we raise an UNCERTAIN
> exception (that is an exception not currently in anyone's exception list) ,
> and let a long winded, known to be correct, hunk of SW produce the IEEE
> 754-2008 compliant result.
>

That's a good idea!

>