[LLVMdev] [RFC] Add a simple soft-float class

[Sean Silva]

On Wed, Jun 18, 2014 at 11:12 AM, Owen Anderson <resistor at mac.com> wrote:

>
> On Jun 18, 2014, at 9:30 AM, Sean Silva <chisophugis at gmail.com> wrote:
>
> On Tue, Jun 17, 2014 at 10:11 PM, Rafael Avila de Espindola <
> rafael.espindola at gmail.com> wrote:
>
>> If we require the host to have sse2 or other IEEE 754 conformant
>>  implementation, would it be possible to use hardware float?
>>
>
> I don't think that IEEE 754 actually guarantees bit-exact results in all
> cases.
>
>
>
> IEEE 754 accuracy guarantees are expressed in terms of ULPs:
> Units-in-the-Last-Place.
>

Yeah, that's what I was thinking of, but...

 Most of the primitive operations are accurate to +/- 0.5 ULPs, which is
> equivalent to saying that there is a single bit-accurate result (for a
> given rounding mode).
>

... I wasn't aware that the primitives were spec'd with sufficiently tight
ULP bounds for bit-exact results. Good to know!


>  Once you get into transcendentals, you start getting operations with much
> wider ULP bounds.
>

Yeah, this was the specific counter-example that I was aware of.


>
> —Owen
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140618/3a2e43fa/attachment.html>