[LLVMdev] [RFC] Add a simple soft-float class
Sean Silva
chisophugis at gmail.com
Wed Jun 18 10:18:52 PDT 2014
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
>
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