[Libre-soc-dev] we need two XLENs

Jacob Lifshay programmerjake at gmail.com
Fri Sep 9 16:11:20 BST 2022

On Fri, Sep 9, 2022, 07:57 Richard Wilbur <richard.wilbur at gmail.com> wrote:

> > On Sep 8, 2022, at 20:10, Jacob Lifshay via Libre-soc-dev <
> libre-soc-dev at lists.libre-soc.org> wrote:
> >
> > imho it's a waay better idea to have the two variables, a post-analysis
> > isn't sufficient because of things like combined fp arithmetic and fp
> > format conversion, (e.g. fsqrt with srcelwid=64 and destelwid=f16) where
> > rounding correctly requires fsqrt to know what the actual destination
> > format is, if it just rounds to f64 and then rounds again to f16, it'll
> > give the wrong result in some cases.
> The infamous double round?

yes. interestingly i tried for a while to demonstrate that with fsqrt but
didn't find any examples, that said, i'm 100% sure they exist since there's
lots of inputs and, even if somehow fsqrt magically never fails (i highly
doubt that fsqrt never fails), there are plenty of other unary ops in
fptrans, i'm sure we can find some...


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