Re: [gentoo-dev] Announcing RISC-V

[Alexis Ballier]

On Wed, 29 May 2019 10:27:34 -0700 (PDT)
Palmer Dabbelt <palmer@××××××.com> wrote:

> On Mon, 20 May 2019 02:44:18 PDT (-0700), aballier@g.o wrote:
> > On Sat, 18 May 2019 20:47:28 +0200
> > Michał Górny <mgorny@g.o> wrote:
> >
> >> On Fri, 2019-05-03 at 23:34 +0200, Andreas K. Huettel wrote:
> >> > * We will initially add two profiles to profile.desc:
> >> >   default/linux/riscv/17.0/rv64gc/lp64d (non-multilib, 64bit
> >> > hardfloat) default/linux/riscv/17.0/rv64gc (multilib lp64d/lp64,
> >> > i.e. hard/softfloat)
> >>
> >> I still don't understand the purpose of this multilib.  If you have
> >> a hardfloat CPU, why would you ever build some of the software
> >> softfloat?
> >
> > One reason I could imagine is that the hardfloat isn't IEEE 754
> > compliant. Searching through the RISC-V spec, it does not seem to be
> > the case here (ie: it is required to be compliant) so I'm also
> > wondering what is the point here.
>
> The RISC-V floating-point extensions are IEEE-754 compliant, but
> they're optional.  We have chips without floating-point units, but
> right now all the Linux capable chips have FPUs.  As far as I know
> there are no Linux binaries that anyone cares about that are compiled
> for systems without hardware floating-point units, but I may be wrong
> about that one.


It was my understanding that FPU is not optional for rv64gc, is that
correct ?

> The non-FPU systems are much more interesting in embedded land, where
> lots of users don't have FPUs.  That's less relevant for Gentoo, but
> I do use crossdev embedded toolchains.


You'll probably not be using multilib here but rather a specific CHOST
and/or flags to enable softfloat everywhere.


[...]


Alexis.