Re: GCC changes for Fedora + riscv64

[Jeff Law <law at redhat dot com>]

On 04/09/2018 12:04 PM, Jim Wilson wrote:
> On 04/08/2018 08:22 AM, Jeff Law wrote:
>> On 03/31/2018 12:27 PM, Richard W.M. Jones wrote:
>>> I'd like to talk about what changes we (may) need to GCC in
>>> Fedora to get it working on 64-bit RISC-V, and also (more
>>> importantly) to ask your advice on things we don't fully
>>> understand yet.  However, I don't know even what venue you'd
>>> prefer to discuss this in.
> 
> A discussion here is fine with me.  I know of a few issues.
> 
> I have a work-in-progress --with-multilib-list patch in PR 84797 but it
> isn't quite right yet, and needs to work more like the patch in PR
> 85142, which isn't OK to check in.
> 
> There is a problem with atomics.  We only have builtins for the ones
> that can be implemented with a single instruction.  Adding -latomic
> unconditionally might fix it, but won't work for gcc builds and the gcc
> testsuite unless we also add paths pointing into the libatomic build
> dir.  I'm also concerned that this might cause build problems, if we end
> up trying to link with libatomic before we have built it.  The simplest
> solution might be to just add expanders for all of the missing atomics,
> even if they require multiple instructions, just like how all of the
> mainstream linux targets currently work.
> 
> There is a problem with the linker not searching the right set of dirs
> by default.  That is more a binutils problem than a gcc problem, but the
> linker might need some help from gcc to fix it, as the linker doesn't
> normally take -march and -mabi options.
> 
> There is a problem with libffi, which has RISC-V support upstream, but
> not in the FSF GCC copy.  This is needed for go language support.  There
> was also a dispute about go something naming, as to whether it should be
> riscv64 or riscv, with one person doing a port choosing the former and
> another person doing another port choosing the latter.
> 
> Those are all of the Linux specific ones I can remember at the moment. I
> might have missed some.
Are you guys using qemu user mode emulation for testing purposes?  When
I've set up a suitable riscv64 rootfs and try to do anything nontrivial
in it with qemu user mode emulation it immediately complains that my
kernel is too old -- which is quite odd as I've got a dozen or so of
these kinds of environments set up for testing which don't issue that
complaint.

I'd like to avoid full system emulation just from a cost standpoint..

Jeff