[Buildroot] [PATCH 1/1] erlang: support builds when gcc __atomic_* exist

[Frank Hunleth]

Hi Thomas,

On Wed, Jul 28, 2021 at 5:35 PM Thomas Petazzoni
<thomas.petazzoni at bootlin.com> wrote:
>
> On Sat,  8 May 2021 18:24:19 -0400
> Frank Hunleth <fhunleth at troodon-software.com> wrote:
>
> > While Erlang will use it's own atomic operations, it can also use gcc
> > __atomic_* builtins. This is now listed in Erlang's HOWTO/INSTALL.md.
> >
> > This change was necessary on RISC-V, since Erlang didn't have a built-in
> > implementation, but it was able to use gcc's __atomic_* functions.
> >
> > Signed-off-by: Frank Hunleth <fhunleth at troodon-software.com>
> > ---
> >  package/erlang/Config.in | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
>
> Thanks Frank for the patch. I have applied to master. However, I think
> the patch may be incomplete. Indeed, on some CPU architectures, the
> atomic intrinsics are implemented in a separate library provided by
> gcc, called libatomic, and one need to link against it to use those
> intrinsics.
>
> Could you try to do a build of Erlang on SPARCv8 for example ?
>
> I suppose it will fail, and if it does, could you add some logic in
> erlang.mk to link against libatomic when BR2_TOOLCHAIN_HAS_LIBATOMIC=y
> (of course assuming the Erlang build system doesn't do that by itself,
> of course).

As you expected, a SPARCv8 build fails. I updated erlang.mk to pass
linker flags to link to libatomic, but still received errors. The
errors were of type:

/tmp/cc1Mog0n.s:230: Error: Architecture mismatch on "cas [%g3],%g2,%g1".
/tmp/cc1Mog0n.s:230: (Requires v9|v9a|v9b|v9c|v9d|v9e|v9v|v9m|m8;
requested architecture is v8.)

I ran out of time this morning to track this down (sorry), but it's
sounding like Erlang is doing something that's not causing function
calls into libatomic.

Right now, I'm thinking that trying to be more generic about what
platforms Erlang supports may not be worth it. I.e., change this:

        default y if BR2_i386 || BR2_x86_64 || BR2_powerpc || \
                BR2_sparc_v9 || BR2_arm || BR2_aarch64 || BR2_mipsel || \
                BR2_TOOLCHAIN_HAS_ATOMIC

to

        default y if BR2_i386 || BR2_x86_64 || BR2_powerpc || \
                BR2_sparc_v9 || BR2_arm || BR2_aarch64 || BR2_mipsel || \
                BR2_RISCV_64

What do you think?

Thanks,
Frank

Thanks,
Frank