[Buildroot] [PATCH] erlang: selectively enable libatomic_ops

[Yann E. MORIN <yann.morin.1998@free.fr>]

Frank, All,

On 2014-12-22 12:49 -0500, Frank Hunleth spake thusly:
> On Mon, Dec 22, 2014 at 12:13 PM, Yann E. MORIN <yann.morin.1998@free.fr> wrote:
> > Franck, All,
> >
> > On 2014-12-22 04:04 -0500, Frank Hunleth spake thusly:
> >> Erlang can use either the system libatomic_ops or its own version.
> >> However, if using the system version, it must find implementations for
> >> AO_compare_and_swap() and other calls.  Since these are not available on
> >> all platforms, selectively enable the use of the system libatomic_ops.
> >>
> >> Fixes:
> >> http://autobuild.buildroot.net/results/eba/eba5bf85e9ff98916cd3638b5f13ab5bd54564d8/
> >> http://autobuild.buildroot.net/results/c74/c743cfca629fb332bbb1327cf4ea360f0a4e4bfe/
> >> http://autobuild.buildroot.net/results/095/095348d4d309c2a408a3b297b4325348ed1dd7e8/
> >>
> >> Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
> >> ---
> >>  package/erlang/Config.in | 10 ++++++++++
> >>  package/erlang/erlang.mk |  4 +++-
> >>  2 files changed, 13 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/package/erlang/Config.in b/package/erlang/Config.in
> >> index 56830da..9caf1ec 100644
> >> --- a/package/erlang/Config.in
> >> +++ b/package/erlang/Config.in
> >> @@ -26,6 +26,16 @@ config BR2_PACKAGE_ERLANG_SMP
> >>         If you do not need SMP support, turning this option off reduces
> >>         compile time and the size of the Erlang installation.
> >>
> >> +config BR2_PACKAGE_ERLANG_USE_LIBATOMIC_OPS
> >> +     bool
> >> +     select BR2_PACKAGE_LIBATOMIC_OPS
> >> +     default y if BR2_ARM_CPU_ARMV7A || BR2_x86_64
> >
> > Well, we already have BR2_PACKAGE_LIBATOMIC_ARCH_SUPPORTS that defines
> > which architecture libatomic_ops supports.
> >
> > So, unless AO_compare_and_swap() is not available on all those
> > architectures (and it seems it is), you should use that symbol instead.
> 
> That was my first thought, but if you look at the configs for the
> failed builds, BR2_PACKAGE_LIBATOMIC_ARCH_SUPPORTS=y is defined. E.g.:
> 
>    http://autobuild.buildroot.net/results/eba/eba5bf85e9ff98916cd3638b5f13ab5bd54564d8/config
> 
> Unfortunately, configure only prints the following to the build log so
> it's not immediately obvious which function was missing:
> 
>    checking for a usable libatomic_ops implementation... no
>    configure: error: No usable libatomic_ops implementation found
> 
> My assumption is that AO_compare_and_swap() is missing since that's
> what I saw on a BR2_ARM_CPU_ARMV4 platform, but it could be something
> else on BR2_ARM_CPU_ARMV5 (the config that I referenced above).

Well, looking at output/build/erlang-17.4/erts/config.log, one can see:
    conftest.c:84:2: error: #error No compare_and_swap

which is consistent with the corresponding check, that looks for some
variant of AO_compare_and_swap.

So you were right, it's AO_compare_and_swap that is missing.

Let's see why...

Any definition of AO_compare_and_swap is enclosed in a pre-processor
block, conditional on AO_ARM_HAVE_LDREX.

AO_ARM_HAVE_LDREX is defined for anything armv6+ (except armv6m which is
thumb-only, but I doubt we have support for that, and that Linux even
runs on that processor).

So for ARM, we need armv6+ to use libatomic_ops.

However, I fail to see the reasoning for singling-out armv7a (and armv6)
and x86_64. Is there no other architecture that has AO_compare_and_swap()
or any of its variants? What about armv8 (AArch64), i386, sparc and the
others?

Maybe we could add this instead:

    config BR2_PACKAGE_ERLANG_USE_LIBATOMIC_OPS
        bool
        default y
        depends on BR2_PACKAGE_LIBATOMIC_ARCH_SUPPORTS
        # On ARM, only armv6+ has the necessary AO_compare_and_swap()
        depends on BR2_ARM_CPU_ARMV6 || BR2_ARM_CPU_ARMV7A || !(BR2_arm || BR2_armeb)
        help
          Use the system libatomic_ops on platforms that provide
          implementations for AO_store(), AO_load(), and AO_compare_and_swap().
          Platforms without support for those functions must still use the
          Erlang-provided implementation.

> >>  config BR2_PACKAGE_ERLANG_MEGACO
> >>       bool "install megaco application"
> >>       help
> >> diff --git a/package/erlang/erlang.mk b/package/erlang/erlang.mk
> >> index 0b8619e..007053a 100644
> >> --- a/package/erlang/erlang.mk
> >> +++ b/package/erlang/erlang.mk
> >> @@ -22,9 +22,11 @@ ERLANG_CONF_ENV += erl_xcomp_sysroot=$(STAGING_DIR)
> >>
> >>  ERLANG_CONF_OPTS = --without-javac
> >>
> >> -ifeq ($(BR2_PACKAGE_LIBATOMIC_OPS),y)
> >> +ifeq ($(BR2_PACKAGE_ERLANG_USE_LIBATOMIC_OPS),y)
> >>  ERLANG_DEPENDENCIES += libatomic_ops
> >>  ERLANG_CONF_OPTS += --with-libatomic_ops=$(STAGING_DIR)/usr
> >> +else
> >> +ERLANG_CONF_OPTS += --without-libatomic_ops

It would also be good to add a comment that says something like:

    # --without-libatomic_ops means: do not use an external libatomic_ops;
    # atomic operations use an internal implementation instead.

Regards,
Yann E. MORIN.

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