Problem on static libraries enabled with LinkTimeOptimization

[Lei YU]

An update on this about a "better" workaround.

To use gcc-ar gcc-ranlib with autoconf, we could "define" the tools in
configure.ac before `AM_PROG_AR` to override the default ones.

    AC_SUBST([AR], [${ac_tool_prefix}gcc-ar])
    AC_SUBST([RANLIB], [${ac_tool_prefix}gcc-ranlib])

See an example at
https://gerrit.openbmc-project.xyz/#/c/openbmc/phosphor-fan-presence/+/12962/

This change is smaller and does not involves bitbake recipes anymore.

Any feedback/comment is welcome :D

On Wed, Sep 12, 2018 at 5:26 PM Lei YU <mine260309 at gmail.com> wrote:
>
> This email is to describe a problem about static libraries enabled with
> LinkTimeOptimization (-flto), and asking for suggestions how to fix the
> problem.
>
> ## Background
>
> When a repo enables lto, and if the repo builds a static library, it gets
> build error, e.g. for [phosphor-fan-presence#11][1]
>
>     arm-openbmc-linux-gnueabi-ar: .libs/libfan_la-utility.o: plugin
> needed to handle lto object
>     arm-openbmc-linux-gnueabi-ar: .libs/libfan_la-timer.o: plugin
> needed to handle lto object
>     arm-openbmc-linux-gnueabi-libtool: link:
> arm-openbmc-linux-gnueabi-ranlib .libs/libfan.a
>     arm-openbmc-linux-gnueabi-ranlib:
> .libs/libfan.a(libfan_la-utility.o): plugin needed to handle lto
> object
>     arm-openbmc-linux-gnueabi-ranlib:
> .libs/libfan.a(libfan_la-timer.o): plugin needed to handle lto object
>
> And eventually it causes undefined references during linking.
> The same for phosphor-time-manager.
>
>
> ## Root Cause
>
> ar needs to load liblto_plugin to handle lto objects, and our build
> environment does not tell ar to load the plugin.
>
> According to https://gcc.gnu.org/wiki/LinkTimeOptimizationFAQ:
> > In order to handle archives of these objects you have to use the gcc
> > wrappers: gcc-ar, gcc-nm and gcc-ranlib.
>
> So the simplest way to load the plugin is to use gcc-ar instead of ar.
>
>
> ## Question
>
> Most of phosphor repos in OpenBMC uses autotool to configure, and it depends
> on environment variables to configure the repo.
> By default, AR is defined as `${TARGET_PREFIX}ar`, I do not find a way to
> change it.
>
> So my question is, how to handle this case? E.g. how to specify gcc-ar in
> autoconf?
>
>
> ## References
>
> * I have an *ugly* workaround to override AR by define it in `EXTRA_OECONF`,
>    see https://gerrit.openbmc-project.xyz/#/c/openbmc/openbmc/+/12721/ for
>    details.
> * bmcweb uses CMake and it specifies gcc-ar in [CMakefiles][2].
>
> [1]: https://github.com/openbmc/phosphor-fan-presence/issues/11
> [2]: https://github.com/openbmc/bmcweb/blob/master/CMakeLists.txt#L70