[Buildroot] [PATCH 1/1] boost: thread needs atomic if GCC has bug 64735

[Thomas Petazzoni <thomas.petazzoni@bootlin.com>]

Hello Fabrice,

Have you had the chance to look into the below issue and questions ?
boost is now the #1 failure reason in the autobuilders, so it is
becoming important to fix it soon.

Thanks!

Thomas

On Sat, 14 Jul 2018 23:05:47 +0200, Thomas Petazzoni wrote:
> Hello Fabrice,
> 
> On Fri, 13 Jul 2018 19:30:26 +0200, Fabrice Fontaine wrote:
> > When gcc has bug 64735, lock-free atomic ints are not supported:
> >  - lockfree boost::atomic_flag : no
> > 
> > As a result, boost thread needs boost atomic:
> > output/host/usr/bin/nios2-linux-readelf -d output/staging/usr/lib/libboost_thread.so
> > 
> > Dynamic section at offset 0x2cee0 contains 32 entries:
> >   Tag        Type                         Name/Value
> >  0x00000001 (NEEDED)                     Shared library: [libboost_system.so.1.67.0]
> >  0x00000001 (NEEDED)                     Shared library: [libboost_atomic.so.1.67.0]
> > 
> > Fixes:
> >  - http://autobuild.buildroot.net/results/5a7db292f1365f27e32695527701d5b827f60092
> >  - http://autobuild.buildroot.net/results/413dff87f5329d3c5180167a8711cdedea5dec67
> >  - http://autobuild.buildroot.net/results/a7eb4cbcdbd9412c344f45336dec58c82e84dab9
> > 
> > Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>  
> 
> You'll probably find me annoying, but I'd like to have more details on
> how this is related to BR2_TOOLCHAIN_HAS_GCC_BUG_64735.
> 
> I looked into the Boost build system a bit, and here is what I
> collected.
> 
> In libs/thread/build/Jamfile.v2, there is the following snippet:
> 
>         if ! [ configure.builds has_atomic_flag_lockfree 
>             : $(properties) : "lockfree boost::atomic_flag" ]  { 
>            result += <library>/boost/atomic//boost_atomic ; 
>         } 
> 
> So, the boost_atomic library gets added to the build if
> has_atomic_flag_lockfree isn't set. This macro is set by this line in
> the same file:
> 
> exe has_atomic_flag_lockfree : ../build/has_atomic_flag_lockfree_test.cpp ;
> 
> So presumably, it builds has_atomic_flag_lockfree_test.cpp, and
> depending on that decides if atomic_flag_lockfree is available or not.
> This ./libs/thread/build/has_atomic_flag_lockfree_test.cpp file contains:
> 
> 
> #include "../../../boost/atomic.hpp"
> #include "../../../boost/static_assert.hpp"
> 
> int main(int argc, char *argv[])
> {
>   BOOST_STATIC_ASSERT(BOOST_ATOMIC_FLAG_LOCK_FREE);
>   return 0;
> }
> 
> So basically, it tests if BOOST_ATOMIC_FLAG_LOCK_FREE is non-zero. And
> then, things get crazy, because the definition of
> BOOST_ATOMIC_FLAG_LOCK_FREE depends on lots of other macros.
> 
> But to me, there is no evidence that this is related to gcc bug 64735.
> 
> To illustrate this, you can build the following defconfig. It uses a
> gcc 8.x NIOS2 toolchain, which is not affected by
> BR2_TOOLCHAIN_HAS_GCC_BUG_64735, and still libboost_thread implies
> building libboost_atomic. Therefore, the problem is not related to
> BR2_TOOLCHAIN_HAS_GCC_BUG_64735 :-/
> 
> BR2_nios2=y
> BR2_TOOLCHAIN_EXTERNAL=y
> BR2_TOOLCHAIN_EXTERNAL_CUSTOM=y
> BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD=y
> BR2_TOOLCHAIN_EXTERNAL_URL="http://toolchains.bootlin.com/downloads/releases/toolchains/nios2/tarballs/nios2--glibc--bleeding-edge-2018.06-1.tar.bz2"
> BR2_TOOLCHAIN_EXTERNAL_GCC_8=y
> BR2_TOOLCHAIN_EXTERNAL_HEADERS_4_14=y
> BR2_TOOLCHAIN_EXTERNAL_CUSTOM_GLIBC=y
> BR2_TOOLCHAIN_EXTERNAL_CXX=y
> BR2_INIT_NONE=y
> BR2_SYSTEM_BIN_SH_NONE=y
> # BR2_PACKAGE_BUSYBOX is not set
> BR2_PACKAGE_BOOST=y
> BR2_PACKAGE_BOOST_THREAD=y
> # BR2_TARGET_ROOTFS_TAR is not set
> 
> Best regads,
> 
> Thomas



-- 
Thomas Petazzoni, CTO, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com