On Mon, 11 Sept 2023 at 17:22, Jonathan Wakely <jwak...@redhat.com> wrote:
> On Mon, 11 Sept 2023 at 14:57, Christophe Lyon > <christophe.l...@linaro.org> wrote: > > > > > > > > On Mon, 11 Sept 2023 at 15:12, Jonathan Wakely <jwak...@redhat.com> > wrote: > >> > >> On Mon, 11 Sept 2023 at 13:36, Christophe Lyon > >> <christophe.l...@linaro.org> wrote: > >> > > >> > > >> > > >> > On Mon, 11 Sept 2023 at 12:59, Jonathan Wakely <jwak...@redhat.com> > wrote: > >> >> > >> >> On Sun, 10 Sept 2023 at 20:31, Christophe Lyon > >> >> <christophe.l...@linaro.org> wrote: > >> >> > > >> >> > Some targets like arm-eabi with newlib and default settings rely on > >> >> > __sync_synchronize() to ensure synchronization. Newlib does not > >> >> > implement it by default, to make users aware they have to take > special > >> >> > care. > >> >> > > >> >> > This makes a few tests fail to link. > >> >> > >> >> Does this mean those features are unusable on the target, or just > that > >> >> users need to provide their own __sync_synchronize to use them? > >> > > >> > > >> > IIUC the user is expected to provide them. > >> > Looks like we discussed this in the past :-) > >> > In https://gcc.gnu.org/legacy-ml/gcc-patches/2016-10/msg01632.html, > >> > see the pointer to Ramana's comment: > https://gcc.gnu.org/ml/gcc-patches/2015-05/msg02751.html > >> > >> Oh yes, thanks for the reminder! > >> > >> > > >> > The default arch for arm-eabi is armv4t which is very old. > >> > When running the testsuite with something more recent (either as > default by configuring GCC --with-arch=XXX or by forcing -march/-mcpu via > dejagnu's target-board), the compiler generates barrier instructions and > there are no such errors. > >> > >> Ah yes, that's fine then. > >> > >> > For instance, here is a log with the defaults: > >> > > https://git.linaro.org/toolchain/ci/base-artifacts/tcwg_gnu_embed_check_gcc/master-arm_eabi.git/tree/00-sumfiles?h=linaro-local/ci/tcwg_gnu_embed_check_gcc/master-arm_eabi > >> > and a log when we target cortex-m0 which is still a very small cpu > but has barriers: > >> > > https://git.linaro.org/toolchain/ci/base-artifacts/tcwg_gnu_embed_check_gcc/master-thumb_m0_eabi.git/tree/00-sumfiles?h=linaro-local/ci/tcwg_gnu_embed_check_gcc/master-thumb_m0_eabi > >> > > >> > I somehow wanted to get rid of such errors with the default > configuration.... > >> > >> Yep, that makes sense, and we'll still be testing them for newer > >> arches on the target, so it's not completely disabling those parts of > >> the testsuite. > >> > >> But I'm still curious why some of those tests need this change. I > >> think the ones I noted below are probably failing for some other > >> reasons. > >> > > Just looked at 23_containers/span/back_assert_neg.cc, the linker says > it needs > > arm-eabi/libstdc++-v3/src/.libs/libstdc++.a(debug.o) to resolve > > ./back_assert_neg-back_assert_neg.o (std::__glibcxx_assert_fail(char > const*, int, char const*, char const*)) > > and indeed debug.o has a reference to __sync_synchronize > > Aha, that's just because I put __glibcxx_assert_fail in debug.o, but > there are no dependencies on anything else in that file, including the > _M_detach member function that uses atomics. > indeed > This would also be solved by -Wl,--gc-sections :-) > :-) > I think it would be better to move __glibcxx_assert_fail to a new > file, so that it doesn't make every assertion unnecessarily depend on > __sync_synchronize. I'll do that now. > Sounds like a good idea, thanks. > We could also make the atomics in debug.o conditional, so that debug > mode doesn't depend on __sync_synchronize for single-threaded targets. > Does the arm4t arch have pthreads support in newlib? I didn't bother > No ( grep _GLIBCXX_HAS_GTHREADS $objdir/arm-eabi/libstdc++-v3/include/arm-eabi/bits/c++config returns: /* #undef _GLIBCXX_HAS_GTHREADS */ making the use of atomics conditional, because performance is not > really a priority for debug mode bookkeeping. But the problem here > isn't just a slight performance overhead of atomics, it's that they > aren't even supported for arm4t. > OK thanks. So finally, this uncovered at least a "bug" that __glibcxx_assert_fail should be in a dedicated object file :-) I'll revisit my patch once you have moved __glibcxx_assert_fail Thanks, Christophe
