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. 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. 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 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.
