[PATCH v2] libstdc++: Remove dependency on std::atomic<unsigned> in tzdb.cc
Torbjorn SVENSSON
torbjorn.svensson@foss.st.com
Wed Jul 1 12:49:36 GMT 2026
Hi,
When trying to build r17-2056-g44e33b2d621d6d, I get the following compilation error:
libtool: compile: /build/r17-2056-g44e33b2d621d6dgcc-final/./gcc/xgcc -shared-libgcc -B/build/r17-2056-g44e33b2d621d6dgcc-final/./gcc -nostdinc++ -L/build/r17-2056-g44e33b2d621d6dgcc-final/arm-none-eabi/thumb/v7/nofp/libstdc++-v3/src -L/build/r17-2056-g44e33b2d621d6dgcc-final/arm-none-eabi/thumb/v7/nofp/libstdc++-v3/src/.libs -L/build/r17-2056-g44e33b2d621d6dgcc-final/arm-none-eabi/thumb/v7/nofp/libstdc++-v3/libsupc++/.libs -B/build/r17-2056-g44e33b2d621d6d/arm-none-eabi/bin/ -B/build/r17-2056-g44e33b2d621d6d/arm-none-eabi/lib/thumb/v7/nofp -B/build/r17-2056-g44e33b2d621d6d/arm-none-eabi/lib/ -isystem /build/r17-2056-g44e33b2d621d6d/arm-none-eabi/include -isystem
/build/r17-2056-g44e33b2d621d6d/arm-none-eabi/sys-include -mthumb -march=armv7 -mfloat-abi=soft -I/build/gcc_src/libstdc++-v3/../libgcc -I/build/r17-2056-g44e33b2d621d6dgcc-final/arm-none-eabi/thumb/v7/nofp/libstdc++-v3/include/arm-none-eabi -I/build/r17-2056-g44e33b2d621d6dgcc-final/arm-none-eabi/thumb/v7/nofp/libstdc++-v3/include -I/build/gcc_src/libstdc++-v3/libsupc++ -std=gnu++20 -fno-implicit-templates -Wall -Wextra -Wwrite-strings -Wcast-qual -Wabi=19 -fdiagnostics-show-location=once -ffunction-sections -fdata-sections -frandom-seed=tzdb.lo -fimplicit-templates -g -O2 -mthumb -march=armv7 -mfloat-abi=soft -c /build/gcc_src/libstdc++-v3/src/c++20/tzdb.cc -o tzdb.o
...
/build/gcc_src/libstdc++-v3/src/c++20/tzdb.cc: In static member function 'static const std::chrono::tzdb& std::chrono::tzdb_list::_Node::_S_replace_head(std::shared_ptr<std::chrono::tzdb_list::_Node>, std::shared_ptr<std::chrono::tzdb_list::_Node>)':
/build/gcc_src/libstdc++-v3/src/c++20/tzdb.cc:1617:22: error: 'struct std::chrono::tzdb_list::_Node::NumLeapSeconds' has no member named 'set_locked'
1617 | num_leap_seconds.set_locked(new_head_ptr->db.leap_seconds.size(), lock);
| ^~~~~~~~~~
Makefile:575: recipe for target 'tzdb.lo' failed
make[9]: *** [tzdb.lo] Error 1
make[9]: *** Waiting for unfinished jobs....
I am configured GCC with the following options:
/build/gcc_src/configure \
--target=arm-none-eabi \
--prefix=/build/r17-2056-g44e33b2d621d6d \
--libexecdir=/build/r17-2056-g44e33b2d621d6d/lib \
--infodir=/build/r17-2056-g44e33b2d621d6d/share/doc/gcc-arm-none-eabi/info \
--mandir=/build/r17-2056-g44e33b2d621d6d/share/doc/gcc-arm-none-eabi/man \
--htmldir=/build/r17-2056-g44e33b2d621d6d/share/doc/gcc-arm-none-eabi/html \
--pdfdir=/build/r17-2056-g44e33b2d621d6d/share/doc/gcc-arm-none-eabi/pdf \
--enable-checking=release \
--enable-languages=c,c++ \
--enable-plugins \
--disable-decimal-float \
--disable-libffi \
--disable-libgomp \
--disable-libmudflap \
--disable-libquadmath \
--disable-libssp \
--disable-libstdcxx-pch \
--disable-nls \
--disable-shared \
--disable-threads \
--disable-tls \
--with-gnu-as \
--with-gnu-ld \
--with-newlib \
--with-headers=yes \
--with-python-dir=share/gcc-arm-none-eabi \
--with-sysroot=/build/r17-2056-g44e33b2d621d6d/arm-none-eabi \
--with-zstd=no \
--build=x86_64-linux-gnu \
--host=x86_64-linux-gnu \
--with-gmp=/build/host-libs/usr \
--with-mpfr=/build/host-libs/usr \
--with-mpc=/build/host-libs/usr \
--with-isl=/build/host-libs/usr \
'--with-host-libstdcxx=-static-libgcc -Wl,-Bstatic,-lstdc++,-Bdynamic -lm' \
'--with-pkgversion=r17-2056-g44e33b2d621d6d' \
--with-multilib-list=rmprofile,aprofile
On 2026-06-30 19:41, Jonathan Wakely wrote:
> My r17-471-ge79f0f818c0e42 change to optimize handling of leap seconds
> introduced a hard dependency on std::atomic<unsigned>, which causes
> problems for targets without atomic word operations, like Cortex-M0.:
> https://gcc.gnu.org/pipermail/gcc-patches/2026-June/719704.html
>
> This patch replaces the num_leap_seconds variable with a struct which
> decides whether to use std::atomic_ref<unsigned> or perform all accesses
> while holding a lock on the pre-existing mutex used for the tzdb_list
> singleton.
>
> The workaround is a bit ugly, because it assumes that there is only one
> caller of num_leap_seconds.set and that the list_mutex() is locked by
> that caller iff the tzdb_list doesn't use atomic<shared_ptr<>>. To make
> the assumption explicit, there are two different functions used to
> update the value, depending on whether the mutex is used or not.
>
> libstdc++-v3/ChangeLog:
>
> * src/c++20/tzdb.cc (_Node::NumLeapSeconds): New class.
> (_Node::num_leap_seconds): New static variable.
> (num_leap_seconds): Remove.
> (__detail::__recent_leap_second_info): Replace uses of
> num_leap_seconds with _Node::num_leap_seconds.
> (_Node::_S_replace_head): Likewise.
> ---
>
> v2: Replace NumLeapSeconds::set by two members, set_atomically and
> set_locked.
>
> Tested x86_64-linux.
>
> libstdc++-v3/src/c++20/tzdb.cc | 119 ++++++++++++++++++++++++++-------
> 1 file changed, 94 insertions(+), 25 deletions(-)
>
> diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
> index 5793155b6d89..9e601fc176f3 100644
> --- a/libstdc++-v3/src/c++20/tzdb.cc
> +++ b/libstdc++-v3/src/c++20/tzdb.cc
> @@ -67,6 +67,7 @@
> #endif
>
> #if USE_ATOMIC_SHARED_PTR && ! USE_ATOMIC_LIST_HEAD
> +// Cannot use atomic<shared_ptr<T>> without lock-free atomic<T*>.
> # error Unsupported combination
> #endif
>
> @@ -200,6 +201,10 @@ namespace std::chrono
>
> // This is here because _Node is a friend so can call private constructor.
> static const leap_second fixed_leaps[];
> +
> + // This is a member so that it can access fixed_leaps.
> + struct NumLeapSeconds;
> + static NumLeapSeconds num_leap_seconds;
> };
>
> // Implementation of the private constructor used for the singleton object.
> @@ -1301,12 +1306,85 @@ namespace
> // The expiry date corresponding to the list above.
> // tzdata 2026a leapseconds list expires at 2026-12-28 00:00:00 UTC
> constexpr seconds fixed_expiry{1798416000u};
> -
> - // This holds the most up-to-date number of leap seconds known at runtime.
> - // Initially zero, updated when _S_read_leap_seconds() is called.
> - constinit atomic<unsigned> num_leap_seconds{0};
> }
>
> +// This holds the most up-to-date number of leap seconds known at runtime.
> +// Initially zero, updated when _S_read_leap_seconds() is called.
> +struct tzdb_list::_Node::NumLeapSeconds
> +{
> + // Called by __recent_leap_second_info to read num_leap_seconds.
> + unsigned
> + get()
> + {
> +#if ATOMIC_INT_LOCK_FREE == 2
> + atomic_ref<unsigned> ref(count);
> + auto num = ref.load(memory_order::relaxed);
> +
> + if (num == std::size(_Node::fixed_leaps))
> + // A leapseconds file has been read and has no new leap seconds.
> + return num;
> +
> + if (num == 0)
> + // No leapseconds file has been read yet.
> + return 0;
> +
> + // The tzdb_list has been initialized and contains a tzdb object with
> + // new leap seconds, which the caller is going to use.
> + // The relaxed load above does not synchronize with anything, so to
> + // ensure that the get_tzdb_list() in the caller will see a tzdb object
> + // set by _S_replace_head, we load num_leap_seconds again with acquire
> + // ordering:
> + return ref.load(memory_order::acquire);
> +#else
> + lock_guard<mutex> l(list_mutex()); // This ensures acquire ordering.
> + return count;
> +#endif
> + }
> +
> + // Called by __recent_leap_second_info to set num_leap_seconds when
> + // we have determined there are no new leap seconds in a leapseconds file.
> + void
> + set_to_fixed_size()
> + {
> +#if ATOMIC_INT_LOCK_FREE == 2
> + atomic_ref<unsigned> ref(count);
> + unsigned expected = 0;
> + ref.compare_exchange_strong(expected, std::size(_Node::fixed_leaps),
> + memory_order::relaxed);
> +#else
> + lock_guard<mutex> l(list_mutex());
> + if (count == 0)
> + count = std::size(_Node::fixed_leaps);
> +#endif
> + }
> +
> + // Called by _Node::_S_replace_head
> + // The two versions are named differently so that caller has to be explicit
> + // about which version it calls, based on whether the mutex is held.
> +#if ATOMIC_INT_LOCK_FREE == 2
> + void
> + set_atomically(unsigned val)
> + {
> + atomic_ref<unsigned> ref(count);
> + // The release op here synchronizes with the acquire op in get().
> + ref.store(val, memory_order::release);
> + }
> +#else
> + void
> + set_locked(unsigned val, const lock_guard<mutex>&)
> + {
> + // XXX The only caller of this function locks list_mutex() so we would
> + // deadlock if we locked it again here.
> + count = val;
> + }
> +#endif
> +
> +private:
> + unsigned count = 0;
> +};
> +
> +constinit tzdb_list::_Node::NumLeapSeconds tzdb_list::_Node::num_leap_seconds;
> +
> namespace __detail
> {
> // Called by chrono::__detail::__get_leap_second_info in <chrono>
> @@ -1368,19 +1446,11 @@ namespace
>
> constexpr auto num_fixed_leaps = std::size(_Node::fixed_leaps);
>
> - auto num_leaps = num_leap_seconds.load(memory_order::relaxed);
> + auto num_leaps = _Node::num_leap_seconds.get();
> if (num_leaps == num_fixed_leaps)
> // A leapseconds file has been read and has no new leap seconds:
> return update_info(_Node::fixed_leaps);
> - else if (num_leaps != 0)
> - // The tzdb_list has been initialized and contains a tzdb object
> - // with new leap seconds, which we want to use here.
> - // The relaxed load above does not synchronize with anything, so to
> - // ensure that the get_tzdb_list() below will see a tzdb object set
> - // by _S_replace_head, we load num_leap_seconds again with acquire
> - // ordering:
> - (void) num_leap_seconds.load(memory_order::acquire);
> - else
> + else if (num_leaps == 0)
> {
> // The tzdb_list has not been initialized yet, so we don't know
> // the correct number of leap seconds.
> @@ -1389,11 +1459,9 @@ namespace
> // to parse all of tzdata.zi and initialize a whole tzdb object.
> if (_Node::_S_read_leap_seconds().first.size() == num_fixed_leaps)
> {
> - // There are no new leap seconds. remember that so that the next
> + // There are no new leap seconds. Remember that so that the next
> // call to this function can just use fixed_leaps.
> - num_leap_seconds.compare_exchange_strong(num_leaps,
> - num_fixed_leaps,
> - memory_order::relaxed);
> + _Node::num_leap_seconds.set_to_fixed_size();
> return update_info(_Node::fixed_leaps);
> }
> // else there are new leap seconds. We init tzdb_list so that the
> @@ -1525,8 +1593,13 @@ namespace
> new_head_ptr->next = curr;
> }
> // XXX small window here where _S_head_cache still points to previous tzdb.
> + _S_cache_list_head(new_head_ptr);
> +
> + // This allows __recent_leap_second_info() to know that it can use
> + // get_tzdb_list()->begin()->leap_seconds to get new leap seconds.
> + num_leap_seconds.set_atomically(new_head_ptr->db.leap_seconds.size());
> #else
> - lock_guard<mutex> l(list_mutex());
> + lock_guard<mutex> lock(list_mutex());
> if (const _Node* h = _S_head_owner.get())
> {
> if (h->db.version == new_head_ptr->db.version)
> @@ -1534,14 +1607,10 @@ namespace
> new_head_ptr->next = _S_head_owner;
> }
> _S_head_owner = std::move(new_head);
> -#endif
> _S_cache_list_head(new_head_ptr);
>
> - // This allows __recent_leap_second_info() to know that it can use
> - // get_tzdb_list()->begin()->leap_seconds to get new leap seconds.
> - // The release op here synchronizes with the acquire op there.
> - num_leap_seconds.store(new_head_ptr->db.leap_seconds.size(),
> - memory_order::release);
> + num_leap_seconds.set_locked(new_head_ptr->db.leap_seconds.size(), lock);
Looks like the pre-processor condition here is missaligned with the preprocessor condition for the definition of set_locked().
Hope this helps to figure out how to get trunk buildable again.
Kind regards,
Torbjörn
> +#endif
>
> return new_head_ptr->db;
> }
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