On Tue, Jun 3, 2025 at 4:40 PM Jonathan Wakely <jwak...@redhat.com> wrote:
> On Tue, 3 Jun 2025 at 14:46, Jonathan Wakely <jwak...@redhat.com> wrote:
> >
> > For some 32-bit targets Glibc supports changing the size of time_t to be
> > 64 bits by defining _TIME_BITS=64. That causes an ABI change which
> > would affect std::chrono::system_clock::to_time_t. Because to_time_t is
> > not a function template, its mangled name does not depend on the return
> > type, so it has the same mangled name whether it returns a 32-bit time_t
> > or a 64-bit time_t. On targets where the size of time_t can be selected
> > at preprocessing time, that can cause ODR violations, e.g. the linker
> > selects a definition of to_time_t that returns a 32-bit value but a
> > caller expects 64-bit and so reads 32 bits of garbage from the stack.
> >
> > This commit adds always_inline to to_time_t when time_t has been changed
> > from a 32-bit type to a 64-bit type by defining _TIME_BITS=64. This
> > ensures that callers expecting a 64-bit time_t can't link to a
> > definition returning a 32-bit time_t.
> >
> > We use the internal Glibc macro __USE_TIME64_REDIRECTS to detect the
> > case where time_t defaults to 32-bit for the target but has been
> > explicitly changed to 64-bit by the user.
> >
> > libstdc++-v3/ChangeLog:
> >
> > PR libstdc++/99832
> > * include/bits/chrono.h (system_clock::to_time_t): Add always
> > inline_attribute for 64-bit time_t on 32-bit target.
> > * testsuite/20_util/system_clock/time64.cc: New test.
> > ---
> >
> > Tested x86_64-linux (-m64 and -m32, with recent glibc).
> >
> > libstdc++-v3/include/bits/chrono.h | 3 +++
> > .../testsuite/20_util/system_clock/time64.cc | 21 +++++++++++++++++++
> > 2 files changed, 24 insertions(+)
> > create mode 100644 libstdc++-v3/testsuite/20_util/system_clock/time64.cc
> >
> > diff --git a/libstdc++-v3/include/bits/chrono.h
> b/libstdc++-v3/include/bits/chrono.h
> > index fad216203d2f..5c6ee759b381 100644
> > --- a/libstdc++-v3/include/bits/chrono.h
> > +++ b/libstdc++-v3/include/bits/chrono.h
> > @@ -1244,6 +1244,9 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
> > now() noexcept;
> >
> > // Map to C API
> > +#ifdef __USE_TIME64_REDIRECTS
>
> Florian suggested not relying on this internal glibc macro (which was
> not present in the first versions of glibc to support _TIME_BITS=64).
>
> We can just make the always_inline attribute unconditional, which does
> no harm. It's a tiny function that just extracts an integer from the
> time and does an integer division to convert nanoseconds to seconds,
> so always_inline is appropriate. And this way no targets will get a
> dependency on any to_time_t symbol, with any mangling or any return
> type.
>
> Existing objects which were already compiled before the attribute was
> added will still work, because the function is inline so those objects
> will already have a COMDAT definition of the symbol (or will have
> inlined it anyway).
>
> The only case that can't work is linking together existing objects
> which were compiled with and without -D_TIME_BITS=64 before libstdc++
> knew how to support that macro, but we can't fix those, the objects
> compiled with -D_TIME_BITS=64 might need to be recompiled.
>
Does from_time_t have the same problem, where arguments have different
width?
>
> > + [[__gnu__::__always_inline__]]
> > +#endif
> > static std::time_t
> > to_time_t(const time_point& __t) noexcept
> > {
> > diff --git a/libstdc++-v3/testsuite/20_util/system_clock/time64.cc
> b/libstdc++-v3/testsuite/20_util/system_clock/time64.cc
> > new file mode 100644
> > index 000000000000..3cbf80e0f06e
> > --- /dev/null
> > +++ b/libstdc++-v3/testsuite/20_util/system_clock/time64.cc
> > @@ -0,0 +1,21 @@
> > +// { dg-options "-D_FILE_OFFSET_BITS=64 -D_TIME_BITS=64 -O0 -g0" }
> > +// { dg-do compile { target *-*-linux-gnu } }
> > +// { dg-require-effective-target c++20 }
> > +// { dg-require-effective-target ilp32 }
> > +// { dg-final { scan-assembler-not "system_clock9to_time_t" } }
> > +
> > +#include <chrono>
> > +
> > +template<typename T>
> > +std::time_t
> > +test()
> > +{
> > + using std::chrono::system_clock;
> > +
> > + if constexpr (sizeof(T) == 8)
> > + return system_clock::to_time_t(system_clock::now());
> > + else // _TIME_BITS=64 had no effect, maybe an old Glibc
> > + return 0;
> > +}
> > +
> > +auto t = test<std::time_t>();
> > --
> > 2.49.0
> >
>
>
