On Tue, 3 Jun 2025, 16:07 Tomasz Kaminski, <tkami...@redhat.com> wrote:
>
>
> 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?
>
No, the mangled name always depends on parameters types (except for extern
"C" functions).
So when time_t is long that function generates a symbol with a different
name to the case where time_t is long long.
>
>> > + [[__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
>> >
>>
>>
