libstdc++: Make system_clock::to_time_t always_inline [PR99832]

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 so that all callers inline
the conversion to time_t, and will do so using whatever type time_t
happens to be in that translation unit.

Existing objects compiled before this change will either have inlined
the function anyway (which is likely if compiled with any optimization
enabled) or will contain a COMDAT definition of the inline function and
so still be able to find it at link-time.

The attribute is also added to system_clock::from_time_t, because that's
an equally simple function and it seems reasonable for them to both be
always inlined.

libstdc++-v3/ChangeLog:

	PR libstdc++/99832
	* include/bits/chrono.h (system_clock::to_time_t): Add
	always_inline attribute to be agnostic to the underlying type of
	time_t.
	(system_clock::from_time_t): Add always_inline for consistency
	with to_time_t.
	* testsuite/20_util/system_clock/99832.cc: New test.

0 files changed, 0 insertions, 0 deletions