Bug 99832 - std::chrono::system_clock::to_time_t needs ABI tag for 32-bit time_t
Summary: std::chrono::system_clock::to_time_t needs ABI tag for 32-bit time_t
Status: NEW
Alias: None
Product: gcc
Classification: Unclassified
Component: libstdc++ (show other bugs)
Version: 11.0
: P3 normal
Target Milestone: 14.2
Assignee: Not yet assigned to anyone
URL:
Keywords: ABI
Depends on:
Blocks:
 
Reported: 2021-03-30 16:01 UTC by Jonathan Wakely
Modified: 2024-05-07 07:39 UTC (History)
2 users (show)

See Also:
Host:
Target:
Build:
Known to work:
Known to fail:
Last reconfirmed: 2023-06-10 00:00:00


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Description Jonathan Wakely 2021-03-30 16:01:09 UTC
chrono::system_clock defines:

      static std::time_t
      to_time_t(const time_point& __t) noexcept

This is a non-template function and its mangled name does not depend on the return type. The mangled name is:

_ZNSt6chrono3_V212system_clock9to_time_tERKNS_10time_pointIS1_NS_8durationIlSt5ratioILl1ELl1000000000EEEEEE

For a target that allows time_t to be either 32-bit or 64-bit, we need this mangled name to reflect the type of time_t.
Comment 1 Jonathan Wakely 2024-01-31 11:49:04 UTC
Maybe something like this:

diff --git a/libstdc++-v3/config/os/gnu-linux/os_defines.h b/libstdc++-v3/config/os/gnu-linux/os_defines.h
index 0af29325388..f7c73560831 100644
--- a/libstdc++-v3/config/os/gnu-linux/os_defines.h
+++ b/libstdc++-v3/config/os/gnu-linux/os_defines.h
@@ -84,7 +84,13 @@
 // Since glibc 2.34 all pthreads functions are usable without linking to
 // libpthread.
 #  define _GLIBCXX_GTHREAD_USE_WEAK 0
-# endif
+// Since glibc 2.34 using -D_TIME_BITS=64 will enable 64-bit time_t
+// for "legacy ABIs", i.e. ones that historically used 32-bit time_t.
+// This internal glibc macro will be defined iff new 64-bit time_t is in use.
+#  ifdef __USE_TIME_BITS64
+#   define _GLIBCXX_TIME_BITS64 1
+#  endif
+# endif // glibc 2.34
 #endif // __linux__
 
 #endif
diff --git a/libstdc++-v3/include/bits/chrono.h b/libstdc++-v3/include/bits/chrono.h
index 579c5a266be..a63782b92ff 100644
--- a/libstdc++-v3/include/bits/chrono.h
+++ b/libstdc++-v3/include/bits/chrono.h
@@ -1242,6 +1242,9 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
       now() noexcept;
 
       // Map to C API
+#ifdef _GLIBCXX_TIME_BITS64
+      [[__gnu__::__abi_tag__("__time64")]]
+#endif
       static std::time_t
       to_time_t(const time_point& __t) noexcept
       {
@@ -1249,6 +1252,9 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
                           (__t.time_since_epoch()).count());
       }
 
+#ifdef _GLIBCXX_TIME_BITS64
+      [[__gnu__::__abi_tag__("__time64")]]
+#endif
       static time_point
       from_time_t(std::time_t __t) noexcept
       {

Alternatively, in <bits/c++config.h> do:

#define _GLIBCXX_TIME_BITS64_ABI_TAG

and then in config/os/gnu-linux/os_defines.h:


#  ifdef __USE_TIME_BITS64
#   undef _GLIBCXX_TIME_BITS64_ABI_TAG
#   define _GLIBCXX_TIME_BITS64_ABI_TAG [[__gnu__::__abi_tag__("__time64")]]
#  endif

Then the chrono code can just use that unconditionally instead of using #ifdef

I think for musl, newer versions use 64-bit time_t unconditionally. I'm not sure if we can (or need to) use the abi_tag there.
Comment 2 Jonathan Wakely 2024-01-31 12:05:27 UTC
(In reply to Jonathan Wakely from comment #1)
> +// Since glibc 2.34 using -D_TIME_BITS=64 will enable 64-bit time_t
> +// for "legacy ABIs", i.e. ones that historically used 32-bit time_t.
> +// This internal glibc macro will be defined iff new 64-bit time_t is in
> use.

This is correct for current glibc releases, but in glibc master __USE_TIME_BITS64 is defined unconditionally to 0 or 1 and tells you the size of time_t, not whether it switched to 64-bit counter to the legacy ABI:
https://inbox.sourceware.org/libc-alpha/20240118131801.600373-1-adhemerval.zanella@linaro.org/

Yay.
Comment 3 Richard Biener 2024-05-07 07:39:56 UTC
GCC 14.1 is being released, retargeting bugs to GCC 14.2.