linux: Add helper function to optimize 64-bit time_t fallback support

These helper functions are used to optimize the 64-bit time_t support on
configurations that requires support for 32-bit time_t fallback
(!__ASSUME_TIME64_SYSCALLS).  The idea is once the kernel advertises that
it does not have 64-bit time_t support, glibc will stop to try issue the
64-bit time_t syscall altogether.

For instance:

  #ifndef __NR_symbol_time64
  # define __NR_symbol_time64 __NR_symbol
  #endif
  int r;
  if (supports_time64 ())
    {
      r = INLINE_SYSCALL_CALL (symbol, ...);
      if (r == 0 || errno != ENOSYS)
        return r;

      mark_time64_unsupported ();
    }
  #ifndef __ASSUME_TIME64_SYSCALLS
  <32-bit fallback syscall>
  #endif
  return r;

On configuration with default 64-bit time_t this optimization should be
optimized away by the compiler resulting in no overhead.

sysdeps/unix/sysv/linux/Makefile

@@ -60,7 +60,8 @@ sysdep_routines += adjtimex clone umount umount2 readahead sysctl \
 		   personality epoll_wait tee vmsplice splice \
 		   open_by_handle_at mlock2 pkey_mprotect pkey_set pkey_get \
 		   timerfd_gettime timerfd_settime prctl \
-		   process_vm_readv process_vm_writev clock_adjtime
+		   process_vm_readv process_vm_writev clock_adjtime \
+		   time64-support
 
 CFLAGS-gethostid.c = -fexceptions
 CFLAGS-tee.c = -fexceptions -fasynchronous-unwind-tables

sysdeps/unix/sysv/linux/time64-support.c

@@ -0,0 +1,23 @@
+/* Auxiliary definitions for 64-bit time_t support.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <time64-support.h>
+
+#ifndef __ASSUME_TIME64_SYSCALLS
+int __time64_support = 1;
+#endif

sysdeps/unix/sysv/linux/time64-support.h

@@ -0,0 +1,70 @@
+/* Auxiliary definitions for 64-bit time_t support.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdbool.h>
+#include <atomic.h>
+
+/* These helper functions are used to optimize the 64-bit time_t support on
+   configurations that requires support for 32-bit time_t fallback
+   (!__ASSUME_TIME64_SYSCALLS).  The idea is once the kernel advertises that
+   it does not have 64-bit time_t support, glibc will stop to try issue the
+   64-bit time_t syscall altogether.
+
+   For instance:
+
+     #ifndef __NR_symbol_time64
+     # define __NR_symbol_time64 __NR_symbol
+     #endif
+     int r;
+     if (supports_time64 ())
+       {
+         r = INLINE_SYSCALL_CALL (symbol, ...);
+         if (r == 0 || errno != ENOSYS)
+	   return r;
+
+         mark_time64_unsupported ();
+       }
+     #ifndef __ASSUME_TIME64_SYSCALLS
+     <32-bit fallback syscall>
+     #endif
+     return r;
+
+   On configuration with default 64-bit time_t this optimization should be
+   optimized away by the compiler resulting in no overhead.  */
+
+#ifndef __ASSUME_TIME64_SYSCALLS
+extern int __time64_support attribute_hidden;
+#endif
+
+static inline bool
+supports_time64 (void)
+{
+#ifdef __ASSUME_TIME64_SYSCALLS
+  return true;
+#else
+  return atomic_load_relaxed (&__time64_support) != 0;
+#endif
+}
+
+static inline void
+mark_time64_unsupported (void)
+{
+#ifndef __ASSUME_TIME64_SYSCALLS
+  atomic_store_relaxed (&__time64_support, 0);
+#endif
+}