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.

1 files changed, 23 insertions, 0 deletions

diff --git a/sysdeps/unix/sysv/linux/time64-support.c b/sysdeps/unix/sysv/linux/time64-support.c
new file mode 100644
index 0000000..9f03c9a
--- /dev/null
+++ b/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