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

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

[Adhemerval Zanella]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=02c91eb6115872219a85bb93724affd7fcdedf7a

commit 02c91eb6115872219a85bb93724affd7fcdedf7a
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Thu Aug 20 09:18:15 2020 -0300

    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.

Diff:
---
 sysdeps/unix/sysv/linux/Makefile         |  3 +-
 sysdeps/unix/sysv/linux/time64-support.c | 23 +++++++++++
 sysdeps/unix/sysv/linux/time64-support.h | 70 ++++++++++++++++++++++++++++++++
 3 files changed, 95 insertions(+), 1 deletion(-)

diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile
index 9b2a253032..5079f33655 100644
--- a/sysdeps/unix/sysv/linux/Makefile
+++ b/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
diff --git a/sysdeps/unix/sysv/linux/time64-support.c b/sysdeps/unix/sysv/linux/time64-support.c
new file mode 100644
index 0000000000..9f03c9adda
--- /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
diff --git a/sysdeps/unix/sysv/linux/time64-support.h b/sysdeps/unix/sysv/linux/time64-support.h
new file mode 100644
index 0000000000..ebba155f98
--- /dev/null
+++ b/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
+}