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

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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.
This commit is contained in:
Adhemerval Zanella 2020-08-20 09:18:15 -03:00
parent 756c306502
commit 02c91eb611
3 changed files with 95 additions and 1 deletions
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3
sysdeps/unix/sysv/linux/Makefile
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@ -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

23
sysdeps/unix/sysv/linux/time64-support.c Normal file
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@ -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

70
sysdeps/unix/sysv/linux/time64-support.h Normal file
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@ -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
}