/* force-elision.h: Automatic enabling of elision for mutexes

   Copyright (C) 2015-2018 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

   <http://www.gnu.org/licenses/>.  */

/* Automatically enable elision for existing user lock kinds.  */

#define FORCE_ELISION(m, s)						\

  if (__pthread_force_elision)						\

    {									\

      /* See concurrency notes regarding __kind in			\

	 struct __pthread_mutex_s in					\

	 sysdeps/nptl/bits/thread-shared-types.h.			\

									\

	 There are the following cases for the kind of a mutex		\

	 (The mask PTHREAD_MUTEX_ELISION_FLAGS_NP covers the flags	\

	 PTHREAD_MUTEX_ELISION_NP and PTHREAD_MUTEX_NO_ELISION_NP where	\

	 only one of both flags can be set):				\

	 - both flags are not set:					\

	 This is the first lock operation for this mutex.  Enable	\

	 elision as it is not enabled so far.				\

	 Note: It can happen that multiple threads are calling e.g.	\

	 pthread_mutex_lock at the same time as the first lock		\

	 operation for this mutex.  Then elision is enabled for this	\

	 mutex by multiple threads.  Storing with relaxed MO is enough	\

	 as all threads will store the same new value for the kind of	\

	 the mutex.  But we have to ensure that we always use the	\

	 elision path regardless if this thread has enabled elision or	\

	 another one.							\

									\

	 - PTHREAD_MUTEX_ELISION_NP flag is set:			\

	 Elision was already enabled for this mutex by a previous lock	\

	 operation.  See case above.  Just use the elision path.	\

									\

	 - PTHREAD_MUTEX_NO_ELISION_NP flag is set:			\

	 Elision was explicitly disabled by pthread_mutexattr_settype.	\

	 Do not use the elision path.					\

	 Note: The flag PTHREAD_MUTEX_NO_ELISION_NP will never be	\

	 changed after mutex initialization.  */			\

      int mutex_kind = atomic_load_relaxed (&((m)->__data.__kind));	\

      if ((mutex_kind & PTHREAD_MUTEX_ELISION_FLAGS_NP) == 0)		\

	{								\

	  mutex_kind |= PTHREAD_MUTEX_ELISION_NP;			\

	  atomic_store_relaxed (&((m)->__data.__kind), mutex_kind);	\

	}								\

      if ((mutex_kind & PTHREAD_MUTEX_ELISION_NP) != 0)			\

	{								\

	  s;								\

	}								\

    }