compiler.h

   1 #ifndef _COMPILER_H
   2 #define _COMPILER_H
   3
   4 /*
   5  * compiler.h
   6  *
   7  * Compiler definitions.
   8  *
   9  * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
  10  *
  11  * This library is free software; you can redistribute it and/or
  12  * modify it under the terms of the GNU Lesser General Public
  13  * License as published by the Free Software Foundation; either
  14  * version 2.1 of the License, or (at your option) any later version.
  15 *
  16  * This library is distributed in the hope that it will be useful,
  17  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  18  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  19  * Lesser General Public License for more details.
  20  *
  21  * You should have received a copy of the GNU Lesser General Public
  22  * License along with this library; if not, write to the Free Software
  23  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  24  *
  25  * IBM's contributions to this file may be relicensed under LGPLv2 or later.
  26  */
  27
  28 #define likely()        __builtin_expect(!!(x), 1)
  29 #define unlikely()      __builtin_expect(!!(x), 0)
  30
  31 #define barrier()       asm volatile("" : : : "memory");
  32
  33 /*
  34  * Instruct the compiler to perform only a single access to a variable
  35  * (prohibits merging and refetching). The compiler is also forbidden to reorder
  36  * successive instances of ACCESS_ONCE(), but only when the compiler is aware of
  37  * particular ordering. Compiler ordering can be ensured, for example, by
  38  * putting two ACCESS_ONCE() in separate C statements.
  39  *
  40  * This macro does absolutely -nothing- to prevent the CPU from reordering,
  41  * merging, or refetching absolutely anything at any time.  Its main intended
  42  * use is to mediate communication between process-level code and irq/NMI
  43  * handlers, all running on the same CPU.
  44  */
  45 #define ACCESS_ONCE(x)  (*(volatile typeof(x) *)&x)
  46
  47 #endif /* _COMPILER_H */