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 MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
  12  * OR IMPLIED.  ANY USE IS AT YOUR OWN RISK.
  13  *
  14  * Permission is hereby granted to use or copy this program
  15  * for any purpose,  provided the above notices are retained on all copies.
  16  * Permission to modify the code and to distribute modified code is granted,
  17  * provided the above notices are retained, and a notice that the code was
  18  * modified is included with the above copyright notice.
  19  */
  20
  21 #define likely(x)       __builtin_expect(!!(x), 1)
  22 #define unlikely(x)     __builtin_expect(!!(x), 0)
  23
  24 #define barrier()       asm volatile("" : : : "memory");
  25
  26 /*
  27  * Instruct the compiler to perform only a single access to a variable
  28  * (prohibits merging and refetching). The compiler is also forbidden to reorder
  29  * successive instances of ACCESS_ONCE(), but only when the compiler is aware of
  30  * particular ordering. Compiler ordering can be ensured, for example, by
  31  * putting two ACCESS_ONCE() in separate C statements.
  32  *
  33  * This macro does absolutely -nothing- to prevent the CPU from reordering,
  34  * merging, or refetching absolutely anything at any time.  Its main intended
  35  * use is to mediate communication between process-level code and irq/NMI
  36  * handlers, all running on the same CPU.
  37  */
  38 #define ACCESS_ONCE(x)  (*(volatile typeof(x) *)&x)
  39
  40 #endif /* _COMPILER_H */