LXR linux/tools/include/linux/compiler.h

   1#ifndef _TOOLS_LINUX_COMPILER_H_
   2#define _TOOLS_LINUX_COMPILER_H_
   3
   4/* Optimization barrier */
   5/* The "volatile" is due to gcc bugs */
   6#define barrier() __asm__ __volatile__("": : :"memory")
   7
   8#ifndef __always_inline
   9# define __always_inline        inline __attribute__((always_inline))
  10#endif
  11
  12#define __user
  13
  14#ifndef __attribute_const__
  15# define __attribute_const__
  16#endif
  17
  18#ifndef __maybe_unused
  19# define __maybe_unused         __attribute__((unused))
  20#endif
  21
  22#ifndef __packed
  23# define __packed               __attribute__((__packed__))
  24#endif
  25
  26#ifndef __force
  27# define __force
  28#endif
  29
  30#ifndef __weak
  31# define __weak                 __attribute__((weak))
  32#endif
  33
  34#ifndef likely
  35# define likely(x)              __builtin_expect(!!(x), 1)
  36#endif
  37
  38#ifndef unlikely
  39# define unlikely(x)            __builtin_expect(!!(x), 0)
  40#endif
  41
  42#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
  43
  44#include <linux/types.h>
  45
  46/*
  47 * Following functions are taken from kernel sources and
  48 * break aliasing rules in their original form.
  49 *
  50 * While kernel is compiled with -fno-strict-aliasing,
  51 * perf uses -Wstrict-aliasing=3 which makes build fail
  52 * under gcc 4.4.
  53 *
  54 * Using extra __may_alias__ type to allow aliasing
  55 * in this case.
  56 */
  57typedef __u8  __attribute__((__may_alias__))  __u8_alias_t;
  58typedef __u16 __attribute__((__may_alias__)) __u16_alias_t;
  59typedef __u32 __attribute__((__may_alias__)) __u32_alias_t;
  60typedef __u64 __attribute__((__may_alias__)) __u64_alias_t;
  61
  62static __always_inline void __read_once_size(const volatile void *p, void *res, int size)
  63{
  64        switch (size) {
  65        case 1: *(__u8_alias_t  *) res = *(volatile __u8_alias_t  *) p; break;
  66        case 2: *(__u16_alias_t *) res = *(volatile __u16_alias_t *) p; break;
  67        case 4: *(__u32_alias_t *) res = *(volatile __u32_alias_t *) p; break;
  68        case 8: *(__u64_alias_t *) res = *(volatile __u64_alias_t *) p; break;
  69        default:
  70                barrier();
  71                __builtin_memcpy((void *)res, (const void *)p, size);
  72                barrier();
  73        }
  74}
  75
  76static __always_inline void __write_once_size(volatile void *p, void *res, int size)
  77{
  78        switch (size) {
  79        case 1: *(volatile  __u8_alias_t *) p = *(__u8_alias_t  *) res; break;
  80        case 2: *(volatile __u16_alias_t *) p = *(__u16_alias_t *) res; break;
  81        case 4: *(volatile __u32_alias_t *) p = *(__u32_alias_t *) res; break;
  82        case 8: *(volatile __u64_alias_t *) p = *(__u64_alias_t *) res; break;
  83        default:
  84                barrier();
  85                __builtin_memcpy((void *)p, (const void *)res, size);
  86                barrier();
  87        }
  88}
  89
  90/*
  91 * Prevent the compiler from merging or refetching reads or writes. The
  92 * compiler is also forbidden from reordering successive instances of
  93 * READ_ONCE, WRITE_ONCE and ACCESS_ONCE (see below), but only when the
  94 * compiler is aware of some particular ordering.  One way to make the
  95 * compiler aware of ordering is to put the two invocations of READ_ONCE,
  96 * WRITE_ONCE or ACCESS_ONCE() in different C statements.
  97 *
  98 * In contrast to ACCESS_ONCE these two macros will also work on aggregate
  99 * data types like structs or unions. If the size of the accessed data
 100 * type exceeds the word size of the machine (e.g., 32 bits or 64 bits)
 101 * READ_ONCE() and WRITE_ONCE()  will fall back to memcpy and print a
 102 * compile-time warning.
 103 *
 104 * Their two major use cases are: (1) Mediating communication between
 105 * process-level code and irq/NMI handlers, all running on the same CPU,
 106 * and (2) Ensuring that the compiler does not  fold, spindle, or otherwise
 107 * mutilate accesses that either do not require ordering or that interact
 108 * with an explicit memory barrier or atomic instruction that provides the
 109 * required ordering.
 110 */
 111
 112#define READ_ONCE(x) \
 113        ({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
 114
 115#define WRITE_ONCE(x, val) \
 116        ({ union { typeof(x) __val; char __c[1]; } __u = { .__val = (val) }; __write_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
 117
 118#endif /* _TOOLS_LINUX_COMPILER_H */
 119