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#ifdef __ANDROID__
  13/*
  14 * FIXME: Big hammer to get rid of tons of:
  15 *   "warning: always_inline function might not be inlinable"
  16 *
  17 * At least on android-ndk-r12/platforms/android-24/arch-arm
  18 */
  19#undef __always_inline
  20#define __always_inline inline
  21#endif
  22
  23#define __user
  24
  25#ifndef __attribute_const__
  26# define __attribute_const__
  27#endif
  28
  29#ifndef __maybe_unused
  30# define __maybe_unused         __attribute__((unused))
  31#endif
  32
  33#ifndef __packed
  34# define __packed               __attribute__((__packed__))
  35#endif
  36
  37#ifndef __force
  38# define __force
  39#endif
  40
  41#ifndef __weak
  42# define __weak                 __attribute__((weak))
  43#endif
  44
  45#ifndef likely
  46# define likely(x)              __builtin_expect(!!(x), 1)
  47#endif
  48
  49#ifndef unlikely
  50# define unlikely(x)            __builtin_expect(!!(x), 0)
  51#endif
  52
  53#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
  54
  55#include <linux/types.h>
  56
  57/*
  58 * Following functions are taken from kernel sources and
  59 * break aliasing rules in their original form.
  60 *
  61 * While kernel is compiled with -fno-strict-aliasing,
  62 * perf uses -Wstrict-aliasing=3 which makes build fail
  63 * under gcc 4.4.
  64 *
  65 * Using extra __may_alias__ type to allow aliasing
  66 * in this case.
  67 */
  68typedef __u8  __attribute__((__may_alias__))  __u8_alias_t;
  69typedef __u16 __attribute__((__may_alias__)) __u16_alias_t;
  70typedef __u32 __attribute__((__may_alias__)) __u32_alias_t;
  71typedef __u64 __attribute__((__may_alias__)) __u64_alias_t;
  72
  73static __always_inline void __read_once_size(const volatile void *p, void *res, int size)
  74{
  75        switch (size) {
  76        case 1: *(__u8_alias_t  *) res = *(volatile __u8_alias_t  *) p; break;
  77        case 2: *(__u16_alias_t *) res = *(volatile __u16_alias_t *) p; break;
  78        case 4: *(__u32_alias_t *) res = *(volatile __u32_alias_t *) p; break;
  79        case 8: *(__u64_alias_t *) res = *(volatile __u64_alias_t *) p; break;
  80        default:
  81                barrier();
  82                __builtin_memcpy((void *)res, (const void *)p, size);
  83                barrier();
  84        }
  85}
  86
  87static __always_inline void __write_once_size(volatile void *p, void *res, int size)
  88{
  89        switch (size) {
  90        case 1: *(volatile  __u8_alias_t *) p = *(__u8_alias_t  *) res; break;
  91        case 2: *(volatile __u16_alias_t *) p = *(__u16_alias_t *) res; break;
  92        case 4: *(volatile __u32_alias_t *) p = *(__u32_alias_t *) res; break;
  93        case 8: *(volatile __u64_alias_t *) p = *(__u64_alias_t *) res; break;
  94        default:
  95                barrier();
  96                __builtin_memcpy((void *)p, (const void *)res, size);
  97                barrier();
  98        }
  99}
 100
 101/*
 102 * Prevent the compiler from merging or refetching reads or writes. The
 103 * compiler is also forbidden from reordering successive instances of
 104 * READ_ONCE, WRITE_ONCE and ACCESS_ONCE (see below), but only when the
 105 * compiler is aware of some particular ordering.  One way to make the
 106 * compiler aware of ordering is to put the two invocations of READ_ONCE,
 107 * WRITE_ONCE or ACCESS_ONCE() in different C statements.
 108 *
 109 * In contrast to ACCESS_ONCE these two macros will also work on aggregate
 110 * data types like structs or unions. If the size of the accessed data
 111 * type exceeds the word size of the machine (e.g., 32 bits or 64 bits)
 112 * READ_ONCE() and WRITE_ONCE()  will fall back to memcpy and print a
 113 * compile-time warning.
 114 *
 115 * Their two major use cases are: (1) Mediating communication between
 116 * process-level code and irq/NMI handlers, all running on the same CPU,
 117 * and (2) Ensuring that the compiler does not  fold, spindle, or otherwise
 118 * mutilate accesses that either do not require ordering or that interact
 119 * with an explicit memory barrier or atomic instruction that provides the
 120 * required ordering.
 121 */
 122
 123#define READ_ONCE(x) \
 124        ({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
 125
 126#define WRITE_ONCE(x, val) \
 127        ({ union { typeof(x) __val; char __c[1]; } __u = { .__val = (val) }; __write_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
 128
 129#endif /* _TOOLS_LINUX_COMPILER_H */
 130