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  */
  57 typedef __u8  __attribute__((__may_alias__))  __u8_alias_t;
  58 typedef __u16 __attribute__((__may_alias__)) __u16_alias_t;
  59 typedef __u32 __attribute__((__may_alias__)) __u32_alias_t;
  60 typedef __u64 __attribute__((__may_alias__)) __u64_alias_t;
  61
  62 static __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
  76 static __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 */