1 #ifndef _TOOLS_LINUX_COMPILER_H_
2 #define _TOOLS_LINUX_COMPILER_H_
4 /* Optimization barrier */
5 /* The "volatile" is due to gcc bugs */
6 #define barrier() __asm__ __volatile__("": : :"memory")
8 #ifndef __always_inline
9 # define __always_inline inline __attribute__((always_inline))
14 #ifndef __attribute_const__
15 # define __attribute_const__
18 #ifndef __maybe_unused
19 # define __maybe_unused __attribute__((unused))
23 # define __packed __attribute__((__packed__))
31 # define __weak __attribute__((weak))
35 # define likely(x) __builtin_expect(!!(x), 1)
39 # define unlikely(x) __builtin_expect(!!(x), 0)
42 #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
44 #include <linux/types.h>
46 static __always_inline void __read_once_size(const volatile void *p, void *res, int size)
49 case 1: *(__u8 *)res = *(volatile __u8 *)p; break;
50 case 2: *(__u16 *)res = *(volatile __u16 *)p; break;
51 case 4: *(__u32 *)res = *(volatile __u32 *)p; break;
52 case 8: *(__u64 *)res = *(volatile __u64 *)p; break;
55 __builtin_memcpy((void *)res, (const void *)p, size);
60 static __always_inline void __write_once_size(volatile void *p, void *res, int size)
63 case 1: *(volatile __u8 *)p = *(__u8 *)res; break;
64 case 2: *(volatile __u16 *)p = *(__u16 *)res; break;
65 case 4: *(volatile __u32 *)p = *(__u32 *)res; break;
66 case 8: *(volatile __u64 *)p = *(__u64 *)res; break;
69 __builtin_memcpy((void *)p, (const void *)res, size);
75 * Prevent the compiler from merging or refetching reads or writes. The
76 * compiler is also forbidden from reordering successive instances of
77 * READ_ONCE, WRITE_ONCE and ACCESS_ONCE (see below), but only when the
78 * compiler is aware of some particular ordering. One way to make the
79 * compiler aware of ordering is to put the two invocations of READ_ONCE,
80 * WRITE_ONCE or ACCESS_ONCE() in different C statements.
82 * In contrast to ACCESS_ONCE these two macros will also work on aggregate
83 * data types like structs or unions. If the size of the accessed data
84 * type exceeds the word size of the machine (e.g., 32 bits or 64 bits)
85 * READ_ONCE() and WRITE_ONCE() will fall back to memcpy and print a
86 * compile-time warning.
88 * Their two major use cases are: (1) Mediating communication between
89 * process-level code and irq/NMI handlers, all running on the same CPU,
90 * and (2) Ensuring that the compiler does not fold, spindle, or otherwise
91 * mutilate accesses that either do not require ordering or that interact
92 * with an explicit memory barrier or atomic instruction that provides the
96 #define READ_ONCE(x) \
97 ({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
99 #define WRITE_ONCE(x, val) \
100 ({ union { typeof(x) __val; char __c[1]; } __u = { .__val = (val) }; __write_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
102 #endif /* _TOOLS_LINUX_COMPILER_H */