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A memory barrier, also known as a membar, memory fence or fence instruction, is a type of barrier instruction that causes a central processing unit (CPU) or compiler to enforce an ordering constraint on memory operations issued before and after the barrier instruction. This typically means that operations issued prior to the barrier are guaranteed to be performed before operations issued after the barrier. Memory barriers are necessary because most modern CPUs employ performance optimizations that can result in out-of-order execution. This reordering of memory operations (loads and stores) normally goes unnoticed within a single thread of execution, but can cause unpredictable behaviour in concurrent programs and device drivers unless carefully controlled. The exact nature of an ordering constraint is hardware dependent and defined by the architecture's memory ordering model. Some architectures provide multiple barriers for enforcing different ordering constraints. Memory barriers are typically used when implementing low-level machine code that operates on memory shared by multiple devices. Such code includes synchronization primitives and lock-free data structures on multiprocessor systems, and device drivers that communicate with computer hardware. ==An illustrative example== When a program runs on a single-CPU machine, the hardware performs the necessary bookkeeping to ensure that the program executes as if all memory operations were performed in the order specified by the programmer (program order), so memory barriers are not necessary. However, when the memory is shared with multiple devices, such as other CPUs in a multiprocessor system, or memory mapped peripherals, out-of-order access may affect program behavior. For example, a second CPU may see memory changes made by the first CPU in a sequence which differs from program order. The following two-processor program gives an example of how such out-of-order execution can affect program behavior: Initially, memory locations x and f both hold the value 0. The program running on processor #1 loops while the value of f is zero, then it prints the value of x. The program running on processor #2 stores the value 42 into x and then stores the value 1 into f. Pseudo-code for the two program fragments is shown below. The steps of the program correspond to individual processor instructions.Processor #1: while (f == 0); // Memory fence required here print x; Processor #2: x = 42; // Memory fence required here f = 1; One might expect the print statement to always print the number "42"; however, if processor #2's store operations are executed out-of-order, it is possible for f to be updated ''before'' x, and the print statement might therefore print "0". Similarly, processor #1's load operations may be executed out-of-order and it is possible for x to be read ''before'' f is checked, and again the print statement might therefore print an unexpected value. For most programs neither of these situations are acceptable. A memory barrier can be inserted before processor #2's assignment to f to ensure that the new value of x is visible to other processors at or prior to the change in the value of f. Another can be inserted before processor #1's access to x to ensure the value of x is not read prior to seeing the change in the value of f.For another illustrative example (a non-trivial one that arises in actual practice), see double-checked locking. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Memory barrier」の詳細全文を読む スポンサード リンク
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