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Concept
Memory ordering
Résumé
Memory ordering describes the order of accesses to computer memory by a CPU. The term can refer either to the memory ordering generated by the compiler during compile time, or to the memory ordering generated by a CPU during runtime.
In modern microprocessors, memory ordering characterizes the CPU's ability to reorder memory operations – it is a type of out-of-order execution. Memory reordering can be used to fully utilize the bus-bandwidth of different types of memory such as caches and memory banks.
On most modern uniprocessors memory operations are not executed in the order specified by the program code. In single threaded programs all operations appear to have been executed in the order specified, with all out-of-order execution hidden to the programmer – however in multi-threaded environments (or when interfacing with other hardware via memory buses) this can lead to problems. To avoid problems, memory barriers can be used in these cases.
Most programming languages have some notion of a thread of execution which executes statements in a defined order. Traditional compilers translate high-level expressions to a sequence of low-level instructions relative to a program counter at the underlying machine level.
Execution effects are visible at two levels: within the program code at a high level, and at the machine level as viewed by other threads or processing elements in concurrent programming, or during debugging when using a hardware debugging aid with access to the machine state (some support for this is often built directly into the CPU or microcontroller as functionally independent circuitry apart from the execution core which continues to operate even when the core itself is halted for static inspection of its execution state). Compile-time memory order concerns itself with the former, and does not concern itself with these other views.
During compilation, hardware instructions are often generated at a finer granularity than specified in the high-level code.
Source officielle
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