Prefetch instruction

In computing, a cache control instruction is a hint embedded in the instruction stream of a processor intended to improve the performance of hardware caches, using foreknowledge of the memory access pattern supplied by the programmer or compiler. They may reduce cache pollution, reduce bandwidth requirement, bypass latencies, by providing better control over the working set. Most cache control instructions do not affect the semantics of a program, although some can.

Several such instructions, with variants, are supported by several processor instruction set architectures, such as ARM, MIPS, PowerPC, and x86.

Also termed data cache block touch, the effect is to request loading the cache line associated with a given address. This is performed by the PREFETCH instruction in the x86 instruction set. Some variants bypass higher levels of the cache hierarchy, which is useful in a 'streaming' context for data that is traversed once, rather than held in the working set. The prefetch should occur sufficiently far ahead in time to mitigate the latency of memory access, for example in a loop traversing memory linearly. The GNU Compiler Collection intrinsic function __builtin_prefetch can be used to invoke this in the programming languages C or C++.

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