Cache control 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++. A variant of prefetch for the instruction cache. This hint is used to prepare cache lines before overwriting the contents completely. In this example, the CPU needn't load anything from main memory. The semantic effect is equivalent to an aligned memset of a cache-line sized block to zero, but the operation is effectively free. This hint is used to discard cache lines, without committing their contents to main memory. Care is needed since incorrect results are possible. Unlike other cache hints, the semantics of the program are significantly modified. This is used in conjunction with allocate zero for managing temporary data. This saves unneeded main memory bandwidth and cache pollution. This hint requests the immediate eviction of a cache line, making way for future allocations.