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Hardware-controlled prefetching primarily offers two advantages over software-controlled prefetching. First, old code does not need to be recompiled to take advantage of prefetching. However, this dissertation has demonstrated that the compiler technology for automatically inserting prefetches can be quite successful and is straightforward to implement. Therefore since prefetching compilers should be readily available in the future, this does not appear to be a compelling argument. In particular, scientific programmers usually care enough about performance that they are willing to recompile their code. The second advantage of hardware-controlled prefetching is that it adds no instruction overhead. However, as we have already seen in Chapters and , the instruction overhead of software-controlled prefetching is typically quite small, so this also appears not to be much of an advantage.

Hardware-controlled prefetching has some important disadvantages compared to software-controlled prefetching. First, it is limited only to constant-stride access patterns, and therefore cannot prefetch the indirect references which our compiler can handle (as demonstrated in Section ). Since the compiler is also quite successful at prefetching the constant-stride cases (as we have demonstrated), software-controlled prefetching is likely to offer better coverage than hardware-controlled prefetching. We would expect this trend to continue in the future as the compiler becomes more sophisticated. Second, although hardware-based schemes have no software cost, they may have a significant hardware cost, consuming chip area and possibly affecting cycle time. Therefore since software-controlled prefetching has been shown to be quite effective, offers a broader coverage of misses, and is much simpler to implement in the processor, it appears to be a better solution than hardware-controlled prefetching.

Sat Jun 25 15:13:04 PDT 1994