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Won't CISC architectures always have the benefit of being able to have dedicated silicon for those complex instructions and thus do them faster than many smalle
by tormeh 4y ago
Won't CISC architectures always have the benefit of being able to have dedicated silicon for those complex instructions and thus do them faster than many smaller instructions? I understand RISC-V does instruction fusing, which provides a lot of the same benefits, but I'm surprised ARM gets around this.
- zozbot234 4y agoDedicated silicon for custom instructions is now quite favored actually, because of the well known "dark silicon" problem. I.e. most of the die actually has to be powered down at any given time, to stay within power limits. Hence why RISC-V is designed to make custom, even complex instructions very easy to add. ("Complex" is actually good for a pure custom accelerator, because it means a lot of otherwise onerous dispatch work can be pulled out of the binary code layer and wired directly into the silicon. The problem with old CISC designs is that those instructions didn't find enough use, and were often microcoded anyway so trying to use them meant the processor still had to do that costly dispatch.)
- marcosdumay 4y agoThe simpler your instructions, the more use-cases they have. But yes, the less benefit you get from specializing them too. If you can break your specialized instruction into a few much more generic ones, it's probably a gain.