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Proposal for instruction set - now on Github
Author: Agner Date: 2016-08-04 14:37
Joe Duarte wrote:
Fast encryption is going to be a central requirement of any computing device for decades. So the lack of AES and similar instructions is a disadvantage.
You are probably right that AES instructions would be useful.
Parsing text and bytecode is a huge part of what computers do every day, so I think a new ISA should take a clean look at how to speed up parsing (and JIT'ing). You don't have any counterpart to the SSE 4.2 string comparisons, and I wouldn't want to punt that to FPGAs either.
Text strings are rarely so long that it really matters. It will take less time to parse a string than to read it from an SSD anyway.

x86 AES and SSE4.2 instructions are still only 128 bits while most other vector instructions are 256 - and soon 512 - bits. This is because they are costly to implement. In most cases, it will be faster to parse a string using simple compare instructions with the maximum vector length than a SSE4.2 instruction limited to 128 bits.

Have you been able to guess at performance improvements from the new ISA? Holding everything else equal, the foundry technology, etc. would ForwardCom be faster or more efficient than a comparable Broadwell-E? I'm thinking of your new loops, for example, and other features. What sense do you have of the performance difference?
Instruction fetch and decoding is a bottleneck on all Intel processors. They are putting a micro-operation cache after the decoder to fix this, but each micro-op uses many bits so physical distances on the silicon chip is a limitation. The ForwardCom instruction format will be more efficient than this because it can decode fast and it is compact. x86 has a lot of suboptimal designs because it needs to preserve backwards compatibility with a long legacy of short-sighted designs and patches. They even keep supporting undocumented opcodes from the old 8086.

It is difficult to predict how much you can gain from having longer vectors. Some applications will be unable to use long vectors. The special vector loop will be an advantage whenever the loop count is not certain to be a multiple of the vector size.

 
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