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Comment by phire | Hacker News Reader

Yes... but then you are kind of wasting a pipeline stage on nothing more than length decoding.

I suspect a design with a full decoder every 16-bits might actually win on everything but gate count, mostly because it can deal with variable length instructions and variable number of μops per instruction in the same step. A decoder that doesn't output a μop because it was clobbered by a previous instruction, can be handled the same was as a decoder that didn't output a μop because of μop fusion.

Actually, that approach might actually eliminate the need for the extra pipeline stage (just at the cost of gates).

It's certainly not a deal breaker. But it's a valid criticism of the ISA.

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Ok, so you doubled the number of decoders, how is that not significantly better than x86?

I'm not even sure you have a point with regards to it being a valid criticism. Doubling the silicon area for instruction decoding probably costs nothing, because if you have a simple decompression stage, the maximum number of decoders is already doubled in the first place, because you're hypothetically encoding twice as many instructions to begin with. If you can double the decoders in the decompression stage, you can probably get rid of a separate decoding stage altogether and thereby reduce the cost to literally nothing.

Look, it might not be obvious but in university I once had to design an ASIP and then do the floor plan with Cadence and the area of the SRAM dwarfed everything to the point where my ASIP was a tiny vertical column in-between two SRAM chips. I personally was shocked by the fact that I struggled to even find my ASIP on the floor plan, because it was maybe ten standard cells wide in-between the SRAM blocks. Like, ridiculously tiny to the point where it is hard for me to even care about the area the ASIP took up.

I said easily less than half a pipeline not a full stage. Everything kind of shifts around a bit because of that, and it ends up being a pretty different design than a fixed width front end because of it (hence qualcomm's objections), but it's not clearly worse.

And for better than aarch64 density, it seems to make a lot of sense.

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