SCBNDSI redundancy?

[nwf]

SCBNDSI has a 5-bit immediate, uimm, and a gated (by s) shift-by-4 operation, in

riscv-cheri/src/insns/scbnds_32bit.adoc

Line 43 in 1c43ce2

`immediate = ZeroExtend(s ? uimm<<4 : uimm)`

which means that there are two ways to request bound lengths of 0 (unshifed or shifted 0) and 16 (unshifed 16, shifted 1). Perhaps "shifted 0" and "shifted 1" could be put to better use (as requests for 512 and 528, say)?

[arichardson]

Avoiding redundant encodings here would be nice and I agree that having shifted zero be 512 would make a lot of sense here. Not sure how much that increases decoder complexity, but if we are already special casing, maybe we could also have shifted 1 be something more useful like 1024 or 4096?

[davidchisnall]

I'd like to see some distribution of object sizes before thinking about adding that. And it will probably be different on 32- and 64-bit systems. It might be worth marking those as explicitly invalid encodings that are reserved for future expansion.

[arichardson]

I'd like to see some distribution of object sizes before thinking about adding that. And it will probably be different on 32- and 64-bit systems. It might be worth marking those as explicitly invalid encodings that are reserved for future expansion.

That is a good argument - I believe @jonwoodruff did some analysis here.
For now I've opened a PR to reserve these encodings.

[jonwoodruff]

These are data samples from Spec2006, averaging distributions from each of 10 benchmarks.

This first set is for flat "bits of precision". It starts at 1 bit because we assume signed values. 12 reaches 100% because we're just measuring the used sizes in RISC-V code generation from CHERI ISA-v9, which only has a 12-bit immediate.

	1	2	3	4	5	6	7	8	9	10	11	12
incoffseti	0.14%	41.46%	47.54%	53.55%	62.63%	69.90%	72.67%	76.13%	87.26%	97.36%	98.02%	100.00%

And here are results with a "scale bit" that shifts by 4, which was the point that hit the highest number of cases. Presumably this is because it facilitates pointer-aligned arithmetic. Unsurprisingly, this matches choices in the Morello ISA.

	1	2	3	4	5	6	7	8	9	10	11	12
incoffseti	0.14%	40.06%	49.83%	66.03%	72.97%	85.50%	96.57%	97.22%	98.09%	98.29%	98.53%	100.00%

I don't know if this informs how effective adding an additional value, e.g. 512, to the spectrum. I can dig up the old scripts and add that condition in. There is at most 10% on the table (the jump from 9 bits to 10 bits; we would get a single value from the 10-bit space, but probably the most common one).

[arichardson]

Just to clarify, it looks like this is the data for incoffset? I would imagine the values used for setbounds are somewhat different.

[tariqkurd-repo]

#418 is merged, so can we close this now?

[arichardson]

I agree the immediate issue is fixed, but maybe we should keep this open as a future enhancement to allow for larger values?