--- (---.delete@this.redheron.com) on August 2, 2022 10:25 am wrote:
> NvaxPlus (spam.delete@this.spam.com) on August 2, 2022 8:45 am wrote:
> > I'm curious if anyone here has any good resources (papers, or things of that sort) on empirical
> > data about what works and what doesn't in an ISA. It seems to me that a lot of arguments about
> > instruction set design rely on intuition and received wisdom. Sometimes that's helpful and sometimes
> > it isn't. I've been doing a (admittedly, cursory) literature review on this subject and was
> > surprised just how seemingly little publicly available information there is.
> >
> > When I say "empirical data" I mean specifically things like, how large immediates you
> > actually need, what kind branching ops actually get used (i.e. do we need the full
> > gamut of integer comparisons, or are equals/does not equal zero enough?), &c.
> >
> > The question re: immediate size is especially interesting to me. The only real data I could find on
> > this is from H&P and as far as I could see they don't really delve into their methodology. What they
> > do show suggests that the log2 of immediates is bimodal which kind of intuitively makes sense to me.
>
> The recent data on, eg, branch prediction, show what sort of lengths are
> *common*. For example fig 3 of https://arxiv.org/pdf/2106.04205.pdf
>
> Of course you need some sort of out for the rare cases that go beyond this, but at that
> point the usual "aesthetic" considerations kick in, with some people preferring load-type
> solutions (GOT and so on), others preferring multi-step-creation solutions.
>
> A second issue that is frequently overlooked is that constants
> are used for different things, with different ranges.
> There are at least
> - branch offsets
> - logical constants
> - immediates (eg for adding/subtracting loop counters)
> - FP/vector constants.
>
> ARMv8, as the most recent carefully designed ISA (yes, RISC-V is more recent...)

This is way funnier than it should be. It also isn't wrong.