Frankly, the length agnostic stuff is a mistake that I hope hardware designers will eventually see the light on, like delay slots.
It reallt isn't. You just make the default SIMD-width agnostic and anything less portable opt-in.
You can still specialize for a specific width pn scalabe vector ISAs.
> The intended programming model is just too different from traditional ISAs, and there are algorithms that are nearly impossible to write efficiently for it.
Such as?
> All the ones I've seen wrap it up as a bastardized fixed length ISA, and even ARM's own guidance basically recommends that approach.
google highway doesn't. And while Arm is stuck with 128-bit SVE, because they alsp have to implement NEON as fast as possible to be competitive, RVV already has a large diversitly of hardware with different vector length available 128,256,512,1024.
Such as?
It seems to me like you want tp use CNTP on a bitset that tells you, which rows are relevant, skipping them if CNT is 0? Is that what you where describing?
Anyway, essentially yes. My previous comment didn't mention all of the context. The join enforces that the result set is the intersection of the individual column sets, so it gets increasingly sparse as individual columns are computed. So I just maintain a bit tree that says which columns could populate the result set and skip computing the other lanes, which depends on the vector width and benefits from knowing it at compile time.
With RVV (and SVE I assume) there are a wider range of realistic options - at least 128, 256 and 512. The RVV spec allows up to 65536! Also it's totally reasonable to want a single binary to work with all of them so then you're into compiling parts of your code multiple times with runtime dispatch which is a right pain.
I haven't looked into how Highway does it but I don't really know you you write length-agnostic code in high level languages. It's easy in assembly, but it sucks if you have to do it in assembly.
There is a bit of boilerplate to get dynamic dispatch working, but apart from that it's quite simple to use.
Also to note that is C not C++.