Proposal: Remove multi-dimensional memories from core primitives

[rachitnigam]

I've pondering this for a while. If I understand correctly, multi-dimensional memories are not directly mappable when doing synthesis, which probably means that at some point, the underlying synthesis toolchain remaps that accesses to multi-dimensional memories to single dimension ones.

If this is true, I think we should move multi-dimensional memories out of the core primitives and implement a pass that lower multi-dimensional memories into single dimensional memories to make the cost of this transformation explicit.

I'd also like the CIRCT folks to chime in if possible: @mikeurbach @stephenneuendorffer

[stephenneuendorffer]

Makes sense to me. This is one of those things that is likely an important hardware lowering for many platforms.

[mikeurbach]

I agree, it seems best to make this explicit in the IR rather than deferring to an external toolchain. I was thinking of something similar, in the context of scheduling. Lowering from a multi-dimensional memory to a single dimension may introduce hardware components related to linearizing indices, and these should definitely be taken into account in scheduling, or anything else with a cost model.

Perhaps it would be nice to add the transformation first and deprecate multi-dimensional memories, before fully removing them? In CIRCT, @mortbopet has developed a pass that does exactly this, so we already have this option coming in to Calyx from CIRCT workflows.

[EclecticGriffin]

Also makes sense to me! (fwiw the interpreter & debugger use flattened versions internally anyway)

[sampsyo]

Indeed! One baby step we could take here would be reimplementing the std_mem_d* components in pure Calyx, which would let us remove the primitives while preserving compatibility… except that we don't yet have any support for parameterized Calyx components.

[cgyurgyik]

except that we don't yet have any support for parameterized Calyx components.

Yeah I recall this being brought up awhile back, with this being a main limitation.

[rachitnigam]

We don’t have to implement them in Calyx if we can’t. We should move them out of the core primitives.

[cgyurgyik]

Isn't this a common enough abstraction though that will take some burden off the frontend generators? Like I think the right approach is templated memories with a correspond pass to flatten them, rather than entirely remove them.

[rachitnigam]

No I literally mean put them in a different file that is not core. When we have all the other fancy stuff we can use it but core should be tiny.