[comb-to-synth] Implement Sklanskey Tree and Architecture Selection based on Attribute

[cowardsa]

Introduce the Sklanskey Tree lowering option for comb.add and make it the default option (used in the PULP Platform library of arithmetic circuits). The Sklanskey Tree is as fast as a Kogge-Stone adder but tradesoff greater fan-out for less logic (preference will depend on target technology).

To aide testing purposes - make architecture selection optionally controlled by an attribute on the operator - in future this could be expanded to provide a way in which analysis passes could modify operator implementation selection, without interacting with the lowering pass itself. Open to suggestions if this is not a preferred method for passing "synthesis" controls?

comb.add %arg0, %arg1, %arg2 {synth.arch = "SKLANSKEY"} : i4

[uenoku]

LGTM, using attributes for testing is great idea! I want to make sure that these attributes are not part of public API so could you add explicitly include test as attribute name? like synth.test.arch maybe. I think eventually it might make sense to expose these architectures as stand-alone operations synth.add.sklanskey if an user really wants to use specific architecture in a similar way to LLVM intrinsic.

[uenoku]

LGTM! Thank you for making the change.

[seldridge]

This is causing nightly debug builds to timeout when running the tests added to comb-lowering-lec.mlir. See: https://github.com/llvm/circt/actions/runs/18129142989/job/51613927639 The tests seem fine, but we may need to either disable them for debug builds or modify the lit timeout.

[seldridge]

☝️ I bumped the timeout to fix this in: 2bc2607

[cowardsa]

Thanks for catching this @seldridge - we could also split the test across multiple files to avoid the timeout issues - then can reduce the limit back down - would that be preferable?