[HW][circt-synth] Implement AggregateToComb pass and add to circt-synth pipeline #8068
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Would it make sense to have aggregate to comb lowering in a separate pass? To me, it seems like it could also be useful outside of the AIG lowering path. |
Generally yes, I agree. One concern is that the lowering pattern for |
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include/circt/Dialect/HW/Passes.td
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| let description = [{ | ||
| This pass lowers aggregate *operations* to comb operations within modules. | ||
| This pass does not lower ports, as ports are handled by FlattenIO. This pass | ||
| will also change the behavior of out-of-bounds access of arrays. |
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Can we mention here how it changes the behavior? If I understand correctly it's just a refinement, so we're all good and wouldn't even need to mention it here?
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Can we mention here how it changes the behavior?
Yes. I'll do.
If I understand correctly it's just a refinement
Yes, it's refinement so it should be fine from the semantics perspective but i would at least mention in the doc. Maybe in the future we might want to run post-synthesis simulation on arcilator but only pre-synthesis IR can catch out-of-bounds access in this case.
| %c3_i2 = hw.constant 3 : i2 | ||
| // NOTE: If the index is out of bounds, the result value is undefined. | ||
| // In LEC such value is lowered into unbounded SMT variable and cause | ||
| // the LEC to fail. So just asssume that the index is in bounds. |
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Yeah, for these kind of things, a translation validation tool would be quite nice 🙂
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Yeah without circt-lec it has been almost impossible to implement lowering pattern with confidence for the correctness. Thank you @maerhart @frog-in-the-well @TaoBi22 for outstanding work!
This PR adds a new pass
HWAggregateToCombthat lowers array operations to comb operations. The pass converts:hw.array_getto a mux tree using comb operationshw.array_createandhw.array_concattocomb.concathw.aggregate_constanttocomb.concatof individual constantsThe PR also moves two utility functions from CombToAIG to the Comb dialect:
extractBits: Extracts individual bits from a valueconstructMuxTree: Builds a multiplexer tree from selectors and leaf nodesThe pass is required to run before CombToAIG to ensure array operations are
properly lowered to AIG. Strictly speaking other than array_get operations can be preserved
but array values prevent optimizations at AIG level. So for the simplicity, I think it's reasonable
to lower array operations within circt-synth pipeline.
I currently added the pass under
HW/TransformsconsideringFlattenIOis very similar but the pass might better belong to Conversion.