[LLHD] Make Deseq pass emit seq.firreg instead of seq.compreg #8662
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The LLHD Deseq pass detects clocks and optional asynchronous resets. It currently, mistakenly, creates `seq.compreg` ops with those clocks and resets. The `seq.compreg` op turns out to have a _synchronous_ reset though, making the lowering invalid. Use the `seq.firreg` op instead, as it allows the reset type to be specified explicitly. This requries an additional `comb.mux` to be created for potential enable signals, which `seq.firreg` does not support directly. We should really nail down the semantics of `seq.compreg` in a follow-up commit. The documentation is very vague, and Deseq is likely not the last pass mistakenly assuming the explicit reset of `seq.compreg` is assumed to be asynchronous.
maerhart
approved these changes
Jul 8, 2025
| @@ -239,7 +239,6 @@ def FirRegOp : SeqOp<"firreg", | |||
| ); | |||
| let results = (outs AnyType:$data); | |||
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| let skipDefaultBuilders = 1; | |||
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Why is this necessary? Aren't you just using the second custom builder or am I missing something?
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That's what I was trying initially, but it turns out that the second custom builder requires the reset and reset value to be non-null. So you'd have to call either the first or second builder depending on whether you want a reset or not. Having the default builder available feels like a good thing.
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The LLHD Deseq pass detects clocks and optional asynchronous resets. It currently, mistakenly, creates
seq.compregops with those clocks and resets. Theseq.compregop turns out to have a synchronous reset though, making the lowering invalid. Use theseq.firregop instead, as it allows the reset type to be specified explicitly. This requries an additionalcomb.muxto be created for potential enable signals, whichseq.firregdoes not support directly.We should really nail down the semantics of
seq.compregin a follow-up commit. The documentation is very vague, and Deseq is likely not the last pass mistakenly assuming the explicit reset ofseq.compregis assumed to be asynchronous.