[SimToSV] support lowering stdout/stderr

[nanjo712]

Support lowering stdout/stderr in SimToSV.

[fzi-hielscher]

Thanks, @nanjo712.
I'm not sure if it is going to work out, but it would be nice if we could treat streams more like normal values here. That is, add a type conversion from sim::OutputStreamType to I32 and add operation conversion patterns for the two ops. We will have to insert an UnrealizedConversionCastOp when creating the FWriteOp. This should then simply fold away after conversion. It could become more of a problem when we add support for the GetFileOp.

[uenoku]

One tricky point with FIRRTL's fwrite semantics is we can literally make the file name dynamic, so it would be necessary to get file descriptor in the same cycle/always block.

[nanjo712]

Sorry for the delayed follow-up on this PR — I was caught up with some personal matters and wasn’t able to address the review comments in time.

I’ve now updated the implementation to reflect the feedback. I’d appreciate it if you could take another look.

[fzi-hielscher]

Sorry for the delayed follow-up on this PR — I was caught up with some personal matters and wasn’t able to address the review comments in time.

I've added the print operations two years ago, I'm in no hurry. 😉

We should (hopefully) be able to resolve the conversion casts on-the-fly without needing materializations and reconciliation:
First, mark them as dynamically illegal if they carry any type we need to convert.

target.addDynamicallyLegalOp<:UnrealizedConversionCastOp>([&](UnrealizedConversionCastOp op){
      return typeConverter.isLegal(op);
 });

Then add a conversion pattern for them that checks if the converted input types match the converted result types. If so, we can directly replace the op with the values provided by the adaptor.

[nanjo712]

Thank you for your feedback. I think your suggestion is indeed a good one, and I have revised the code accordingly.

[fzi-hielscher]

Nice, thanks.