Add support for the blif construct .subckt

[SebastianBoe]

Hi,

from what I see in the blif parser, it appears that the .subckt
construct is not supported yet.

I would like this supported because Yosys generated .subckt
commands when the -noflatten option was used.

Symptom:
top.blif:42: fatal error: unknown directive

Workaround:
Don't use subckt. (For the yosys flow, don't use the -noflatten option)

PS: Thanks for creating this project. Since there exists a workaround, it might make sense
to close this as won't fix.

[cseed]

This would be reasonable to add. It might also be useful for non-yosys tools that want to generate .blif files directly. I'm going to leave it open as an enhancement. Thanks for the suggestion!

[porglezomp]

I ran into this issue while working with yosys.
Another workaround that might be helpful to people who aren't happy with avoiding -noflatten for any reason. Instead of writing:

synth_ice40 … -blif <name.blif>

you can flatten your design after synthesis and write it then.

synth_ice40 …
flatten
write_blif -gates -attr -param <name.blif>

[whitequark]

FYI Arachne is essentially not maintained at this point as nextpnr is a far more capable replacement.