Nexus: Cell type 'CCU2'...not supported by this device. Encountered when using 'synth_nexus -abc9' in Yosys.

[tcal-x]

This may or may not be a nextpnr bug, but I'm starting here to hopefully get a pointer to the real issue.

In a current LiteX setup, if you cd to litex-boards/litex-boards/targets/ and run

$ ./lattice_crosslink_nx_evn.py --build --toolchain=oxide --yosys-abc9

it will eventually fail in nextpnr-nexus with

ERROR: Cell type 'CCU2' instantiated as 'main_crg_por_count_FD1P3JX_Q_D_CCU2_S1_CIN_CCU2_COUT' is not supported by this device.

If you don't include the --yosys-abc9, it succeeds.

I'll attach a zip with the Yosys output json from each case, the pdc file, and the command line to rerun nextpnr-nexus: demo.zip

One diff I noticed between the json files is this, which might be completely irrelevant:

// without -abc9 (the version that works):
    "CCU2": {
      "attributes": {
        "abc9_box": "00000000000000000000000000000001",
        "blackbox": "00000000000000000000000000000001",
        "src": "/usr/local/bin/../share/yosys/nexus/cells_sim.v:197.1-243.10"
      },

// with -abc9 (the version that results in the error):
    "CCU2": {
      "attributes": {
        "blackbox": "00000000000000000000000000000001",
        "src": "/usr/local/bin/../share/yosys/nexus/cells_sim.v:197.1-243.10"
      },

Versions used:

"nextpnr-nexus" -- Next Generation Place and Route (Version nextpnr-0.6-154-g535709a9)
Yosys 0.36+42 (git sha1 70d35314d, clang 14.0.0-1ubuntu1.1 -fvisibility-inlines-hidden -fmessage-length=0 -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -fPIC -Os)

[gatecat]

Thanks for reporting!

I've improved the bad error message, occurring because of a failure in carry chain packing, in #1262

However, this doesn't fix the root cause, which seems to be that ABC9 is now converting the correct missing carry input on the start of a carry chain, to a carry input connected to zero (which can't necessarily be packed). That would require a Yosys MCVE and fix, which I will look at over the next few days if no-one else gets there first.

[tcal-x]

Thanks @gatecat for the quick response. I think I have a pretty small reproducer.