Incorrect constant K-LUT primitive instantiation

[Junius00]

Expected Behaviour

To implement global GND and VCC signals, I understand that VTR makes use of K-LUTs to generate these constant signals. In theory, the LUT mask should be all '0's or all '1's, which outputs a '0' or '1' regardless of LUT inputs.

Current Behaviour

When using --gen_post_synthesis_netlist, the constant LUT for a global GND is instantiated as follows:

LUT_K #(
        .K(5),
        .LUT_MASK(32'b11111111111111111111111111111110)
    ) \lut_gnd  (
        .in({
            1'bX,
            1'bX,
            1'bX,
            1'bX,
            1'bX
         }),
        .out(\lut_gnd_output_0_0 )
    );

Which yields 2 problems:

1. The LUT mask is not as expected (all '0'), but rather only the last output bit is set to '0'.
2. The LUT_K primitive present in vtr_flow/primitives.v uses the input in as an array index of the LUT_MASK parameter. Setting them as 1'bX generates an output signal of 1'bX as a result, which messes with all modules relying on this global GND output.

Possible Solution

The only possible solution I've managed to get proper Verilog simulations out of is to change the inputs to all '0's and then ensure that the last bit in the LUT mask is equal to the desired constant output:

• Any undefined signal in in will cause an undefined out.
• If the in signal is fixed at a certain position, then only that bit in the LUT mask will matter.

Steps to Reproduce

1. Run VTR with --gen_post_synthesis_netlist with any circuit that requires global GND/VCC (e.g., simple addition with a constant).
2. Inspect the *_post_synthesis.v file.

Context

When trying to verify that the final implementation of VTR matches the initial Verilog design provided with a testbench, the undefined output of this constant K-LUT has a tendency to cause all the post-implementation module's outputs to be undefined, which leads to incorrect simulation results.

Your Environment

• VTR revision used: latest commit ce706d579de8d370994fb3d3eb1a469d98707866
• Operating System and version: Ubuntu 22.04.4 LTS
• Compiler version: cmake version 3.22.1, GNU Make 4.3

[vaughnbetz]

Thanks @Junius00 ! @KennethKent @amirarjmand93 @tangxifan @duck2 : this looks high priority to fix, and from Junius' description it should be a matter of changing the output for gnd/vcc LUTs. Any volunteer to fix this? I'm surprised we haven't hit this before.

[vaughnbetz]

Just pinging people on this ... @Junius00 : do you want to make a PR to fix this? Our tests should catch if the fix has a bug, so if CI and the nightly tests pass it should be safe to merge.

[Junius00]

Hi @vaughnbetz, a new semester's started for me so unfortunately I don't think I'll have the time to look into this :-(