Crash when compiling Skywater ReRAM model: index out of bounds in bitset

[robtaylor]

Description

OpenVAF crashes with an index out of bounds panic when compiling the Skywater PDK ReRAM model.

Steps to Reproduce

1. Obtain the Skywater PDK ReRAM model from: https://github.com/google/skywater-pdk/tree/main/libraries/sky130_fd_pr_reram
2. Compile with OpenVAF:

openvaf sky130_fd_pr_reram__reram_cell.va -o reram.osdi

Expected Behavior

The model should compile successfully or produce a meaningful error message.

Actual Behavior

OpenVAF crashes with:

Panic occurred in file 'lib/bitset/src/lib.rs' at line 133
index out of bounds: the len is 3 but the index is 67108863

Stack Trace

   0: bitset::BitSet<T>::insert::ha8ec904b50560df2
      at lib/bitset/src/lib.rs:133
   0: sim_back::context::Context::compute_outputs::h29cfda05f1d089c8
      at openvaf/sim_back/src/context.rs:120
   0: sim_back::dae::builder::Builder::new::h7e312b05da5b0018
      at openvaf/sim_back/src/dae/builder.rs:70
   0: sim_back::dae::DaeSystem::new::h734b86705eda2457
      at openvaf/sim_back/src/dae.rs:60
   0: sim_back::CompiledModule::new::h416a6863298a9f8a
      at openvaf/sim_back/src/lib.rs:180

Analysis

The crash occurs in sim_back::context::Context::compute_outputs when trying to insert index 67108863 (0x3FFFFFF) into a bitset of length 3. The value 67108863 appears to be an invalid/uninitialized value or a sentinel that's being treated as a valid index.

Model Features

The ReRAM model uses several features that may be relevant:

• @(initial_step) event for initialization
• $bound_step() for time step control
• $abstime for accessing simulation time
• Complex transient state with manual Euler integration
• analog function definitions

Environment

• OpenVAF version: 23.5.0
• Platform: macOS (aarch64)
• Model: sky130_fd_pr_reram__reram_cell.va v2.0.3

Attachments

The model source is available at:
https://github.com/google/skywater-pdk/blob/main/libraries/sky130_fd_pr_reram/latest/cells/reram_cell/sky130_fd_pr_reram__reram_cell.va