[Pack] Pack Pattern Root IPIN

[amin1377]

To identify the root block of a chain, VPR iterates over all cluster-level IPINs associated with the pack pattern and considers all blocks connected to those pins (via get_all_connected_primitive_pins) as potential starting points.

This approach typically works well. However, issues arise when the pack pattern spans blocks that support multiple modes, for example, in a shift register pattern where registers have different modes. In such cases, a separate pattern must be defined for each mode. As a result, the same cluster-level IPIN may be connected to multiple blocks (e.g., registers in various modes), not all of which belong to the specific pack pattern being considered.

To address this, I added filtering to get_all_connected_primitive_pins so it only returns blocks that already belong to the current pattern, avoiding invalid starting points.

Without this fix, in an architecture where a flip-flop has multiple modes, the chain is only correctly formed if the first mode defined in the architecture file corresponds to the one used in the netlist. This is because the root block is currently retrieved using list_of_pack_patterns->chain_root_pins[0][0].

[AmirhosseinPoolad]

Thanks for the changes Amin! I'll let Vaughn comment on the actual functionality changes, but meanwhile I had a couple of style related comments and nitpicks.

[amin1377]

@AmirhosseinPoolad Thanks for reviewing the code! I’ve addressed your comments. Please resolve the conversations if you think they’ve been properly handled. Let me know if you have any further suggestions.

[AlexandreSinger]

LGTM