Resizing Problems

[AcKoucher]

Description

Currently, RepairDesign uses a strategy of resizing cells to a target slew in order to perform Post Placement Optimization. With the current methodology, the Resizer ends up "fixing" things that are not actually violations of any rule leading to over-sizing.

Note: The API that resizes to the target slew is also used in RepairSetup and RepairHold.

(Some) Current Problems

• Cells are resized regardless of violations (No DRC, Setup/Hold violations)
• FFs are being touched.
• Special cells are also touched. They should be checked with the cell footprint.
• Clock buffers are used for data network.

Suggested Solution: Enhancement Plan

Phase 1

• [Deferred] Suppress sequential sizing (#5601)

• [ ] Sizing should not swap cells with incompatible electrical properties.

  • [x] Add footprint support (sta: add cell footprint support & add it to equiv check)
    • [Unneeded as discussed in the previous PR] Improve memory by avoid storing a string for each LibertyCell.
    • [Done and reverted.] Check cell_footprint by default in sta equivalency check. (Remove the footprint check from equivCells)
  • [ ] Add user_function_class support (sta: Parse and store user_function_class from Liberty.)
  • [ ] Honor user_function_class if available && provide option to honor cell_footprint during all sizing optimization.

• [x] Use adequate buffer selection strategy for different operations (#5752)

  • [x] Data path sizing should use only data buffers (With exception of hold repairing due to tech specific properties)
  • [x] Clock nets repairing should use the same list of buffers created by CTS

Phase 2

• [ ] Add FO4
  • [ ] Gates that don't respect FO4 should be resized.
  • [ ] Gates with cap ratio that don't violate FO4 shouldn't be further sized
• [ ] Redo repair_design sizing using FO4
  • [ ] Do sequential sizing based on area increase (nominal buffer area threshold)

Phase 3

• [ ] Remove target slew resizing from setup fixing
• [ ] Remove target slew resizing from hold fixing
• [ ] Make setup fixing or power recovery with global sizing

Additional Context

No response