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# flipflop + inverter + rebuffer + output buffer

Tool: Clock Tree Synthesis

Subcategory: Unexpected optimization

## Conversation

### oharboe
Is the "rebuffer" buffer superfluous here?

The "rebuffer" buffer does not exist in placement, only after CTS.

`make DESIGN_CONFIG=designs/asap7/mock-array/Element/config.mk gui_final`:

![image](https://github.com/The-OpenROAD-Project/OpenROAD/assets/2798822/7a197d43-9270-4233-bebd-620b9cfd5fc8)


`make DESIGN_CONFIG=designs/asap7/mock-array/Element/config.mk gui_place`:

![image](https://github.com/The-OpenROAD-Project/OpenROAD/assets/2798822/7d45e527-9062-4a14-b4d3-24125d3c0f46)


### maliberty
It is there for timing optimization as the path has negative slack which is addressed post-CTS.  Is there a reason to think it superfluous?

### oharboe
No extra buffers after removing `set_load -pin_load 10 [all_outputs]`:

Untar  [cts-nobuffer.tar.gz](https://github.com/The-OpenROAD-Project/OpenROAD/files/14554686/cts-nobuffer.tar.gz)

```

$ ./run-me-mock-array_Element-asap7-base.sh

OpenROAD v2.0-12488-g85f541bb6

openroad> report_checks -to io_lsbOuts_7

Startpoint: _755_ (rising edge-triggered flip-flop clocked by clock)

Endpoint: io_lsbOuts_7 (output port clocked by clock_vir)

Path Group: clock_vir

Path Type: max



  Delay    Time   Description

---------------------------------------------------------

   0.00    0.00   clock clock (rise edge)

  74.98   74.98   clock network delay (propagated)

   0.00   74.98 ^ _755_/CLK (DFFHQNx2_ASAP7_75t_R)

  45.97  120.95 v _755_/QN (DFFHQNx2_ASAP7_75t_R)

  13.06  134.01 ^ _435_/Y (INVx4_ASAP7_75t_R)

  18.04  152.05 ^ output265/Y (BUFx2_ASAP7_75t_R)

   0.01  152.06 ^ io_lsbOuts_7 (out)

         152.06   data arrival time



 300.00  300.00   clock clock_vir (rise edge)

  70.00  370.00   clock network delay (ideal)

 -20.00  350.00   clock uncertainty

   0.00  350.00   clock reconvergence pessimism

-240.00  110.00   output external delay

         110.00   data required time

---------------------------------------------------------

         110.00   data required time

        -152.06   data arrival time

---------------------------------------------------------

         -42.06   slack (VIOLATED)

```


### maliberty
> How do I stop "don't touch" from removing the output buffer so I get the comparison I'm after?

Set don't touch after the output buffer is created.

> If I delete set_load -pin_load 10 [all_outputs], the extra buffer goes away.



Probably the timing improves and the net no longer needs repair.  Similarly with the set_driving_cell changing the timing.