| # flipflop + inverter + rebuffer + output buffer |
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| Tool: Clock Tree Synthesis |
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| Subcategory: Unexpected optimization |
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| ## Conversation |
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| ### oharboe |
| Is the "rebuffer" buffer superfluous here?
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| The "rebuffer" buffer does not exist in placement, only after CTS.
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| `make DESIGN_CONFIG=designs/asap7/mock-array/Element/config.mk gui_final`:
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| `make DESIGN_CONFIG=designs/asap7/mock-array/Element/config.mk gui_place`:
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| ### 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? |
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| ### oharboe |
| No extra buffers after removing `set_load -pin_load 10 [all_outputs]`:
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| Untar [cts-nobuffer.tar.gz](https://github.com/The-OpenROAD-Project/OpenROAD/files/14554686/cts-nobuffer.tar.gz)
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| ```
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| $ ./run-me-mock-array_Element-asap7-base.sh
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| OpenROAD v2.0-12488-g85f541bb6
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| openroad> report_checks -to io_lsbOuts_7
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| Startpoint: _755_ (rising edge-triggered flip-flop clocked by clock)
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| Endpoint: io_lsbOuts_7 (output port clocked by clock_vir)
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| Path Group: clock_vir
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| Path Type: max
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| Delay Time Description
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| ---------------------------------------------------------
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| 0.00 0.00 clock clock (rise edge)
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| 74.98 74.98 clock network delay (propagated)
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| 0.00 74.98 ^ _755_/CLK (DFFHQNx2_ASAP7_75t_R)
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| 45.97 120.95 v _755_/QN (DFFHQNx2_ASAP7_75t_R)
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| 13.06 134.01 ^ _435_/Y (INVx4_ASAP7_75t_R)
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| 18.04 152.05 ^ output265/Y (BUFx2_ASAP7_75t_R)
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| 0.01 152.06 ^ io_lsbOuts_7 (out)
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| 152.06 data arrival time
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| 300.00 300.00 clock clock_vir (rise edge)
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| 70.00 370.00 clock network delay (ideal)
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| -20.00 350.00 clock uncertainty
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| 0.00 350.00 clock reconvergence pessimism
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| -240.00 110.00 output external delay
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| 110.00 data required time
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| ---------------------------------------------------------
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| 110.00 data required time
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| -152.06 data arrival time
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| ---------------------------------------------------------
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| -42.06 slack (VIOLATED)
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| ```
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| ### maliberty |
| > How do I stop "don't touch" from removing the output buffer so I get the comparison I'm after?
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| Set don't touch after the output buffer is created.
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| > If I delete set_load -pin_load 10 [all_outputs], the extra buffer goes away.
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| Probably the timing improves and the net no longer needs repair. Similarly with the set_driving_cell changing the timing. |
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