The-OpenROAD-Project
MPL should support incremental placement in designs with FIXED macros
Description
In https://github.com/The-OpenROAD-Project/OpenROAD/commit/259a4a6f8e084e19edb7d2bc46b31aa27f2536d3, it seems mpl was changed such that it cannot place UNPLACED macros if there are already existing FIXED macros in the FP area. If a user attempts to do mpl in this case, this logic will return an error: https://github.com/The-OpenROAD-Project/OpenROAD/blob/359623a96826d2137a82ffcd90b6d94627f2466a/src/mpl/src/clusterEngine.cpp#L157-L166
[ERROR MPL-0050] Found fixed instance a/b/c/d/inst inside the floorplan area.
My understanding is that the original mpl algorithm could handle this scenario. Is that understanding correct, and if so, is there a workaround to support this in the new mpl? I notice there are checks to ignore fixed macros, which makes me think this is possible to support it, e.g. https://github.com/The-OpenROAD-Project/OpenROAD/blob/359623a96826d2137a82ffcd90b6d94627f2466a/src/mpl/src/clusterEngine.cpp#L787-L793
The use case is: The RTL is modified but the reference FP DEF with fixed macros is not. Changes to add more macros in RTL and will require incremental placement, but I would like any original placement to be respected. If this is not currently possible, it would be great to support this use case.
Suggested Solution
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Additional Context
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@mikesinouye We never actually had the full support for partial macro placement i.e., having some macros placed and run mpl for others. Although these ignore checks exist in the clustering algorithm, the mechanism to handle fixed macros inside the annealer was never actually implemented.
The changes in 259a4a6 were to cover cases in which there are special fixed cells inside the core, but outside the area specified for the macro placement - these should not be touched by mpl.
I have some ideas to handle the pre-placed macros scenario.
We never actually had the full support for partial macro placement
I think he was referring to TritonMacroPlacer (old mpl)
@mikesinouye I just realized this is a duplicated version of #6033. I'll update you through the other issue once we have updates.
@AcKoucher Thanks, we ran into this issue again the other day so this would be great to support.