nexus: LUT and FF clustering #863
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Signed-off-by: Maciej Kurc <[email protected]>
Signed-off-by: Maciej Kurc <[email protected]>
Signed-off-by: Maciej Kurc <[email protected]>
…rameter controlling the ratio of FFs that got glued to carry-chain clusters. Signed-off-by: Maciej Kurc <[email protected]>
gatecat
reviewed
Nov 22, 2021
| ff->constr_y = lut->constr_y; | ||
| ff->constr_z = lut->constr_z + 2; | ||
| ff->constr_abs_z = lut->constr_abs_z; | ||
| root->constr_children.push_back(ff); |
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this case also needs to be checking that the FF can legally be added to the cluster (i.e. CLK/LSR/CE sharing etc)
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Oh, you are right. I missed that... I'll push a fix soon.
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Alright, should be fixed by now.
…configuration Signed-off-by: Maciej Kurc <[email protected]>
gatecat
approved these changes
Nov 24, 2021
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This PR adds to the nexus architecture inference of directly connected LUT+FF pairs followed by clustering them together.
For each directly connected LUT and FF nextpnr now creates a new cluster. Moreover it glues FFs to existing clusters comprised of LUTs like wide LUTs and carry-chains.
The new behavior is enabled by default but can be disabled by the
--no-pack-lutffoption. Another option--carry-lutff-ratioallows to specify a ratio of clustering together FFs and LUTs which are part of carry-chains.The added clustering approach has been analyzed on a single design (the SoC from https://chromium.googlesource.com/chromiumos/platform/hps-firmware) targeting LIFCL-17 FPGA using 500 chosen seeds. A timeout of 5min was set for each run so that when it expired a run was considered a failure. The conclusions are the following:
The following two charts show router2 progression in terms of overused wire count vs. iteration count. Each trace is terminated either with a green circle (indicating a success) or a red "x" (indicating a failure). Plots are clipped to 200 iterations for better clarity.
No clustering:
With clustering:
I'm also attaching CSV files with the gathered statistical results:
no-clustering.csv
with-clustering.csv
As it can be seen in the pictures with the LUT and FF clustering enabled the router is able to converge much quicker and gets stuck more rarely.
The option to specify clustering ratio for FFs and existing carry-chain clusters is meant to mitigate situations when placement of such large clusters of fully populated slices cannot be legalized.