Phase 15.18 placement optimization

I'm working on a Virtex 5 project containing a 13-stage pipeline clocked at

200 MHz. The data path gets quite wide in the latter stages; but only 11% of available slices are used; so the FPGA is relatively empty. Every other step in the build process rips through quickly, except the above named MAP stage, which accounts for 98% of total build time. Routing is over in a flash, and the result meets timing. I would like to reduce the build time and I was wondering how to go about it. One idea was to create LOC constraints on logic, perhaps based on where the tools themselves placed it after a previous optimization run. Any other suggestions?


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Andrew Holme
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For Virtex 5 and newer parts, Map always places the design as well as mapping into slices, so "PAR" really only routes. You should expect map to take the longest time of the processes.

If your pipeline is not hand-optimized and requires register-balancing, then map may be spending a lot of time on that (physical synthesis), and using LOC constraints might not help much for execution time. An easy way to try the LOC idea without a lot of effort is to use "guided" placement. This normally works best when you make only small mods to the design between runs. It essentially tries to LOC everything where it was in the prior "golden" run. The problem is when you make enough changes that very little of the design keeps its prior instance names.

-- Gabor

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