Add support for using retime and dretime from abc in synth.tcl

[oharboe]

Description

https://people.eecs.berkeley.edu/~alanmi/abc/abc.htm does have a retiming capability.

https://inst.eecs.berkeley.edu/~eecs151/sp21/files/lec26-abc.pdf discusses the retime and dretime commands.

Suggested Solution

Try them by tweaking synth.tcl.

Additional Context

No response

[oharboe]

Experiment, there's a detectable, but not significant difference #2541

[oharboe]

I wandered through a couple of the End point Slack Histogram buckets in MegaBoom and it looks like the need for retiming is endemic. This is based purely on finding 50+ logic depth and the BOOM having been taped out at 28nm 1000ps. It would be interesting to know the maximum logic depth in BOOM.

A maximum depth of 20 at 1000ps at 28nm?

[maliberty]

Fixed in #3399