clockgate: add -liberty

[widlarizer]

Integrated clock gating cells (ICGs) are ASIC cells designed as part of PDKs and they are described in Liberty files. To avoid requiring per-PDK hacks to PDK-agnostic scripts, instead of requiring in the invocation a specification of a concrete ICG cell name and its interface, this PR allows the user to use -liberty <filename> argument for the clockgate command and automatically selects the most suitable ICGs described in <filename>. A suitable cell is one with matching clock polarity, a reasonable interface based on the Liberty Reference Manual (refer to pin attributes clock_gate_... and cell attribute clock_gating_integrated_cell). Most suitable cells are those with the fewest scan/test inputs to tie low and smallest area. Using existing current libparse, I use logic similar to that in dfflibmap.cc to find these cells and interfaces.

This PR also assumes all test inputs are to be tied low and all enables are active high. I've tested it manually with asap7 and sky130hd to behave as expected

• test
• help string

[povik]

I think we need to convert all cases in which we are confident the ICG attributes are malformed into warnings, and in all other cases ignore the cell and possibly explain with a print. Oftentimes people can't share the Liberty file with us easily, so we should relax what's acceptable.

Other than that I leave some suggestions on simplifying the code.

[widlarizer]

I think what we need is errors that are suppressable as a new category of logging event, in addition to the non-suppressable errors that we have now. I want to terminate and tell the user to fix their Liberty file, and still give them the power to just cope with it otherwise

[KrystalDelusion]

Is there a reason this uses lines like if (auto clk = pin->find("clock_gate_clock_pin")) { and then not using the assigned variable for anything else? Because doing so is giving compiler warnings