If the abc command is called by itself without any arguments, what is the expected behavior?

[leeway64]

Let's say I ran abc without any arguments inside a Yosys script, like this example:

read_verilog -sv OpenROAD-flow-scripts/flow/designs/src/gcd/gcd.v
synth -flatten
abc

If I ran abc without any arguments, what is the expected behavior? Would abc only optimize the circuit combinationally? Would it add any additional gates? Would abc affect timing? What is abc supposed to do in this case?

I'm seeing some interesting behavior with abc in another situation, so I just wanted to check really quick.

Thank you!

[ben-j-c]

From the docs it seems there's a default script that will run on your design.
https://yosyshq.readthedocs.io/projects/yosys/en/latest/cmd/index_passes_techmap.html#cmd-abc

if no -script parameter is given, the following scripts are used:

for -liberty/-genlib without -constr:
strash; &get -n; &fraig -x; &put; scorr; dc2; dretime; strash;
&get -n; &dch -f; &nf {D}; &put

for -liberty/-genlib with -constr:
strash; &get -n; &fraig -x; &put; scorr; dc2; dretime; strash;
&get -n; &dch -f; &nf {D}; &put; buffer; upsize {D};
dnsize {D}; stime -p

for -lut/-luts (only one LUT size):
strash; &get -n; &fraig -x; &put; scorr; dc2; dretime; strash;
dch -f; if; mfs2; lutpack {S}

for -lut/-luts (different LUT sizes):
strash; &get -n; &fraig -x; &put; scorr; dc2; dretime; strash;
dch -f; if; mfs2

for -sop:
strash; &get -n; &fraig -x; &put; scorr; dc2; dretime; strash;
dch -f; cover {I} {P}

otherwise:
strash; &get -n; &fraig -x; &put; scorr; dc2; dretime; strash;
&get -n; &dch -f; &nf {D}; &put

Only optimize combinationally?

From reading the help, it doesn't seem to talk about sequential logic, so I would assume. That's also been my experience.
I think you can do a select statement to restrict, if thats what you want.

Additional gates?

Yes, it should modify/add/subtract.

Affect timing?

Yes.

I'm not an expert on this, so take what I say with a grain of salt. Maybe looking at the script syntax of abc could give you more details given the default "otherwise" script.