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!