internal_stats: astnode (sizeof)

[widlarizer]

What are the reasons/motivation for this change?

Requested by @povik. We'd like to see how much of the memory usage of Yosys between passes is directly due to AstNodes. This doesn't count overhead of initialized vectors and such.

Explain how this is achieved.

Adding a counter adjusted on AstNode construction/destruction

If applicable, please suggest to reviewers how they can test the change.

yosys -p "read_verilog tests/sim/dffe.v; internal_stats -json"

[Ravenslofty]

This should have used the scratchpad...

[povik]

Wouldn’t that be inefficient accessing the scratchpad with every construction/destruction?

[Ravenslofty]

I'm not proposing astnodes is a scratchpad variable, I'm proposing that astnodes_count() is a scratchpad variable.

[povik]

But scratchpad is a static key-value store. We would need to periodically update it, or extend what scratchpad is.

In any case we can go back and redo this.