I don't buy it: that would only make sense if you could copy/paste the source or something, and even JS respects function scoping so you can't. You'd have to wait until later anyway, so why not count AST nodes or something instead?
I can't make useful guesses about the V8 developers' reasoning, but I'd assume they considered several options and chose this one for a reason, concerning how much performance is at stake. I don't know about Crankshaft's inner workings, though, so I can't make qualified comments about it. Maybe something like http://jayconrod.com/posts/54/a-tour-of-v8-crankshaft-the-op... can answer some of your questions?
> Graph generation: Crankshaft builds the Hydrogen control flow graph using the AST, scope info, and type feedback data extracted from the full-compiled code. Inlining also happens at this stage. Hydrogen is Crankshaft's high-level architecture-independent intermediate representation.
> Inlining: Performed during the graph generation phase. The inlining heuristic is very simple: basically, if the function being called is known, and it is safe to inline, the function will be inlined. Large functions (>600 source characters including whitespace or 196 AST nodes) will not be inlined. Cumulatively, a total of 196 AST nodes can be inlined in a single function.
So they do use AST nodes as a heuristic; I don't really understand why they would also use "source characters including whitespace" though.
Guess I better keep my comments outside the function body from now on when possible.
