Wow, the v8 optimizer can sometimes be a pretty blunt instrument. Shouldn't it look at the size of the inlined function after it compiles it to some intermediate state that erases things like comments and names?
If I had to suggest a change, I would use the 600-char count as a starting point for one-time functions, and then count the number of calls to a function, counting how many instructions it produces (basically tossing an incrementing variable into the compilation state, and storing it per Function object).
But then again, I've never played with the inside of V8, only the outside. I consider myself lucky to be in that position, because V8 is pretty dang sweet.
http://wingolog.org/archives/2011/07/05/v8-a-tale-of-two-com...
If a function is hot, then it gets promoted to V8's optimizing compiler, which is currently Crankshaft but will soon be replaced by Turbofan. This compiles the AST first into a high-level IR, Hydrogen, which is an architecture-independent SSA-like form vaguely reminiscent of LLVM. Then that's lowered into an architecture-dependent IR, Lithium, where register allocation and instruction scheduling is performed.
http://jayconrod.com/posts/54/a-tour-of-v8-crankshaft-the-op...
If you read the bug for this that someone posted up-thread, the AST information is fully available when inlining in Hydrogen, but a patch to remove the character limit tanked one of their benchmarks. Also, they didn't preclude fixing it in Turbofan, only in Crankshaft, and that's largely because Crankshaft is on its way out.
(I've played a little with the internals of V8, but am not an official developer. Also, one of my friends is the Bay Area TL for V8.)
If comments aren't completely ignored by the parser, then there has to be some case where a comment inside a function has an observable effect?
Surely this must be undocumented features of the language spec?
(Or rather, that's the first step of deciding. It will inline larger methods if they're sufficiently hot).