dns: allow v8 to optimize lookup()

[mscdex]

Reassigning the values of parameter variables causes
v8 to disable function optimization.

[cjihrig]

Should this be callback = callback_?

[mscdex]

Yes, sorry.

[cjihrig]

I thought v8 could still optimize writes to named function arguments, but couldn't handle writes to arguments (need to test though).

[mscdex]

It can't optimize any function that contains any usage of arguments and also mutates parameter variable values. This is mentioned in bluebird's "Optimization killers" and I also saw it while checking for deoptimizations in my own code with node v0.10.

[Fishrock123]

I thought v8 could still optimize writes to named function arguments, but couldn't handle writes to arguments (need to test though).

I was also under this impression. I know writing to arguments is bad, but I always do like fn(param) { param = param || 'thing' }.

[Fishrock123]

This is mentioned in bluebird's "Optimization killers"

Hmm, only says when you also use arguments in the same function. Interesting. Weird.

[cjihrig]

Ah, the problem is that is also reads arguments. I think you can get around that just by enabling strict mode.

[jasnell]

@mscdex ... is this still something you think needs to be done in v0.10?

[mscdex]

@jasnell Well, it's an optimization. If anything it might make lookups a tad faster. If it doesn't apply cleanly to v0.10 anymore I guess I wouldn't worry about it.

[jasnell]

So long as CI passes, LGTM

[jasnell]

Landed in 6502160