Cache Scala.js Linker to enable incremental linking

[lolgab]

Originated from this message on gitter.im.
This should solve the problem.

This PR caches all the linkers separately. For now there are 12 combinations. But adding new parameters could explode the possibilities even more.
I did this so you can jump between different settings of fastOpt/fullOpt, ModuleKind without losing the incremental linking.

[lefou]

Looks good to me.
Do you have any numbers or estimates, what the memory consumption of a cached linker is, and how much speed we gain by caching? My initial though was, that a module-local worker should be enough to cache the linker, but it might be wrong.

[lolgab]

Do you have any numbers or estimates, what the memory consumption of a cached linker is, and how much speed we gain by caching?

I don't. Let me run some tests and I'll let you know.

My initial though was, that a module-local worker should be enough to cache the linker, but it might be wrong.

I don't know if that works because it is common to run fastOpt/fullOpt on the same module.

[lolgab]

Didn't measure the speedup but it's apparent using it.
About memory consumption, a linker instance goes from ~60Mb (fastOpt) to ~120Mb(fullOpt) for a small/medium sized Scala.js application:

These four are for the CommonJSModule/NoModule - fastOpt/fullOpt matrix

[lefou]

Thanks for the numbers, @lolgab ! IMHO, this is already some significant memory, given, that we also keep cached Scala compilers and probably other tools around. Maybe, we can keep WeakReferences of the linker, so that the JVM has some options when in trouble. We already do this in ZincWorkerImpl.

[lolgab]

Didn't know about the WeakReference trick. Nice!
Used it and changed the cache to a mutable.Map following ZincWorkerImpl.

[lefou]

Thank you! I'm going to merge after the CI has finished. Travis-CI is currently disabled, so I'm going to start the CI job on my fork.

[lolgab]

@lefou All the relevant jobs are green ✅

https://travis-ci.org/github/lefou/mill/builds/745613555

[lolgab]

Thank you for the review as usual @lefou 🙂