Meta-issue: Port Backend improvements

[nicolasstucki]

• Port Generate leaner code for branches scala#4963
• Optimize use of hash maps in makeLocal in the back end scala#6070
• Desugar string concat to java.lang.StringBuilder (Port scala#4737 and scala#4944 lampepfl/scala#27)
• Avoid boxing primitives in string concatenation (Port scala#4737 and scala#4944 lampepfl/scala#27)
• upgrade asm to 5.1 -- (Fix IncompatibleClassChangeError under Java 9 lampepfl/scala#24)
• Restrict library and reflect to compact1 profile
• Add bridges for overridden methods in lambda indy call
  • Maybe shouldn't be fixed in backend ("Perhaps this could be revisited for 2.13, which would simplify things for SJS (it wouldn't be a backend change anymore)")
  • Ended up being done in erasure, see Erasure#typedClosure
• Compiler crash in GenBCode referring to a class that extends an annotation
  • Maybe shouldn't be fixed in backend ("I'm fine with the approach of repairing this in the backend for now, but agree that a special case in the typer would be worth trying.")
• Fix code gen with returns in nested try-finally blocks
• Don't emit forwarder in mirror class for bridge methods
  • We should also consider whether we can emit static forwarders before the backend ("This looks rather difficult to fix with the way forwarders are currently done. I think the better solution would be to generate static forwarders earlier in the pipeline and have them transformed normally.")

[nicolasstucki]

@smarter, could you add the one you know should be ported.

[allanrenucci]

Suggestions:

• Backend Refactoring scala#6012
• Optimize use of hash maps in makeLocal in the back end scala#6070

[smarter]

scala/scala#6012

I would rather not port refactorings, it would be a lot of work for not much benefit right now. Longer term we'll have to find some way to synchronize our backend with the scalac one, but right now we should only port simple things I think.

Here are the PRs I had in my TODO list to port:

• Desugar string concat to java.lang.StringBuilder
• Avoid boxing primitives in string concatenation
• upgrade asm to 5.1

[allanrenucci]

Dmitry ported scala/scala#4737 and reverted it: lampepfl/scala@ccbac24

[allanrenucci]

Also we are already using asm version 5.1.0-scala-2

[smarter]

Dmitry ported scala/scala#4737 and reverted it: lampepfl/scala@ccbac24

Let's try doing it again and see if something breaks :)

Also we are already using asm version 5.1.0-scala-2

Yes but the PR also updates the backend to not use deprecated methods, something we haven't done yet.

[smarter]

Yes but the PR also updates the backend to not use deprecated methods, something we haven't done yet.

In fact, the PR does more than that, it fixes an issue preventing the emitted code from running under Java 9 (this is mentioned in scala/scala#5452)

[smarter]

Still a WIP but we should keep an eye on scala/scala#6087, the bug also affects Dotty.

EDIT: actually this should probably be fixed before it reaches the backend, as explained in the PR.

[smarter]

I've opened #3403 to track another possible PR to port.