Optimize AliasAnalysis hotspot

[hubertp]

Pull Request Description

scopeFor is a real hotspot that hasn't been particularly optimized. By using iterator and a simple variable instead of a list and checking its length I was able to get 10% speedup.
Note that it seems tempting to throw the exception within the loop but that seems to create a less optimized code.

Important Notes

I'm consistently getting ~10% speedup on a hello world example (with standard libraries).

For example I'm no longer seeing 20ms spent in scopeFor and most of them are below 10ms:

Before

After

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.

[JaroslavTulach]

Nice finding!

From a strategic perspective we should first address

• Add benchmarks to runtime-compiler #8419

and only then integrate this bugfix. Then we could demonstrate on the graphs how much we do care about static compiler speed.

On the other hand, we do have the Startup_empty_startup benchmark and if that one is improving, then we have an (at least indirect) evidence as well.

[Akirathan]

Very nice improvement. Just out of curiosity: Do you have profiling snapshots? If so, please attach some screenshots from them.

[hubertp]

Yes, I agree we need proper benchmarks. As I talked to @4e6 I think there is a big scope for improving the compiler performance, we haven't even started.

I was hoping that Startup benchmark will confirm my numbers. Started it.

I also added some screenshots. It's a bit hard to show but basically traversing a list is gone.

[hubertp]

hmm, that's weird. it looks like no difference? I'm clearly getting better results locally 🤔

[hubertp]

Maybe the reason is because it is not built against latest develop. I will retry.

[hubertp]

I think something is wrong with the benchmark because I'm clearly getting improvements locally. Not sure what to do with that one for the time being.

[hubertp]

Here are files with VisualVM samping
before and after