JVM backend cleanup opportunities

[SolalPirelli]

Issue for myself based on reading the code and modifying it for the optimizer.

These may or may not be good ideas.

•  We have a whole hierarchy of class/method/labelnode just to keep 1 bit of info per label, whether it's reachable or not. Surely we can use a WeakHashSet or similar? (Or even just a HashSet if the lifetimes work out?)
• Most of the utility classes appear to be historical accident and could be split better
  • AsmUtils in particular is all about logging
  • DottyBackendInterface should not exist
  • Aggressively move stateless stuff out of stateful classes
  • instead of a big switch to map bytecode versions we can do integer addition...
• Do we really need all the BType creation stuff in CoreBTypesFromSymbols, or could we do comparisons on Java type descriptors for instance?
  • CoreBTypes in general seems like it should be named WellKnownBTypes or similar
• Why does the stuff in BTypes need to be in there rather than top-level or somewhere else? Especially inliner stuff
• CallGraph should not let other classes touch its internals, and more generally we should have clearer boundaries around classes
• grep --include=*.scala -Fr '.jvm' compiler | grep -Fv '/jvm'` should return nothing
• simplify emitting supercalls to not need to add explicit inheritance Simplify JVM interface list emission, remove a mini-phase #25203 (comment)

More notes about that last point:

compiler/test/dotty/tools/dotc/classpath/MultiReleaseJarTest.scala:class MultiReleaseJarTest extends dotty.tools.backend.jvm.DottyBytecodeTest {
  Good indication DottyBytecodeTest should be moved up the hierarchy? 

compiler/test/dotty/tools/dotc/classpath/JrtClassPathTest.scala:import dotty.tools.backend.jvm.AsmUtils
  Can be inlined, extremely basic use of ClassReader

compiler/test/dotty/tools/dotc/ConstantFoldingTests.scala:import dotty.tools.backend.jvm._
  Another use of DottyBytecodeTest (it's an e2e constant folding, not bad) 

compiler/test/dotty/tools/dotc/transform/SpecializeTuplesTests.scala:import dotty.tools.backend.jvm.DottyBytecodeTest
compiler/test/dotty/tools/dotc/transform/SpecializeFunctionsTests.scala:import dotty.tools.backend.jvm.DottyBytecodeTest
  More DottyBytecodeTest

compiler/src/dotty/tools/dotc/core/Phases.scala:import dotty.tools.backend.jvm.GenBCode
  Exposes genBCodePhase to:
  - CollectEntryPoints -> removed in PR
  - CollectSuperCalls -> removed in PR
  - GeneratedClassHandler -> as part of ThreadPoolFactory creation, for logging purposes
    -> Could be a String
    -> This is the only creation of ThreadPoolFactory in the entire compiler anyway!
  - GenericSignatures which does "atPhase(genBCodePhase) { sanitizeName(sym.fullName).replace('.', '/') }" -> why?
  - TreeChecker uses it in "isNonMagicalMember" which sounds worrying


compiler/src/dotty/tools/dotc/core/SymbolLoaders.scala:import dotty.tools.backend.jvm.DottyBackendInterface.symExtensions
  .javaClassName

compiler/src/dotty/tools/dotc/config/ScalaSettingsProperties.scala:import dotty.tools.backend.jvm.BackendUtils.classfileVersionMap
  that property should probably be moved somewhere else?
  also, it can be computed instead of being a map, so we could instead code just the min/max instead of having to depend on a map

compiler/src/dotty/tools/dotc/Compiler.scala:import backend.jvm.{CollectSuperCalls, GenBCode}
  OK, Compiler referencing a Phase

compiler/src/dotty/tools/dotc/profile/Profiler.scala:import dotty.tools.backend.jvm.DottyBackendInterface.symExtensions
  .javaBinaryName

compiler/src/dotty/tools/dotc/transform/CollectEntryPoints.scala:import dotty.tools.backend.jvm.GenBCode
  See remark on Phases above

compiler/src/dotty/tools/dotc/transform/TreeChecker.scala:import dotty.tools.backend.jvm.DottyBackendInterface.symExtensions
  See remark on Phases above

compiler/src/dotty/tools/backend/sjs/JSPrimitives.scala:import dotty.tools.backend.jvm.DottyPrimitives
  Should rename DottyPrimitives to ScalaPrimitives and move it next to ScalaPrimitiveOps in backend/

compiler/src/dotty/tools/backend/sjs/JSEncoding.scala:import dotty.tools.backend.jvm.DottyBackendInterface.symExtensions
  .javaSimpleName
  .javaClassName

compiler/src/dotty/tools/debug/JavaEncoding.scala:import dotty.tools.backend.jvm.DottyBackendInterface.symExtensions
  .javaClassName

[SethTisue]

DottyBackendInterface should not exist

that's my understanding as well from @sjrd