[code-reflection] RFR: Testing BytecodeLift and BytecodeGenerator roundtrip stability [v4]

[Adam Sotona]

> Hi,
> I've added `TestLiftSmallCorpus::testDoubleRoundtripStability` to test stability of the `BytecodeLift` and `BytecodeGenerator` roundtrip on JDK classes (aka "Small Corpus"). 
> 
> The test takes ~40k methods from JDK classes and tries to lift them from bytecode, generate, lift again and generate again. Only about 4300 methods pass the roundtrip now, because `BytecodeLift` and `BytecodeGenerator` are still under construction.
> 
> The test then compares "normalized" bytecode from the first round and the second round and fails with printed details if differ. The test actually contains a non-zero error threshold before failure. This threshold can be manually adjusted when working on `BytecodeLift` and `BytecodeGenerator` improvements.
> This approach allows to incrementally fix `BytecodeLift` and `BytecodeGenerator` roundtrip, as well as to extend their coverage.
> 
> This patch also includes several fixes of bugs discovered during work on `TestLiftSmallCorpus`:
> - conditional branches are unwound to avoid growing number of blocks during the round trip
> - conditional branches are inverted to fix roundtrip stability
> - partial fix of block parameters construction
> - partial fix of the locals overrides
> - aggressive local variable slots overrides in `BytecodeGenerator` are temporary disabled until `BytecodeLift` is ready to support it 
> - fixed redundant block argument assignements of unused variables in `BytecodeGenerator`
> - fixed storing of values into invalid slots
> 
> Partial fix means it significantly reduced the error threshold (in order of magnitudes), however there are still some cases failing.
> 
> I would like to integrate this patch, because it introduces a tool to allow subsequent incremental progress.
> 
> Please review.
> 
> Thank you,
> Adam

Adam Sotona has updated the pull request incrementally with one additional commit since the last revision:

  more constrained TestLiftSmallCorpus

-------------

Changes:
  - all: https://git.openjdk.org/babylon/pull/24/files
  - new: https://git.openjdk.org/babylon/pull/24/files/62caefda..239f9c95

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=babylon&pr=24&range=03
 - incr: https://webrevs.openjdk.org/?repo=babylon&pr=24&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/babylon/pull/24.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/24/head:pull/24

PR: https://git.openjdk.org/babylon/pull/24