RFR: 8284579: Improve VarHandle checks for interpreter [v3]

[Claes Redestad]

> A few additional enhancements aiming to improve VH performance in the interpreter:
> 
> - Flatten `TypeAndInvokers`: adds a pointer to `VarHandle` (a small increase 40->48) but removes an object and an indirection on any instance actually used - and might avoid allocating the `MethodHandle[]` unnecessarily on some instances
> - Have `checkExactAccessMode` return the directness of the `VarHandle` so that we can avoid some `isDirect` method calls.
> 
> Baseline, `-Xint`
> 
> Benchmark                                 Mode  Cnt    Score   Error  Units
> VarHandleExact.exact_exactInvocation      avgt   30  478.324 ? 5.762  ns/op
> VarHandleExact.generic_exactInvocation    avgt   30  392.114 ? 1.644  ns/op
> VarHandleExact.generic_genericInvocation  avgt   30  822.484 ? 1.865  ns/op
> 
> 
> Patched, `-Xint`
> 
> Benchmark                                 Mode  Cnt    Score   Error  Units
> VarHandleExact.exact_exactInvocation      avgt   30  437.704 ? 5.320  ns/op
> VarHandleExact.generic_exactInvocation    avgt   30  374.512 ? 3.154  ns/op
> VarHandleExact.generic_genericInvocation  avgt   30  757.054 ? 1.237  ns/op
> 
> 
> No significant performance difference in normal mode.

Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:

  checkExactAccessMode -> checkAccessModeThenIsDirect, privatize throw method, cleanup code generator

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8160/files
  - new: https://git.openjdk.java.net/jdk/pull/8160/files/68f414a1..2a4fbd6d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8160&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8160&range=01-02

  Stats: 90 lines in 4 files changed: 0 ins; 5 del; 85 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8160.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8160/head:pull/8160

PR: https://git.openjdk.java.net/jdk/pull/8160