[foreign-memaccess+abi] RFR: 8274648: Improve logic for acquiring by reference parameters in downcall handles [v2]

[Maurizio Cimadamore]

> The current logic for acquiring/releasing scopes associated with by-reference parameters in a downcall handle call is a bit naive, in the sense that it acquires/releases each parameter in isolation (possibly redundantly).
> 
> I've been working on a way to ameliorate the situation by adapting the downcall method handle so that we can pass all addressable parameters (in bulk) to a single acquire/release function. Since this function sees all arguments at once, it can decide to only acquire the unique scopes.
> 
> How to do that proved to be challenging - as simply using a for loop didn't give the performance I expected. Turns out that as soon as we take a backward branch, there is a regression compared with the base case, even when all scopes are the same. The only case where we gain is when all scopes are shared.
> 
> But I (re)discovered a trick; we can special case the acquire/release functions so that they only use a loop for a high number of addressable parameters, and use a switch (with fallthrough) instead for low counts. This makes the logic more scrutable and the performance numbers are stricty better than what we had:
> 
> 
> BEFORE
> 
> Benchmark
> CallOverheadConstant.panama_identity_struct_ref_confined    avgt   30  11.544 ? 0.170  ns/op
> CallOverheadConstant.panama_identity_struct_ref_confined_3  avgt   30  12.213 ? 0.228  ns/op
> CallOverheadConstant.panama_identity_struct_ref_shared      avgt   30  17.214 ? 0.560  ns/op
> CallOverheadConstant.panama_identity_struct_ref_shared_3    avgt   30  33.132 ? 0.934  ns/op
> 
> AFTER
> 
> Benchmark                                                   Mode  Cnt   Score   Error  Units
> CallOverheadConstant.panama_identity_struct_ref_confined    avgt   30  11.613 ? 0.326  ns/op
> CallOverheadConstant.panama_identity_struct_ref_confined_3  avgt   30  11.646 ? 0.333  ns/op
> CallOverheadConstant.panama_identity_struct_ref_shared      avgt   30  17.763 ? 0.639  ns/op
> CallOverheadConstant.panama_identity_struct_ref_shared_3    avgt   30  17.087 ? 0.475  ns/op
> 
> 
> As you can see, in both cases there is no cost for passing multiple addressable arguments backed by the same scope.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Remove unused imports

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

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/590/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/590/files/087188ed..f6d9273b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=590&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=590&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/590.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/590/head:pull/590

PR: https://git.openjdk.java.net/panama-foreign/pull/590