[foreign-memaccess+abi] RFR: 8274648: Improve logic for acquiring by reference parameters in downcall handles [v3]

[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 with a new target base due to a merge or a rebase. The pull request now contains ten commits:

 - Merge branch 'foreign-memaccess+abi' into acquire_linker_optimized
 - Put default label first, and use fallthrough
 - Remove unused imports
 - cleanup code and sprinkle some comments
 - Bump up the number of addressable supported in the fast path
 - Special case calls with up to 4 by-ref parameters.
   Use a switch up to 4 args, then revert to a loop for further args.
 - Merge branch 'foreign-memaccess+abi' into acquire_linker_optimized
 - Simplify code.
   For some reason doesn't perform great unless we skip the very first argument...
 - Reduce number of acquires if possible

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

Changes: https://git.openjdk.java.net/panama-foreign/pull/590/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=590&range=02
  Stats: 126 lines in 2 files changed: 97 ins; 6 del; 23 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