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

[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.

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Commit messages:
 - 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.
 - Merge branch 'foreign-memaccess+abi' into acquire_linker_optimized
 - Simplify code.
 - 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=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274648
  Stats: 127 lines in 2 files changed: 99 ins; 6 del; 22 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