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

[Maurizio Cimadamore]

On Fri, 1 Oct 2021 14:56:33 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> 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.

This pull request has now been integrated.

Changeset: 6a077704
Author:    Maurizio Cimadamore <mcimadamore at openjdk.org>
URL:       https://git.openjdk.java.net/panama-foreign/commit/6a077704882aa6db630ffa82515d3d084a7cc16a
Stats:     126 lines in 2 files changed: 97 ins; 6 del; 23 mod

8274648: Improve logic for acquiring by reference parameters in downcall handles

Reviewed-by: jvernee

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