[foreign-memaccess+abi] RFR: 8313005: Ensure native access check can fold away

[Jorn Vernee]

On Tue, 25 Jul 2023 18:13:12 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

> This patch helps to ensure that native access checks done by restricted methods can completely fold away (see JBS for details).
> 
> This is accomplished by:
> 1. Marking the `module` field of `java.lang.Class` `@Stable`
> 2. Using a constant `JavaLangAccess` instance.
> 
> I've also added 2 new benchmarks that show the improvement (in two separate classes so that the benchmark methods in each class are comparable to each other in terms of numbers).
> 
> Numbers on my machine are as follows:
> 
> Before:
> 
> Benchmark                       Mode  Cnt  Score   Error  Units
> MemorySegmentGetUnsafe.panama   avgt   30  0.817 ± 0.002  ns/op
> MemorySegmentGetUnsafe.unsafe   avgt   30  0.400 ± 0.003  ns/op
> 
> MemorySegmentCopyUnsafe.panama  avgt   30  8.145 ± 0.016  ns/op
> MemorySegmentCopyUnsafe.unsafe  avgt   30  7.744 ± 0.012  ns/op
> 
> 
> After:
> 
> Benchmark                       Mode  Cnt  Score   Error  Units
> MemorySegmentGetUnsafe.panama   avgt   30  0.411 ± 0.002  ns/op
> MemorySegmentGetUnsafe.unsafe   avgt   30  0.404 ± 0.003  ns/op
> 
> MemorySegmentCopyUnsafe.panama  avgt   30  7.737 ± 0.013  ns/op
> MemorySegmentCopyUnsafe.unsafe  avgt   30  7.728 ± 0.011  ns/op

src/java.base/share/classes/jdk/internal/reflect/Reflection.java line 122:

> 120:             static final JavaLangAccess JLA = SharedSecrets.getJavaLangAccess();
> 121:         }
> 122:         Holder.JLA.ensureNativeAccess(module, owner, methodName);

Note that I have to put this here in a holder class instead of inside the enclosing class to avoid bootstrap cycles (resulting in `getJavaLangAccess` returning `null`)

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PR Review Comment: https://git.openjdk.org/panama-foreign/pull/851#discussion_r1273917178