VarHandle with PathElement

[Maurizio Cimadamore]

Hi Manuel,
all var handles created from path elements in Java 22 accept an 
additional "long" offset parameter, which can be used in case where the 
var handle access must be composed with some other var handle/method 
handle. See:

https://download.java.net/java/early_access/jdk22/docs/api/java.base/java/lang/foreign/MemoryLayout.html#variable-length

It seem like we have missed the update on the javadoc for 
Linker.Option.captureCallState unfortunately.

Maurizio

On 18/02/2024 08:24, Manuel Bleichenbacher wrote:
> In the upgrade of code from JDK 21 to JDK 22, I've run into a problem 
> with VarHandle instances built using PathElement. Something must have 
> changed, which I'm not aware of, or I must have been using them 
> incorrectly all along, or there is a bug.
>
> The VarHandle instances can be created but when they are used to 
> access data an exception like this one is thrown:
>
> Exception in thread "main" java.lang.invoke.WrongMethodTypeException: 
> cannot convert MethodHandle(VarHandle,MemorySegment,long)int to 
> (VarHandle,MemorySegment)int
> at 
> java.base/java.lang.invoke.MethodHandle.asTypeUncached(MethodHandle.java:903)
> at java.base/java.lang.invoke.MethodHandle.asType(MethodHandle.java:870)
> at Main.main(Main.java:16)
>
> The code to reproduce the error can be found below. It crashes on 
> "errnoHandle.get(...)".
>
> The code is mostly form the documentation of 
> Linker.Option.captureCallState() (see 
> https://cr.openjdk.org/~mcimadamore/jdk/FFM_22_PR/javadoc/java.base/java/lang/foreign/Linker.Option.html#captureCallState(java.lang.String...)) 
> But it doesn't just affect call state. It affects all kinds of 
> VarHandle instances built with PathElement.
>
> The bug occurred with the OpenJDK build 29, both on macOS and Linux.
>
> Any help is appreciated.
>
> Regards
> Manuel
>
>
> import java.lang.foreign.*;
> import static java.lang.foreign.ValueLayout.JAVA_INT;
>
> public class Main {
>     public static void main(String[] args) throws Throwable {
>         var linker = Linker.nativeLinker();
>         var descriptor = FunctionDescriptor.of(JAVA_INT, JAVA_INT);
>         var ccs = Linker.Option.captureCallState("errno");
>         var handle = 
> linker.downcallHandle(linker.defaultLookup().find("close").get(), 
> descriptor, ccs);
>         var capturedStateLayout = Linker.Option.captureStateLayout();
>         var errnoHandle = 
> capturedStateLayout.varHandle(MemoryLayout.PathElement.groupElement("errno"));
>
>         try (Arena arena = Arena.ofConfined()) {
>             var capturedState = arena.allocate(capturedStateLayout);
>             var res = (int) handle.invokeExact(capturedState, 100);
>             int errno = (int) errnoHandle.get(capturedState);
>             System.out.println("errno = " + errno);
>         }
>     }
> }
>
>
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