Compare and Swap

[Jasper Potts]

Thanks for the quick response Maurizio,

I will investigate that as an option. I have not used VarHandles directly yet but have seen how they are used in the implementation of java.util.concurrent.atomic.AtomicLongArray that I have been using for doing compare and swap with on-heap memory.

It seems like it would be expensive with all the boxing to Object types for call parameters but I guess the JDK might do a good job making it fast. I will need to do some JMH benchmarks and see. I am doing 10s of thousands a second so the performance matters.

Thanks

Jasper

> On Mar 30, 2022, at 2:36 PM, Maurizio Cimadamore <maurizio.cimadamore at oracle.com> wrote:
> 
> Hi Jasper,
> with both ByteBuffers and MemorySegments you can obtain a var handle for the access expression you want.
> 
> For instance, if you want a VarHandle which dereference ints on a MemorySegment, at a given offset, you can do:
> 
> VarHandle handle = MemoryHandles.varHandle(JAVA_INT)
> int i = (int)handle.get(segment, offset);
> 
> Of course you can also use this var handle to perform a compare and swap, it's one of the supported modes - see:
> 
> https://docs.oracle.com/en/java/javase/18/docs/api/java.base/java/lang/invoke/VarHandle.html#compareAndExchange(java.lang.Object...)
> https://docs.oracle.com/en/java/javase/18/docs/api/java.base/java/lang/invoke/VarHandle.html#compareAndSet(java.lang.Object...)
> 
> With ByteBuffer you can also get a var handle, using this:
> 
> https://docs.oracle.com/en/java/javase/18/docs/api/java.base/java/lang/invoke/MethodHandles.html#byteBufferViewVarHandle(java.lang.Class,java.nio.ByteOrder)
> 
> Have you tried these? Calling these on a VarHandle will just call the corresponding unsafe method.
> 
> Cheers
> Maurizio
> 
> On 30/03/2022 22:18, Jasper Potts wrote:
>> Hi,
>> 
>> I would really like to replace our usage of misc.Unsafe but can’t seem to find a way to do the equivalent of compareAndSwapLong() on a MemorySegment. Today we use off-heap ByteBuffers and Unsafe to do atomic compare and swaps. It works great just sad to have to use Unsafe.
>> 
>> Thanks
>> 
>> Jasper