Testing hotspot's Atomic::cmpxchg(jbyte*) ?

[David Holmes]

On 19/08/2016 7:37 PM, Aleksey Shipilev wrote:
> On 08/19/2016 12:26 PM, David Holmes wrote:
>> On 19/08/2016 5:40 PM, Aleksey Shipilev wrote:
>>> On 08/19/2016 10:13 AM, David Holmes wrote:
>>>>> Now it might be cleaner to ditch Java version from Unsafe, and make
>>>>> native entry points like Unsafe_CompareAnd{Exchange,Swap}{Byte,Short}
>>>>> which would call relevant Atomic::cmpxchg-s.
>>>>
>>>> I tried commenting out the Java-ish version and defining one that would
>>>> call Atomic::cmpxchg from unsafe.cpp in the VM. However something is
>>>> complaining about the intrinisics - I removed the
>>>> HotspotIntrinsicCandidate annotation as I don't want any intrinisics,
>>>> but I get a build error:
>>>>
>>>> Compiler intrinsic is defined for method
>>>> [jdk.internal.misc.Unsafe.compareAndSwapByte(Ljava/lang/Object;JBB)Z],
>>>> but the method is not annotated with @HotSpotIntrinsicCandidate.
>>>> Exiting.
>>>>
>>>> No idea where this is coming from or how I can disable it ??
>>>
>>> @HotSpotIntrinsicCandidate marks Java methods that are declared in
>>> vmSymbols.hpp, and the VM code checks it. So, removing @HSIC without
>>> modifying vmSymbols.hpp would blow up like that.
>>>
>>> Anyway, I think you have to leave @HSIC methods untouched, because C2
>>> would intrinsify them directly. To cater for the unsafe.cpp slow path,
>>> do a separate method:
>>>
>>>     @HotSpotIntrinsicCandidate
>>>     public final byte compareAndExchangeByteVolatile(Object o, long
>>> offset,
>>>                                                      byte expected,
>>>                                                      byte x) {
>>>         compareAndExchangeByte0(o, long, expected, x);
>>>     }
>>>
>>>     public final native byte compareAndExchangeByte0(...);
>>>
>>> ...and then do an entry point for it in unsafe.cpp.
>>
>> Okay that was the easy part.
>>
>> Now where do I find out the conditional compilation syntax for the
>> X-VarHandle.java.template file?
>
> The template itself is driven by Spp preprocessor, the same one used
> thorough JDK. It is called during build via
> ./jdk/make/gensrc/GensrcVarHandles.gmk.

Yes but I don't know the syntax.

>
> But why do you need to change VH templates? VH implementations call
> Unsafe, see:
>
>         @ForceInline
>         static $type$ compareAndExchange(FieldInstanceReadWrite handle,
> Object holder, $type$ expected, $type$ value) {
>             return
> UNSAFE.compareAndExchange$Type$Volatile(Objects.requireNonNull(handle.receiverType.cast(holder)),
>                                                handle.fieldOffset,
>
> {#if[Object]?handle.fieldType.cast(expected):expected},
>
> {#if[Object]?handle.fieldType.cast(value):value});
>         }
>
> ...so once you do the Unsafe part above, everything should be linked
> together. The jtreg tests exercise VH byte ops, so you can run them.

I was only making a change to the Byte version so the template needs to 
be specialized for Byte to call CompareAndExchangeByte0.

Thanks,
David

> Thanks,
> -Aleksey
>
>