I agree with Alexey it should be possible to do this under the covers of
existing Unsafe methods.
Paul.
> On 19 Aug 2016, at 06:29, Aleksey Shipilev <aleksey.shipi...@gmail.com> wrote:
>
> On 08/19/2016 03:03 PM, David Holmes wrote:
>> 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.
>
> If you find Spp.java in the forest, there is a comment section about the
> syntax:
> ./jdk/make/src/classes/build/tools/spp/Spp.java
>
>
>>> 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.
>
> I get that.
>
> But I still think X-VarHandle.template change is unnecessary. The code
> for byte CAS generated from X-VarHandle.template calls
> Unsafe.compareAnd{Swap,Exchange}*. Why can't you leave VH template
> alone, and call CompareAndExchangeByte0 from
> Unsafe.compareAndExchangeByteVolatile, like I suggested before?
>
> @HotSpotIntrinsicCandidate
> public final byte compareAndExchangeByteVolatile(Object o,
> long offset,
> byte expected,
> byte x) {
> compareAndExchangeByte0(o, long, expected, x);
> }
>
> public final native byte compareAndExchangeByte0(...);
>
> VH tests have a least one testing mode which does not intrinsify Unsafe
> methods, i.e. -Xint. compareAndExchangeByte0 path would be visited there.
>
> -Aleksey
