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

[Aleksey Shipilev]

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.

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.

Thanks,
-Aleksey