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    <p>This is what I think is happening (keep in mind I'm not familiar with HotSpot):</p> <pre><code>0xf36c9fd0: mov 0x6c(%ecx),%ebp ; vfoo 0xf36c9fd3: test %ebp,%ebp ; vfoo is null? 0xf36c9fd5: je 0xf36c9ff7 ; throw NullPointerException (I guess) 0xf36c9fd7: movl $0x1,0x8(%ebp) ; vfoo.x = 1 0xf36c9fde: mov 0x68(%ecx),%ebp ; sz 0xf36c9fe1: inc %ebx ; i++ 0xf36c9fe2: test %edi,0xf7725000 ; safepoint on end of loop 0xf36c9fe8: cmp %ebp,%ebx ; i &lt; sz? 0xf36c9fea: jl 0xf36c9fd0 0xf3771ad0: mov 0x6c(%ecx),%ebp ; vfoo 0xf3771ad3: test %ebp,%ebp ; vfoo is null? 0xf3771ad5: je 0xf3771b09 ; throw NullPointerException (I guess) 0xf3771ad7: movl $0x1,0x8(%ebp) ; vfoo.x = 1 0xf3771ade: mov 0x6c(%ecx),%ebp ; \ 0xf3771ae1: mov %ebp,0x70(%ecx) ; / bar = vfoo 0xf3771ae4: mov 0x68(%ecx),%edi ; sz 0xf3771ae7: inc %ebx ; i++ 0xf3771ae8: mov %ecx,%eax ; 0xf3771aea: shr $0x9,%eax ; ??? \ Probably replaced later 0xf3771aed: movb $0x0,-0x3113c300(%eax) ; ??? / by some barrier code? 0xf3771af4: test %edi,0xf77ce000 ; safepoint 0xf3771afa: cmp %edi,%ebx ; i &lt; sz ? 0xf3771afc: jl 0xf3771ad0 ; </code></pre> <p>The reason I think the above code stands in for a barrier is that when taking the NullPointerException, the scalable version has a <code>XCHG</code>, which acts as a barrier, while the non-scalable version has a NOP there.</p> <p>The rationale would be that there needs to be a happens-before ordering between the initial load of <code>vfoo</code> and joining the thread. In the volatile case, the barrier would be inside the loop, so it wouldn't need to be elsewhere. What I don't understand is why <code>XCHG</code> isn't used inside the loop. Maybe runtime detection of MFENCE support?</p>
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    1. POWhy reading a volatile and writing to a field member is not scalable in Java?
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    1. USninjalj
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      1. VTUpMod
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    1. COApparently, the `shr`/`movb` instruction pair is exactly the barrier code - it sets the card dirty-byte used by the garbage collector.
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      1. USaxel22
 

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