VarHandles on non-int-sized fields and atomic operations

[David Holmes]

On 25/05/2016 6:43 AM, David Holmes wrote:
> On 25/05/2016 5:29 AM, Aleksey Shipilev wrote:
>> On 05/24/2016 05:43 AM, John Rose wrote:
>>> On May 23, 2016, at 4:20 PM, Martin Buchholz <martinrb at google.com
>>> <mailto:martinrb at google.com>> wrote:
>>>>
>>>> As I said in a previous message, you can implement subword CAS using
>>>> fullword CAS in a loop.
>>>>
>>>> cas8bit(expect, update) {
>>>>  for (;;) {
>>>>    fullword = atomicRead32()
>>>>    if ((fullword &0xff) != expect) return false;
>>>>    if (cas32(fullword, (fullword & ~0xff) | update) return true;
>>>>  }
>>>> }
>>
>> Yes, stupid me! I was under impression that loops are no-no to emulate
>> strong CAS. But we do loops already with LL/SC...
>
> The VM also does it for jbyte variant.
>
>>> Yes, that's the "artisanal" version I would reach for.
>>> It doesn't scale well if there is unrelated activity on nearby bytes.
>>
>> Okay, we are exploring it here:
>>  https://bugs.openjdk.java.net/browse/JDK-8157726
>
> Don't overlook we are past FC for hotspot so this will need approval
> before it can be pushed.

Never mind I see the JEP 193 exemption - though it is a bit odd to see 
an exception for a JEP that is already marked integrated.

David

> David
> -----
>
>> I was able to intrinsify subword accesses on x86_64, and their
>> performance is on par with int versions. Plain Martin-style Java loops
>> are around 2x slower than direct intrinsics in a few basic tests (I
>> expect them to be even slower on contended cases and/or non-x86
>> platforms). But first, we need to hook them up to VarHandles (in
>> progress now).
>>
>> Thanks,
>> -Aleksey
>>
>>