VarHandles on non-int-sized fields and atomic operations

[David Holmes]

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.

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
>
>