VarHandles on non-int-sized fields and atomic operations

[Aleksey Shipilev]

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...

> 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

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