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 <[email protected]
<mailto:[email protected]>> 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
