[jmm-dev] weakCompareAndSet memory semantics
aph at redhat.com
Thu Apr 21 07:52:44 UTC 2016
On 21/04/16 00:10, David Holmes wrote:
> On 21/04/2016 8:59 AM, Justin Sampson wrote:
>> David Holmes wrote:
>>> Side question: why would you replace compareAndSet with
>>> weakCompareAndSet when that would expose those ll/sc spurious
>> The spurious failures are irrelevant because the whole thing is
>> wrapped in a retry loop anyway.
> And in the compareAndSet the spurious failures never escape because it
> also has a retry loop. So my question stands: why would you switch to
> weakCompareAndSet in "busy loops" and how would this be a benefit?
Say you're incrementing a heavily-contended counter. There's no point
spinning in a loop if the store fails or if there is a spurious failure.
You want to do this:
ldax x0, [addr]
add x0, x0, #1
stlx status, x0, [addr]
cbnz status, loop
You can simulate this with weakCompareAndSet.
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