If you're going for atomic swap as a primitive, why not go for Compare-And-Swap?  That way all the various lock-free algorithms that use the CAS machine instruction will translate easily.

CAS(variable,oldValue,newValue) is an atomic operation which returns a boolean value (my ST syntax is rusty at the moment, too much Objective-C):

(variable == oldValue) ifTrue: [variable := newValue. ^true].
" variable not the same as oldValue, no swap performed "
^false

On 2010-Oct-12, at 06:58, Igor Stasenko wrote:

> On 12 October 2010 16:51, Levente Uzonyi <leves at elte.hu> wrote:
>> On Tue, 12 Oct 2010, Igor Stasenko wrote:
>> 
>>> Hello, i just thought, that it would be cool to have a special bytecode,
>>> which guarantees atomicity for swapping values between two variables.
>>> 
>>> To swap two values, you usually do:
>>> 
>>> | var1 var2 temp |
>>> 
>>> temp := var1.
>>> var1 := var2.
>>> var2 := temp.
>>> 
>>> But since its non-atomic, a process can be interrupted and such operation
>>> is not thread-safe.
>>> 
>>> In order to make it thread safe, you must add even more boilerplate:
>>> 
>>> | var1 var2 temp |
>>> 
>>> semaphore critical: [
>>>  temp := var1.
>>>  var1 := var2.
>>>  var2 := temp.
>>> ]
>> 
>> An alternative solution:
>> 
>> | a b |
>> a := 1.
>> b := 2.
>> [
>>        | tmp |
>>        tmp := a.
>>        a := b.
>>        b := tmp ] valueUnpreemptively
>> 
> 
> Yeah, another boilerplate under the hood, also highly dependent from
> scheduling nuances :)
> 
> valueUnpreemptively
> 	"Evaluate the receiver (block), without the possibility of preemption
> by higher priority processes. Use this facility VERY sparingly!"
> 	"Think about using Block>>valueUninterruptably first, and think about
> using Semaphore>>critical: before that, and think about redesigning
> your application even before that!
> 	After you've done all that thinking, go right ahead and use it..."
> 	| activeProcess oldPriority result |
> 	activeProcess := Processor activeProcess.
> 	oldPriority := activeProcess priority.
> 	activeProcess priority: Processor highestPriority.
> 	result := self ensure: [activeProcess priority: oldPriority].
> 	"Yield after restoring priority to give the preempted processes a
> chance to run"
> 	Processor yield.
> 	^result
>> 
>> Levente
>> 
> 
> 
> 
> -- 
> Best regards,
> Igor Stasenko AKA sig.
> 

--
Tom Rushworth