[Newbies] Re: [] whileTrue: [] implementation

[nicolas cellier]

Bert Freudenberg <bert <at> freudenbergs.de> writes:

> 
> On 04.10.2011, at 19:16, Erlis Vidal wrote:
> 
> > Thanks Bert!
> > 
> > The byte code has the answer, I thought that the byte code will map the
source code but I see that's not the
> case, probably to ensure some optimizations.. or to do that "jump" I don't
know how to do that in smalltalk
> 
> For ifTrue: it is purely an optimization, yes. It would work fine by actually
sending that message. It would
> just be slower.
> 
> In the case of whileTrue:, however, it's not just an optimization. The only
way to do loops otherwise would
> be by recursion. But the Squeak VM does not do tail-call elimination, so it
would run out of space pretty
> soon. Generating the loop as a byte code back jump is essential for the system
to work.
> 
> - Bert -
> 


Oh no, we could as well reduce stack depth to (numberOfIteration log: 2) with
silly recursive code like this:


Block>>tryTwiceThen: aBlock
	self value ifFalse: [^aBlock value].
	self value ifFalse: [^aBlock value].
	^
	[self value ifFalse: [^aBlock value].
	self value ifFalse: [^aBlock value].
	true]
		tryTwiceThen: [^aBlock value]

Block>>log2WhileTrue
    ^self tryTwiceThen: [^nil]

| i maxDepth sender |
i := maxDepth := 0.
[i := i+1.
	depth := 0.
	sender := thisContext.
	[(sender := sender sender) isNil] whileFalse: [depth := depth+1].
	maxDepth := maxDepth max: depth.
i<10000] log2WhileTrue.
^maxDepth


Nicolas