[Vm-dev] Re: [Pharo-project] #at:ifAbsent: for Array(s) could be faster

[Eliot Miranda]

On Wed, Nov 2, 2011 at 11:33 AM, Levente Uzonyi <leves at elte.hu> wrote:

> On Wed, 2 Nov 2011, Igor Stasenko wrote:
>
>  Here the original implementation, which Array inherits from
>> SequenceableCollection:
>>
>> at: index ifAbsent: exceptionBlock
>>        "Answer the element at my position index. If I do not contain an
>> element
>>        at index, answer the result of evaluating the argument,
>> exceptionBlock."
>>
>>        (index between: 1 and: self size) ifTrue: [^ self at: index].
>>        ^ exceptionBlock value
>>
>>
>> the thing is, that #at: primivite also doing a range checking and
>> fails if index is invalid, so if index is valid, as result we
>> performing a range checking twice instead of just once.
>>
>> Here the simple implementation how to make things faster:
>> fat: index
>>        "Primitive. Assumes receiver is indexable. Answer the value of an
>>        indexable element in the receiver. Fail if the argument index is
>> not an
>>        Integer or is out of bounds. Essential. See Object documentation
>>        whatIsAPrimitive. Read the class comment for a discussion about
>> that the fact
>>        that the index can be a float."
>>
>>        <primitive: 60>
>>        ^ #plopp
>>
>> fat: index ifAbsent: aBlock
>>        | x |
>>        x := (self fat: index).
>>        x == #plopp ifTrue: [ ^ aBlock value ].
>>        ^ x
>>
>> And speedup is almost 2 times:
>>
>> [ 1000000 timesRepeat: [ #(1 2 4 5 56 67 67) at: 1 ifAbsent: nil ] ]
>> timeToRun
>>
>> 39
>>
>> [ 1000000 timesRepeat: [ #(1 2 4 5 56 67 67) at: 1 ifAbsent: nil ] ]
>> timeToRun
>>
>> 20
>>
>> Even when index is out of range, it is still much faster:
>>
>> [ 1000000 timesRepeat: [ #(1 2 4 5 56 67 67) at: 100 ifAbsent: nil ] ]
>> timeToRun
>>
>> 35
>>
>> [ 1000000 timesRepeat: [ #(1 2 4 5 56 67 67) fat: 100 ifAbsent: nil ]
>> ] timeToRun
>>
>> 24
>>
>> The problem, of course, that trick with #plopp is a dirty hack.
>> Because if array include an #plopp object as element, it will report
>> as it absent,
>> instead of answering it.
>> There is no way to ensure that any arbitrary object are never included
>> as element in array, so this is no-go for generic replacement.
>> But in controlled environment, when you know that none of arrays which
>> you using with #fat:ifAbsent: will ever include #plopp (or pick your
>> own unique object)
>> it may be a good alternative.
>>
>
> I've been thinking about this earlier and there's a much nicer and simpler
> solution, though it requires vm changes :). The idea is to change primitive
> 60 to work for 2 parameters too, so #at:ifAbsent: could also use the
> primitive directly. There were a few changes about #at: recently (mirror
> prims, cog), so I'm not sure it's still easy to change the primitive.
>

his is neat, but it wouldn't be an extension to primitive 60 but a new
primitive that would ignore its top of stack.  primitive 60 can operate
with 2 args,when used as a mirror primitive,  but then it ignores the
receiver.  Your usage would ignore the last argument.

So choose a primitive number and I'll make it so...


>
> Levente
>
>
>
>> --
>> Best regards,
>> Igor Stasenko.
>>
>>
>>
>


-- 
best,
Eliot
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