[Vm-dev] Re: [squeak-dev] replaceFrom:to:with:startingAt:

[Eliot Miranda]

On Mon, Sep 22, 2014 at 1:47 PM, Nicolas Cellier <
nicolas.cellier.aka.nice at gmail.com> wrote:

>
>
> 2014-09-22 22:34 GMT+02:00 Bert Freudenberg <bert at freudenbergs.de>:
>
>>
>> On 22.09.2014, at 22:21, Ron Teitelbaum <ron at usmedrec.com> wrote:
>>
>> >
>> >> From: Bert Freudenberg
>> >> Sent: Monday, September 22, 2014 4:00 PM
>> >>
>> >> I just found out why PNG decoding was broken in SqueakJS [*]. Wouldn't
>> >> have believed that any code actually relied on the following behavior:
>> >>
>> >>      | a |
>> >>      a := (1 to: 20) asArray.
>> >>      a replaceFrom: 11 to: 20 with: a startingAt: 8.
>> >>
>> >>      ==> #(1 2 3 4 5 6 7 8 9 10 8 9 10 8 9 10 8 9 10 8)
>> >>
>> > That's funky!  Hope it wasn't my code!  Seems wrong to not copy the
>> array
>> > before modifying it  (Like your buggy VM did).
>>
>> It's not a copy. It moved the numbers 8-17 to index 11. Which is what I
>> erroneously had assumed to be the desired behavior.
>>
>> - Bert -
>>
>>
> It's a known problem.
> The primitive should use memmove rather than memcpy.
> So the solution is quite simple, it's just that it's waiting for a good
> soul to integrate it.
>

Um, no.  It has ever been so and must therefore remain.  This is the method
(as in the Smalltalk-80 V2 sources) that the primitive optimizes:

 replaceFrom: start to: stop with: replacement startingAt: repStart
"This destructively replaces elements from start to stop in the receiver
starting at index, repStart, in the collection, replacement.  Answer the
receiver.  No range checks are performed - this may be primitively
implemented."

| index repOff |
repOff _ repStart - start.
index _ start - 1.
[(index _ index + 1) <= stop]
whileTrue: [self at: index put: (replacement at: repOff + index)]

So the primitive is correct.  This is memcpy behaviour.  IIRC BitBlt is the
same.  There were some BitBlt ticks played with image fill that depended on
this.

| oc |
oc := OrderedCollection withAll: (1 to: 20).
oc replaceFrom: 11 to: 20 with: oc startingAt: 8.
oc => an OrderedCollection(1 2 3 4 5 6 7 8 9 10 8 9 10 8 9 10 8 9 10 8)

> Ron
>> >
>> >> In my (buggy) VM it answered
>> >>
>> >>      ==> #(1 2 3 4 5 6 7 8 9 10 8 9 10 11 12 13 14 15 16 17)
>> >>
>> >> which apparently did not at all match what The Creators intended ;)
>> >>
>> >> - Bert -
>> >>
>> >> [*] specifically, #decompressBlock:with:
>> >
>> >
>>
>>
>>
>>
>>
>>
>
>
>
>


-- 
best,
Eliot
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.squeakfoundation.org/pipermail/vm-dev/attachments/20140923/c5d6913c/attachment.htm