2014-09-22 23:16 GMT+02:00 Bert Freudenberg <bert at freudenbergs.de>:

>
> On 22.09.2014, at 22:47, 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.
>
>
> What I'm saying is that there is code which will actually fail in subtle
> ways if we change that behavior. The LZW decode *relies* on this. If we
> change the primitive it would fail.
>
>
Ah, oh !
I missread your post!
Relying on an undefined behavior is not a clever thing...
OK, since we never changed the implementation, it's a stable behavior...
but it is for sure unspecified.
There is no limit to the hackish nature of some Squeak code, really...
Hang it in the hall of shame.



> So rather than changing what the "stringReplace" primitive does we might
> rather want to add a "stringMove" primitive.
>
> Btw, BitBlt has "move" semantics and I believe this is even used in some
> places (via #hackBits:).
>
> - Bert -
>
>
>
>
>
>> > 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:
>> >
>> >
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