Re: Classes incompatible with weak references
Re: Classes incompatible with weak references
- Subject: Re: Classes incompatible with weak references
- From: Charles Srstka <email@hidden>
- Date: Wed, 15 Aug 2012 19:01:19 -0500
On Aug 14, 2012, at 2:09 AM, Vincent Habchi <email@hidden> wrote:
> Le 13 août 2012, à 23:47, Mike Abdullah <email@hidden> scripsit:
>
>> An idea I've vaguely wondered about would be turning the isa variable into a tagged pointer. If you know nothing is accessing it directly (to do so was deprecated with the modern runtime), then, say, the last 4 bits of the pointer could be used to record the retain count. Once they're used up (for those rare, highly retained
>
> Using part of pointer to do something else than record an address is perilous. I remember, once again in the old days of the 68000 on the Atari ST, which had a 24-bit external address bus (and not 20 as I wrote before on some other thread), and 32-bit internal address registers, some developers had figured out to use the MSB (unused) of these address registers (pointers) to store extra information. Now, two years later the 68020 with full 32-bit external address bus was released, and suddenly all the code relying on that trick broke on the TT and later machines…
Same thing happened on the original 68k Macintosh. It was bad enough that for quite some time Apple actually had to leave a setting in the control panel to turn off 32-bit addressing, since it broke so much older software.
Charles
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