Question about garbage collection specification

[Dmitry Olshansky via Digitalmars-d]

On 15-Jun-2015 15:49, Steven Schveighoffer wrote:
> On 6/15/15 8:08 AM, Dmitry Olshansky wrote:
>> On 13-Jun-2015 14:32, rsw0x wrote:
>>> http://dlang.org/garbage.html
>>>
>>> Do not take advantage of alignment of pointers to store bit flags in the
>>> low order bits:
>>> p = cast(void*)(cast(int)p | 1);  // error: undefined behavior
>>>
>>> if this restriction is actually imposed - why does
>>> std.bitmanip.tagged{ClassRef,Pointer} even exist?
>>
>> AFAIK the restriction was that pointers _themselves_ have to be stored
>> at word-aligned addresses. This allows GC to scan memory cheaper w/o
>> considering if some misaligned address may contain a pointer.
>
> That doesn't make sense. Why would you want to do this?
>

What exactly? Storing pointer at misaligned memory location - no good 
reason whatsoever. As in how it may happen - explicitly tightly packed 
structs with align(1) might end up doing this.

 From http://dlang.org/phobos/core_memory.html which seems to be more 
recent:

Implementations are free to assume that GC pointers are only stored on 
word boundaries. Unaligned pointers may be ignored entirely.

> The only rational thing I can think of is that you wouldn't want to
> store the result in an *actual* int pointer (lest it be used thinking it
> was valid without masking out the lower bits). But the example is
> storing it in a void *...
>

The example doesn't show what memory location that p refers to thus it's 
not possible to say if it's valid.


-- 
Dmitry Olshansky