On Monday, 15 June 2015 at 12:49:25 UTC, 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?
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 *...
-Steve
I think you miread it.
The memory the pointer points to can be misaligned.
The memory the pointer is in must be aligned.
Note that the second is mandatory is various hardware
architecture to begin with, so it is not like it is a big
constraint. It allow the GC to scan faster. It is all good.
The first one do not make any sense as GC need to support
interior pointers anyway.
