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
