On 3/1/18 3:33 PM, H. S. Teoh wrote:
On Thu, Mar 01, 2018 at 02:52:26PM -0500, Steven Schveighoffer via
Digitalmars-d-learn wrote:
[...]
There are a few in there, which I think are over-the-top. Such as
"don't cast a pointer to a non-pointer",
[...]
Isn't that necessary for a precise GC?
It's not strictly necessary for a precise GC. It's necessary for a
copying GC. In that case, if the GC decides to move your object and
update all the references, it would miss that one. And even there, it's
not necessary that you refrain from casting, it's just necessary to
refrain from using that as an actual pointer in the future. I can't see
how you might print a pointer value without casting it to an integer ;)
What is more important is that you don't use that as the ONLY reference
to the object.
Also, AIUI the current GC already does not scan size_t[] blocks for
pointers, so if you cast a pointer to size_t and put it in such an array
with no other references to your object, you stand the chance of the GC
collecting your object while it is still live.
Again, the key is "no other references". The document referenced above
just says you can't cast.
or "Do not take advantage of alignment of pointers to store bit flags
in the low order bits".
[...]
Won't a precise GC scanning for pointers to aligned objects want to skip
values that can't be an aligned pointer? Though in D's case, being
required to be conservative would negate this.
int[] x = [1,2,3];
void[] y = x;
y = y[1 .. $]; // misaligned pointer.
It's easy to do, and it's done all over the place (buffers anyone?).
-Steve
