On Tuesday, 21 May 2019 at 13:23:54 UTC, Benjamin Schaaf wrote:
On Tuesday, 21 May 2019 at 11:54:08 UTC, Robert M. Münch wrote:
Is there a trick to accomplish 2 when objects are created from
different scopes which need to be kept? So, I have one
function creating the objects and one using them. How can I
keep things on the stack between these two functions?
How is 3 done? Is this only useful for static variables?
I'll try to describe rules 2 and 3 as simply as possible: As
long as you can access the pointer to gc allocated memory in D
it will not be freed.
If you don't actually access the pointer, the compiler may
optimize-out storage of that pointer and the garbage collector
will then not see it. So this statement should read: "As long as
_the GC_ can see the pointer to gc allocated memory in D it will
not be freed". The GC is looking at registers, stack, static
memory, GC allocated memory, ...
So whether that pointer lives on the stack:
int* foo() {
return new int;
}
void bar() {
int* a = foo();
c_fn(a);
}
This is actually not GC safe [*]. The local storage `a` is
optimized out, and the parameter to `c_fn` is passed in a
register is not guaranteed to not be overwritten by `c_fn`. If
after the overwrite, the GC is invoked (e.g. from other thread,
or from deeper call tree in `c_fn`) then the memory may be freed.
LDC, DMD, GDC, all 3 perform that optimization. So more care is
needed here!
https://d.godbolt.org/z/DumVNF
In static or thread local memory:
int* a;
__gshared int* b;
void bar() {
a = new int;
c_fn(a);
b = a;
c_fn(b);
}
Also here, whole-program analysis/optimization may discover that
`a` and `b` are really never used by anyone. Again,
optimizing-out those storage spaces will make this code GC unsafe
[*]. In this case, I think LTO and LTO visibility of the c_fn
implementation would be needed to do that optimization and thus
probably will be safe, for now (!).
-Johan
[*] The unsafety is a little tricky. If `c_fn` stores the pointer
in a place where the GC can see it (registers, D static memory,
...) all is good. But if `c_fn` stores it in, say, a variable on
the C side, removes it from register, and _then_ GC is invoked,
that's when trouble may happen.
