On 9/19/14, 10:24 PM, Walter Bright wrote:
On 9/19/2014 9:47 PM, Andrei Alexandrescu wrote:
We've had this discussion numerous times before - "throw the magic
compiler switch" and D becomes an ARC system and everything is
wonderful. It cannot work. ARC and GC are not equivalent.
I think past discussions have been inconclusive at best and must be
reopened.
For starters,
1. ARC is slow and generates bloated code.
Agreed. It has the cost distributed a different way.
2. To deal with (1), existing ARC systems allow escapes from it. This
has severe implications for memory safety. Rust's entire type system
appears to be designed around trying to deal with this, and AFAIK
they're the only ones who have tried. C++ shared_ptr and ObjectiveC's
ARC are not memory safe. They are not. Not not not, and they don't even
try. :-)
That's fine. You can't have everything.
3. ARC objects require an embedded count. This means they cannot be
mixed with non-ARC objects. This is fundamentally DIFFERENT from how GC
behaves, and we cannot pretend or wish this away or add a compiler
switch to make it go away.
In nogc mode, everything is ARC so I'm not getting this.
None of these points were addressed in previous discussions beyond the
supposition of some nonexistent compiler technology that would make them
go away.
That was indeed a crappy argument, and not advanced by me. I'm not
advocating for that.
D can have ref counted objects, but it will not work with a compiler
switch to switch them back and forth. They'll have to coexist peacefully
with GC objects.
We need to figure out a design. All I'm saying is we must not bring
prejudice to the table. And a very basic point is: there will be a way
to COMPLETELY disable the GC. That is a must.
Andrei
