On Monday, 15 June 2015 at 16:16:28 UTC, Steven Schveighoffer
wrote:
How can the compiler POSSIBLY know whether a pointer points at
GC data in order to make it undefined behavior?
It doesn't need to, the undefined behaviour comes from the
unfulfilled expectations of the GC. For example, a "sufficiently
advanced GC" with enough knowledge of types, including of local
vars, could by chance run just at the moment when the only
instance of the address is stored in an int variable, and
disregard that variable because it's not a pointer. It is of
course unlikely that GCs will ever get as much knowledge about
local variables, or even unnamed temporaries...
I don't see any justification for any of these restrictions.
The "to allow maximum flexibility" justification needs more
explanation, what possible feature could you implement given
this restriction?
It even allows using unions to do exactly the same thing. There
is just no way this cannot be allowed.
For unions, an advanced GC would know the type and would make
sure to treat them conservatively. The restrictions are for the
other cases where the types are references. The GC needs to rely
on them being pointer if it wants to move them, for example.
But I think you are right that the current restrictions are too
strong. After some thoughts, I can't really see any realistic
problem with tagged pointers.
