On 23/05/2013 3:33 AM, Tom Lee wrote:
Can anybody out there offer some clarification about the following?
* What are glue_fns and why are they necessary? How/why do they differ
from shim_fns (used to invoke foreign functions)?
Glue functions are invoked by the compiler when types are dropped or
copied. They manage the lifecycle of the type, and are ubiquitous and
compiler-generated. They have nothing to do with shim functions aside
from them both being compiler-generated (and too-numerous for our taste!)
* What's the purpose of /visit_glue/ in the type_desc struct? The
others seem largely obvious (take/drop doing some sort of
refcounting, free to clean up, but visit ... ?)
Visit is for reflection. See core::reflect (renamed last night to
std::reflect).
* Where are type_descs/tydescs written? The stack? The heap(s)? (i.e.
anything that can be a GC root?) Where in the source code does this
happen?
They're global constant static data. Compiler-generated.
* What exactly is a "safe point" wrt the garbage collector?
The name refers to the idea (in many GCs) of a place in a code-sequence
at which the compiler has to spill references back to stack locations or
otherwise arrive at a state properly described by metadata / frame maps,
such that it can find all roots. It's only relevant in a precise GC.
Since the current strategy for GC is conservative (presently
_everywhere_, though this will eventually be limited to just-the-stack)
there's on such need: the GC just spills all registers on entry and
treats them as possible-roots like very other word on the stack.
If you see the term in our code, it's residue from the most recent
experiment with precise GC, which has been (for the time being) abandoned.
-Graydon
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