[LLVMdev] Using gcroot with value types

Paul Melis llvm at assumetheposition.nl
Mon Apr 26 00:44:49 PDT 2010


Talin wrote:
> I'm a little confused as to the rules for the arguments to llvm.gcroot,
> which says it must be a pointer alloca. I'm not sure whether that means it
> must be an alloca (which is always a pointer by definition) or an alloca
> *of* a pointer.

I'm pretty sure it should be "alloca of a pointer", as the first argument
of llvm.gcroot has type i8**.

However, http://llvm.org/docs/GarbageCollection.html#gcroot, isn't
completely clear on this:

"The first argument must be a value referring to an alloca instruction or
a bitcast of an alloca."

This last sentence seems to rule out passing GEPs on alloca's to llvm.gcroot.

> What I am trying to figure out is how to declare roots contained in value
> types. Suppose for example I have a small struct which contains several
> garbage-collectable references. This struct is a local variable, so it's
> contained in a block of memory which has been allocated via alloca. Now, I
> don't want to add the struct itself as a root, since it isn't allocated on
> the heap. But I want to declare the fields within the struct - which are
> GEPs from the alloca pointer - as roots, since they are references to
> objects on the heap.

As reasonable as the above sounds, I don't think you can mark roots
contained in structs. But if I'm wrong I'd love to know ;-)  Perhaps you
can ask the LDC devs how/if they handle this case, as it seems you can
allocate a class instance on the stack in D.


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