[LLVMdev] Using gcroot with value types

[Paul Melis]

On 04/29/10 21:27, Talin wrote:
> On Wed, Apr 28, 2010 at 12:16 PM, Paul Melis
> <llvm at assumetheposition.nl <mailto:llvm at assumetheposition.nl>> wrote:
>
>     On 04/27/10 00:20, Talin wrote:
>>     On Mon, Apr 26, 2010 at 12:44 AM, Paul Melis
>>     <llvm at assumetheposition.nl <mailto:llvm at assumetheposition.nl>> wrote:
>>
>>         Hi,
>>
>>         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.
>>
>>     Ugh. If all roots have to be singleton pointers, not embedded
>>     within a larger structure, then I'm not sure that what I want to
>>     do is even possible.
>     In principle you could probably mark the alloca's of your value
>     structs as gcroots. Then during collection you would have to make
>     sure you recognize the value types so you don't
>     move/deallocate/... them during a collection. That would still
>     give you the option to trace all live objects by simply traversing
>     the fields of a value struct. You would only have to do this for
>     value structs (indirectly) containing references. Does that make
>     sense?
>
>
> Unfortunately, when I try to do this, the function verifier reports
> the function as broken. It requires that the argument to llvm.gcroot
> be a pointer to an alloca of a pointer. I can't fool it with casts.
Oops, I wasn't specific enough. Marking an alloca of a value type indeed
won't work, as you noticed. Put differently, the idea is that value
types and heap types are treated the same w.r.t. marking them with
gcroot, while the former is allocated using alloca and the latter with
your GC allocation routine. It's only during collection that you have to
make sure not to collect value types.

  // Some value type
  %struct.V = type { i32, float, i8* }

  // Allocate space for a pointer to a V instance, which we mark as GC root
  %v = alloca %struct.V*, align 8                 ; <%struct.V**> [#uses=2]
  %0 = bitcast %struct.V** %v to i8**             ; <i8**> [#uses=1]
  call void @llvm.gcroot(i8** %0, i8* null)

  // Allocate the V instance on the stack and store it's address in the root
  %1 = alloca %struct.V, align 4                  ; <%struct.V*> [#uses=3]
  store %struct.V* %1, %struct.V** %v, align 8


Paul
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