[LLVMdev] getting started with IR needing GC

[Gordon Henriksen]

On 2008-04-28, at 21:19, Lane Schwartz wrote:

> On Mon, Apr 28, 2008 at 2:13 PM, Gordon Henriksen <gordonhenriksen at mac.com 
> > wrote:
>
>>> If so, then a Collector plugin would need to have info about every  
>>> supported backend lays out the runtime stack?
>>
>> Yes. This information is actually available in a target-independent  
>> fashion with LLVM, so the Collector interface is target- 
>> independent. A backend that doesn't use the target-independent back- 
>> end could also implement support for values of gc "...", but this  
>> is quite hypothetical.
>
> Right. So a GC plugin needs to know a few things in order to find GC  
> roots. It needs to know the stack pointer, the frame pointer,  
> possibly static links (if we allow nested functions), and where in  
> the stack frame local variables live.
>
> How do you get access to this data in a platform-agnostic manner?

http://llvm.org/docs/GarbageCollection.html#stack-map

>> Such a runtime will further need a way to crawl the native call  
>> stack and discover the return address of each call frame. LLVM  
>> doesn't provide such a facility.
>
> I guess I'm missing something here. Why does the GC need the return  
> addresses?


Return addresses are directly discoverable from the stack; function  
entry points are not.

— Gordon