llvm dev - Mar 2009 - [LLVMdev] Garbage collection: Multiple copies of a root

Hi,

As far as I understand now, only stack variables can be marked as gc  
roots. Our
compiler however can also generate roots that are not necessarily on  
the stack.
We can solve this by putting it on the stack anyway. So far so good.

The problem is that we want to implement some sort of copying garbage  
collector.
In this case, the roots on the stack must be updated. However, if  
this root is also
stored in an IR register, then this pointer should be updated as  
well. Is there
support for identifying and updating such references in the GC  
framework, or should we generate
code ourselves to reload the updated references from the stack.

Thanks,

Harmen

On 2009-03-20, at 05:09, Harmen van der Spek wrote:
> The problem is that we want to implement some sort of copying  
> garbage collector. In this case, the roots on the stack must be  
> updated. However, if this root is also stored in an IR register,  
> then this pointer should be updated as well. Is there support for  
> identifying and updating such references in the GC framework, or  
> should we generate code ourselves to reload the updated references  
> from the stack.
You should generate code to reload the registers from the stack for  
now. We'd ultimately like to move to a model where LLVM automatically  
inserts the reloads, only as necessary, but are not there yet.

— Gordon

Another open question is who is responsible for inserting calls to the  
collector routine (or signaling a collector thread)?

My impression is that this has to be done in the front-end for now  
since there is no support for doing this at the safe points at the  
moment.

Is this correct?


Regards,
Mattias

On 20 Mar 2009, at 13:09, Gordon Henriksen wrote:
> On 2009-03-20, at 05:09, Harmen van der Spek wrote:
>
>> The problem is that we want to implement some sort of copying
>> garbage collector. In this case, the roots on the stack must be
>> updated. However, if this root is also stored in an IR register,
>> then this pointer should be updated as well. Is there support for
>> identifying and updating such references in the GC framework, or
>> should we generate code ourselves to reload the updated references
>> from the stack.
>
> You should generate code to reload the registers from the stack for
> now. We'd ultimately like to move to a model where LLVM automatically
> inserts the reloads, only as necessary, but are not there yet.
>
> — Gordon
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