[llvm-dev] [GC / Statepoints] Collector supports only base pointers as stack roots

[Manuel Jacob via llvm-dev]

On 2016-01-04 18:27, Philip Reames wrote:
> Fundamentally, the optimizer expects to be able to introduce derived
> pointers (both interior and exterior) in the form of GEPs.  This is
> unavoidable.
Sure.

> If you collector is non-relocating, you can simply ignore the derived
> portion of the relocation record.  We report both the base pointer and
> the derived pointer for each derived pointer.  If you take all the
> reported base pointers, then unique the set, you should have all of
> the objects you need to report as roots.
Sure.

> If your collector is relocating, you'll need to give more information.
>  In particular, I'm not sure you how you can implement a relocating
> collector which doesn't know about derived pointers without some
> really messy hacks.
Can you explain how a collector could possibly handle derived pointers?  
In the meantime I found a solution, but I'm not sure it works in the 
general case and whether it qualifies as a "really messy hack": The 
collector, for each base, relocates the base pointer, computes an offset 
between the old and the new address, and applies that offset to all 
derived pointers from this base.

-Manuel

> Philip
> 
> On 12/30/2015 06:51 PM, Manuel Jacob wrote:
>> Hi,
>> 
>> My collector supports only base pointers as stack roots.  This wasn't 
>> a problem until I tried to run some optimizations before RS4GC, which 
>> introduced (interior) derived pointers.  The statepoint documentation 
>> mentions that these collectors exist, but doesn't mention whether and 
>> how this is currently supported.  What could I do to make it work?
>> 
>> -Manuel