llvm dev - Jul 2008 - [LLVMdev] CollectorRegistry

Hey,

I am a bit confused about the CollectorRegistry.

I am attempting to write a garbage collector for LLVM, and the tiny
example in the docs at
http://llvm.org/releases/2.3/docs/GarbageCollection.html gives this
line:

   CollectorRegistry::Add<MyCollector> X("mygc", "My
bespoke garbage
collector.");

My question is now: Am I supposed to instantiate my collector
manually, and tell LLVM about it, or can the Registry instantiate it,
and it that case, how to go about that? I haven't been able to deduce
a way to get a pointer to my collector from the doxygen docs or the
header files, so I hope that you can help me here. :-)

Yours,
Simon

On 2008-07-23, at 08:58, Simon Ask Ulsnes wrote:
> I am attempting to write a garbage collector for LLVM, and the tiny  
> example in the docs at
http://llvm.org/releases/2.3/docs/GarbageCollection.html
>  gives this line:
>
>   CollectorRegistry::Add<MyCollector>
>   X("mygc", "My bespoke garbage collector.");
>
> My question is now: Am I supposed to instantiate my collector  
> manually, and tell LLVM about it, or can the Registry instantiate  
> it, and it that case, how to go about that? I haven't been able to  
> deduce a way to get a pointer to my collector from the doxygen docs  
> or the header files, so I hope that you can help me here. :-)

Hi Simon,

The compiler framework will instantiate and call into your Collector  
subclass automatically as required. The CollectorRegistry::Add<>  
instance advertises your class to the framework so it can do this.  
Your Collector class needs to have a default constructor in order to  
insantiate the Add<> template.

The framework decides which Collector to use based upon the 'gc'  
attribute of a function:

define void @f() gc "mygc" {
   ...
}

This string must correspond to the first argument to the  
CollectorRegistry::Add<> constructor.

The second argument to the constructor is, I think, "purely  
informational" for collectors. For the related TargetMachineRegistry,  
the second argument appears in --help output.

Hope that helps,
Gordon

Thank you for that clarification.
> The framework decides which Collector to use based upon the 'gc'
> attribute of a function:
>
> define void @f() gc "mygc" {
>   ...
> }
OK, so for instance if I wanted to be able to use the GC from a C
frontend (presumably by using llvm_gc_allocate?), do the C functions
need this attribute as well?

And if so, can this attribute be applied to functions in bitcode
compiled by llvm-gcc, without modifications to the C compiler or the
output bitcode?
I.e., I use a custom driver program similar to lli that loads in
bitcode and my GC, and executes it, and ideally programs would be able
to use the GC without needing bitcode-level modifications (it's
perfectly ok to have to replace calls to malloc, though).
>
> This string must correspond to the first argument to the
> CollectorRegistry::Add<> constructor.
>
> The second argument to the constructor is, I think, "purely
> informational" for collectors. For the related TargetMachineRegistry,
> the second argument appears in --help output.
>
> Hope that helps,
> Gordon
It helps a lot. Thank you for your time. :-)

Simon