[LLVMdev] Initializing GC roots

[Gordon Henriksen]

On Oct 6, 2011, at 17:19, Yiannis Tsiouris wrote:

> Hello all,
> 
> I set: InitRoots = true; in my gc plugin as i want the roots to be
> initialized to the "null" value.
> Is there a way to define which value should be the initial one? For
> example, i would like to initialize my roots to -5 (tagged, null value
> for the GC in my runtime system) instead of 0.
> 
> Ofcourse, i could do it in the frontend (storing -5 to all GC roots),
> but i was wondering if there is another way e.g. by modifying the GC
> plugin and implementing custom lowering for the gcroot intrinsic.

Hello Yiannis,

Yes, what you describe should be possible. In your GCStrategy subclass constructor, make the following change:

 class MyGCStrategy : public GCStrategy {
   MyGCStrategy() {
-    InitRoots = true;
+    CustomRoots = true;
   }
+  
+  bool performCustomLowering(Function &F) {
+    // You can use LowerIntrinsics::InsertRootInitializers as a template.
+  }
 };

LowerIntrinsics::InsertRootInitializers is only 26 lines of code. You can find it here:

    http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/GCStrategy.cpp?annotate=123170#l174

More invasively, you could consider adding a hook to GCStrategy which replaces the indicated call to ConstantPointerNull::get in LowerIntrinsics::InsertRootInitializers:
 
  // Add root initializers.
  bool MadeChange = false;
  
  for (AllocaInst **I = Roots, **E = Roots + Count; I != E; ++I)
    if (!InitedRoots.count(*I)) {
====> StoreInst* SI = new StoreInst(ConstantPointerNull::get(cast<PointerType>(
====>                   cast<PointerType>((*I)->getType())->getElementType())),
                        *I);
      SI->insertAfter(*I);
      MadeChange = true;
    }
  
  return MadeChange;

But such a change would not be strictly necessary.

Hope this helps,
Gordon