Hi Eric,
On Sep 21, 2007, at 04:49, Eric Christopher wrote:
Here are a pair of patches to add an attribute to automatically
gcroot things based on type. It currently works for pointers to
structures only because a) that's all I need, and b) I think that's
all that makes sense really. I could remove the restriction if
someone can come up with a good use case for other types.
Very cool.
Note that we're also zeroing out automatic variables as well so if
we do a collection before they're initialized we don't end up
following garbage.
@@ -1581,6 +1605,17 @@ void TreeToLLVM::EmitAutomaticVariableDe
// Handle annotate attributes
if (DECL_ATTRIBUTES(decl))
EmitAnnotateIntrinsic(AI, decl);
+
+ // Handle gcroot attribute
+ if (POINTER_TYPE_P(TREE_TYPE (decl))
+ && TYPE_GCROOT(TREE_TYPE (decl)))
+ {
+ // We should null out local variables so that a stack crawl
+ // before initialization doesn't get garbage results to follow.
+ const Type *T = cast<PointerType>(AI->getType())-
>getElementType();
+ EmitTypeGcroot(AI, decl);
+ Builder.CreateStore(Constant::getNullValue(T), AI);
+ }
if (TheDebugInfo) {
if (DECL_NAME(decl)) {
This is redundant; LLVM does it for you. See thread starting here:
http://www.mail-archive.com/[email protected]/msg23987.html
The upshot is that the collector knows whether it is necessary to
initialize roots or not, and does so on the program's behalf iff
necessary.
However, if the source language requires null initialization
semantics, then the front-end should emit explicit null initializers
(as the variable comes into scope).
+// Emits code to do something for a type attribute
+void TreeToLLVM::EmitTypeGcroot(Value *V, tree decl) {
+
+ Function *gcrootFun = Intrinsic::getDeclaration(TheModule,
+ Intrinsic::gcroot);
+
+ // The idea is that it's a pointer to type "Value"
+ // which is opaque* but the routine expects i8** and i8*.
+ const PointerType *Ty = PointerType::get(Type::Int8Ty);
+ V = Builder.CreateBitCast(V, PointerType::get(Ty), "tmp");
The llvm.gcroot intrinsic can be declared to take any types you care
to give it in the module, so long as its eventual type is like void
(<ty>**, <ty2>*).
But since you don't have a parent type to use, what you've done
probably makes as much sense as anything.
— Gordon
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