Assert triggered when complex GC roots initialized

[llvmbot]

Bugzilla Link	2911
Resolution	LATER
Resolved on	Oct 25, 2008 10:07
Version	2.3
OS	Windows XP
Reporter	LLVM Bugzilla Contributor
CC	@asl

Extended Description

In some GC systems where fat pointers are used to keep track of references within objects, instances of fat pointers may be rooted on the stack to make them visible to the garbage collector. When the root initializer code in the collector base class hits these it will assert because it is not a pointer and a cast<> operation cannot be performed safely. This happens for example with this code:

%FatPointerType = type { i8*, i32 }
declare void @​llvm.gcroot(i8**, i8*) nounwind

define i32 @​TestFunc() gc "somegc"
{
%local_fatpointer = alloca %FatPointerType
%local_fatpointer_ptr = bitcast %FatPointerType* %local_fatpointer to i8**
call void @​llvm.gcroot(i8** %local_fatpointer_ptr, i8* null)

ret i32 0

}

[llvmbot]

This is by design. That is, the system only supports using pointers as gcroots. I don't see this changing before a major redesign of the GC system (but patches are welcome!).

A workaround would be to create (locally constant) pointers to your structs and pass those to llvm.gcroot:

%fp = alloca %FatPointerType
store %FatPointerType { i8* null, i32 0 }, %FatPointerType* %fp
%fp_root = alloca %FatPointerType*
%fp_root2 = bitcast %FatPointerType** %fp_root to i8**
call void @​llvm.gcroot(i8** %fp_root2, i8* null)
store %FatPointerType* %fp, %FatPointerType** %fp

This will cost an extra indirection in the collector and increase function call overhead.