One of my work projects involved building a .NET runtime with LLVM. It evolved from conservative garbage collection through to an advanced precise collector supporting a multithreaded runtime.
The conservative collector required no support from the runtime as they operate by treating every word-sized value on the stack and heap as a potential reference. All you need for a conservative collector is to be able to know where your stack starts and ends, where your heap begins and ends, and where your globals are.
We went through a couple designs for precise collectors and they all used the same meta-information. A precise collector needs to know what is a reference and what is not so your compiler needs to emit stack maps (tables of offsets where, given your PC in a function, all the live objects are), global root information, as well as object layout information.
Sometimes this becomes more complex. Object layout information included both location of references within type instances as well as whether or not an object is an array. Arrays of value types require both.
.NET also has a type of reference, a managed pointer, which can be used to refer to the middle of an object, such as a pointer to a value type in the middle of an array, which contributes to the parent object's liveliness information.
To sum up, conservative garbage collection can be easy to add. Precise garbage collection much less so. Language specifics can throw a big wrench into things.
The conservative collector required no support from the runtime as they operate by treating every word-sized value on the stack and heap as a potential reference. All you need for a conservative collector is to be able to know where your stack starts and ends, where your heap begins and ends, and where your globals are.
We went through a couple designs for precise collectors and they all used the same meta-information. A precise collector needs to know what is a reference and what is not so your compiler needs to emit stack maps (tables of offsets where, given your PC in a function, all the live objects are), global root information, as well as object layout information.
Sometimes this becomes more complex. Object layout information included both location of references within type instances as well as whether or not an object is an array. Arrays of value types require both.
.NET also has a type of reference, a managed pointer, which can be used to refer to the middle of an object, such as a pointer to a value type in the middle of an array, which contributes to the parent object's liveliness information.
To sum up, conservative garbage collection can be easy to add. Precise garbage collection much less so. Language specifics can throw a big wrench into things.