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by NobleExpress 1193 days ago
C# has interior pointers as well and it is a language with a precise, moving garbage collector. The deal with Go is that is has a concurrent mark-sweep collector, so it is non-moving. Given the non-moving collector, it is significantly easier to reason with than a moving collector. Hence, you would have a lot of unstated assumptions (about whether the collector moves objects or not) baked into the runtime that would need to be fixed. You probably can't use a moving collector in Go like G1 or ZGC or LXR without non-trivial refactoring.
1 comments
fweimer 1192 days ago
Oh, interesting. It's the & type in the CLI (I.12.1.1.2 in ECMA-335).

It makes Java's GCs quite Java-specific in practice because there aren't that languages that even moderately widely used which are both statically typed (to the degree that it's possible to tell pointers apart before generating code) and restrict pointers to point to the start of the object.

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NobleExpress 1192 days ago
Ehh yes and no. Many languages indeed use conservative GCs, but that is because it is significantly easier to implement than precise GCs which require stack maps etc. I don't think it's exactly a question about languages and static types/generating code, it's more of an implementation detail. Conservative GCs can be fast, but they are inherently less space-efficient as you retain more objects than a precise GC would. Of course in some cases a conservative GC _is_ required, for example if your language has heavy interop with C/C++ land.

Also supporting interior pointers is not too cumbersome even with a "Java-style GC" (whatever that exactly means). It requires an additional bit per smallest aligned object size to denote if an object is allocated or not. If you need to check if a pointer is an interior pointer to an object, you walk back in the bitmap and find the last object with a bit set to 1 (i.e. find the allocated object which was right before the pointer you have), get the size of that object and check that the pointer you have is within the range of the start and end of the object.

EDIT: The big issue you would face with directly porting a Java GC into other languages is the weak reference processing semantics. Each language has their own subtle differences which would make it a pain.

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