Implement Reference Counting GC

Reclaim at Zero

Reference counting increments on alias creation, decrements on drop. At zero, recurse into children. Swift ARC and Python's primary mechanism use refcounting. Cycles (a->b->a) need a separate cycle breaker or weak refs. Objective-C and COM also built industrial-strength systems on refcounting before tracing GC went mainstream.

Operations

inc(obj): header count++. dec(obj): count--; if zero, free children then object. Combine with cycle detection (Python's gc module) for completeness. Thread-safe refcounting uses atomic increment and decrement on every pointer copy.

cCopy
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• Atomic refcount for thread-safe sharing
• Cycles leak without extra pass
• Deferred refcounting batches updates in some VMs
• RC + mark-sweep for cycles is hybrid approach

Connecting to real compilers

Production compilers embed this step inside a longer pipeline. GCC flows through cpp, cc1, assembly, and ld; Clang uses the driver, Sema, LLVM IR passes, and a target backend. LLVM bitcode, JVM bytecode, and WASM are other familiar IRs at the same layer. The exercise isolates one pass so you can test it alone before chaining it to the next stage.

Why for this exercise

You will implement reference counting garbage collection. This exercise requires increment and decrement on pointer operations and recursive freeing when counts reach zero.