Swift ARC Uses Reference Counting, Not Heap Scanning — Experiment Confirms

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A developer conducted a source-to-runtime experiment using Swift 6.3.1 to demonstrate that Swift's Automatic Reference Counting (ARC) does not scan the heap like a tracing garbage collector, but instead tracks object lifetimes through compiler-inserted retain and release operations. The experiment traced ownership rules across four layers: Swift source, SIL (Swift Intermediate Language), LLVM IR, and runtime execution. To prove the mechanism, the researcher deliberately removed a single strong_release instruction from the SIL output, which caused the object's deinit to stop firing entirely. The memory impact was significant: peak RSS grew from approximately 2.5 MB to 145 MB under a repeated-allocation workload, confirming that object destruction is triggered solely by reference-count decrements reaching zero. The findings highlight a common misconception about the word 'Automatic' in ARC, clarifying that it refers to compiler automation rather than any background memory-scanning process.

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