Implement Write Barriers for GC

Remembered Sets for Generational GC

Minor GC scans nursery but must also scan old objects pointing into nursery. Tracking every old object is expensive. Write barriers log stores of old->young pointers into a remembered set or card table. HotSpot and Go use variants.

Barrier on Store

After old.field = young, barrier marks card containing old as dirty or logs the slot. Minor GC roots include remembered set entries.

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• Store barriers vs load barriers (concurrent GC)
• Card size trades precision vs scanning cost
• Floating garbage in remembered set is ok
• Barrier code is inlined at every reference store

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 write barriers for generational GC. This exercise asks you to instrument reference stores and maintain a remembered set so minor collections find all nursery pointers.