ChadScript

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Memory and the garbage collector ​

The runtime has its own garbage collector, written in Milo (runtime/gc.milo); there is no GC library to install or link. Its design, from the plan:

  • Conservative roots, precise heap. Registers, the running stack, every suspended fiber stack and the program's writable data segment are scanned conservatively, because Milo frames carry no stack maps. Heap objects are traced precisely: every object has a header word naming its layout (atomic, values, record, struct with a pointer bitmap, or conservative), and marking follows exactly the pointer slots the layout names. Numbers and booleans are never traced.
  • Immix-style allocation. 32 KB blocks of 128-byte lines inside 1 MB chunks, with a bump-pointer fast path inlined into generated code and lazy sweeping of free lines. Objects over 8 KB go to a large-object space. The collector does not move objects.
  • Policy. Collect when the bytes allocated since the last collection exceed twice the live heap (at least 1 MB).
  • Gates. CHAD_GC_STRESS=N collects before every Nth allocation and runs over the whole differential suite in CI; CHAD_GC_VERIFY=1 poisons freed lines and checks traced slots; under AddressSanitizer freed lines are poisoned, so a read of a collected object is reported.

Not yet: evacuation or compaction, returning memory to the OS, a generational nursery.

The runtime it lives in ​

The runtime (runtime/*.milo) is written in Milo, a systems language that compiles to LLVM IR. It provides strings and number formatting (JS-exact), shapes and inline-cache misses, Map/Set, util.inspect-compatible printing, JSON, fs and path, exceptions, and the async machinery: fibers on ucontext, a microtask queue, timers and an I/O loop, and the garbage collector. What Milo cannot express (a few C macros and platform-specific structures) lives in runtime/residue.c, about 20 lines of code, each item commented with why.

The compiler commit is pinned in scripts/milo-pin.sh. The driver runs milo emit-ir on runtime/lib.milo, compiles it with the same clang and flags as the program, and caches the object by content hash.

Next: Testing philosophy.