runtime: allow custom-gc SetFinalizer and clarify KeepAlive

src/runtime/gc_blocks.go

@@ -687,3 +687,7 @@ func ReadMemStats(m *MemStats) {
 	m.Frees = gcFrees
 	m.Sys = uint64(heapEnd - heapStart)
 }
+
+func SetFinalizer(obj interface{}, finalizer interface{}) {
+	// Unimplemented.
+}

src/runtime/gc_custom.go

@@ -20,6 +20,7 @@ package runtime
 // - func free(ptr unsafe.Pointer)
 // - func markRoots(start, end uintptr)
 // - func GC()
+// - func SetFinalizer(obj interface{}, finalizer interface{})
 // - func ReadMemStats(ms *runtime.MemStats)
 //
 //
@@ -51,6 +52,9 @@ func markRoots(start, end uintptr)
 // GC is called to explicitly run garbage collection.
 func GC()
 
+// SetFinalizer registers a finalizer.
+func SetFinalizer(obj interface{}, finalizer interface{})
+
 // ReadMemStats populates m with memory statistics.
 func ReadMemStats(ms *MemStats)
 

src/runtime/gc_leaking.go

@@ -85,6 +85,10 @@ func GC() {
 	// No-op.
 }
 
+func SetFinalizer(obj interface{}, finalizer interface{}) {
+	// No-op.
+}
+
 func initHeap() {
 	// preinit() may have moved heapStart; reset heapptr
 	heapptr = heapStart

src/runtime/gc_none.go

@@ -26,6 +26,10 @@ func GC() {
 	// Unimplemented.
 }
 
+func SetFinalizer(obj interface{}, finalizer interface{}) {
+	// Unimplemented.
+}
+
 func initHeap() {
 	// Nothing to initialize.
 }

src/runtime/runtime.go

@@ -89,11 +89,10 @@ func UnlockOSThread() {
 }
 
 func KeepAlive(x interface{}) {
-	// Unimplemented. Only required with SetFinalizer().
-}
-
-func SetFinalizer(obj interface{}, finalizer interface{}) {
 	// Unimplemented.
+	// TODO: This function needs to be implemented in a way that LLVM doesn't optimize away the x
+	// parameter. This will likely need either a volatile operation or calling an assembly stub
+	// that simply returns.
 }
 
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