compiler,runtime: implement a portable conservative GC

src/runtime/gc_globals_precise.go

@@ -0,0 +1,35 @@
+// +build gc.conservative
+// +build !cortexm
+
+package runtime
+
+import (
+	"unsafe"
+)
+
+//go:extern runtime.trackedGlobalsStart
+var trackedGlobalsStart uintptr
+
+//go:extern runtime.trackedGlobalsLength
+var trackedGlobalsLength uintptr
+
+//go:extern runtime.trackedGlobalsBitmap
+var trackedGlobalsBitmap [0]uint8
+
+// markGlobals marks all globals, which are reachable by definition.
+//
+// This implementation relies on a compiler pass that stores all globals in a
+// single global (adjusting all uses of them accordingly) and creates a bit
+// vector with the locations of each pointer. This implementation then walks the
+// bit vector and for each pointer it indicates, it marks the root.
+//
+//go:nobounds
+func markGlobals() {
+	for i := uintptr(0); i < trackedGlobalsLength; i++ {
+		if trackedGlobalsBitmap[i/8]&(1<<(i%8)) != 0 {
+			addr := trackedGlobalsStart + i*unsafe.Alignof(uintptr(0))
+			root := *(*uintptr)(unsafe.Pointer(addr))
+			markRoot(addr, root)
+		}
+	}
+}