compiler,runtime: implement a portable conservative GC

src/runtime/gc_stack_portable.go

@@ -0,0 +1,39 @@
+// +build gc.conservative
+// +build !cortexm
+
+package runtime
+
+import (
+	"unsafe"
+)
+
+//go:extern runtime.stackChainStart
+var stackChainStart *stackChainObject
+
+type stackChainObject struct {
+	parent   *stackChainObject
+	numSlots uintptr
+}
+
+// markStack marks all root pointers found on the stack.
+//
+// This implementation is conservative and relies on the compiler inserting code
+// to manually push/pop stack objects that are stored in a linked list starting
+// with stackChainStart. Manually keeping track of stack values is _much_ more
+// expensive than letting the compiler do it and it inhibits a few important
+// optimizations, but it has the big advantage of being portable to basically
+// any ISA, including WebAssembly.
+func markStack() {
+	stackObject := stackChainStart
+	for stackObject != nil {
+		start := uintptr(unsafe.Pointer(stackObject)) + unsafe.Sizeof(uintptr(0))*2
+		end := start + stackObject.numSlots*unsafe.Alignof(uintptr(0))
+		markRoots(start, end)
+		stackObject = stackObject.parent
+	}
+}
+
+// trackPointer is a stub function call inserted by the compiler during IR
+// construction. Calls to it are later replaced with regular stack bookkeeping
+// code.
+func trackPointer(ptr unsafe.Pointer)