runtime: don't async preempt NO_LOCAL_POINTERS assembly functions

We don't async preempt assembly functions. We do that by checking
whether the function has a local pointer map, and assume it is
an assembly (or, non-Go) function if there isn't one. However,
assembly functions marked with NO_LOCAL_POINTERS still have local
pointer maps, and we wouldn't identify them. For them, check for
the special pointer map runtime.no_pointers_stackmap as well, and
treat them as not async preemptible.

Change-Id: I1301e3b4d35893c31c4c5a5147a0d775987bd6f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/202337Reviewed-by: Austin Clements <austin@google.com>

src/runtime/preempt.go

@@ -55,6 +55,7 @@ package runtime
 import (
 	"runtime/internal/atomic"
 	"runtime/internal/sys"
+	"unsafe"
 )
 
 type suspendGState struct {
@@ -369,9 +370,12 @@ func isAsyncSafePoint(gp *g, pc, sp uintptr) bool {
 		// functions (except at calls).
 		return false
 	}
-	if funcdata(f, _FUNCDATA_LocalsPointerMaps) == nil {
+	if fd := funcdata(f, _FUNCDATA_LocalsPointerMaps); fd == nil || fd == unsafe.Pointer(&no_pointers_stackmap) {
 		// This is assembly code. Don't assume it's
-		// well-formed.
+		// well-formed. We identify assembly code by
+		// checking that it has either no stack map, or
+		// no_pointers_stackmap, which is the stack map
+		// for ones marked as NO_LOCAL_POINTERS.
 		//
 		// TODO: Are there cases that are safe but don't have a
 		// locals pointer map, like empty frame functions?
@@ -395,3 +399,5 @@ func isAsyncSafePoint(gp *g, pc, sp uintptr) bool {
 
 	return true
 }
+
+var no_pointers_stackmap uint64 // defined in assembly, for NO_LOCAL_POINTERS macro