cmd/internal/obj, runtime: use register map to mark unsafe points

Currently we use stack map index -2 to mark unsafe points, i.e.
PC ranges that is not safe for async preemption. This has a
problem: it cannot mark CALL instructions, because for stack scan
a valid stack map index is needed.

This CL switches to use register map index for marking unsafe
points instead, which does not conflict with stack scan and can
be applied on CALL instructions. This is necessary as next CL
will mark call to morestack nonpreemptible.

For #35470.

Change-Id: I357bf26c996e1fee1e7eebe4e6bb07d62930d3f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/207349Reviewed-by: David Chase <drchase@google.com>

src/cmd/internal/obj/plist.go

@@ -215,12 +215,10 @@ func (ctxt *Link) StartUnsafePoint(p *Prog, newprog ProgAlloc) *Prog {
 	pcdata := Appendp(p, newprog)
 	pcdata.As = APCDATA
 	pcdata.From.Type = TYPE_CONST
-	pcdata.From.Offset = objabi.PCDATA_StackMapIndex
+	pcdata.From.Offset = objabi.PCDATA_RegMapIndex
 	pcdata.To.Type = TYPE_CONST
 	pcdata.To.Offset = -2 // pcdata -2 marks unsafe point
 
-	// TODO: register map?
-
 	return pcdata
 }
 
@@ -232,7 +230,7 @@ func (ctxt *Link) EndUnsafePoint(p *Prog, newprog ProgAlloc, oldval int64) *Prog
 	pcdata := Appendp(p, newprog)
 	pcdata.As = APCDATA
 	pcdata.From.Type = TYPE_CONST
-	pcdata.From.Offset = objabi.PCDATA_StackMapIndex
+	pcdata.From.Offset = objabi.PCDATA_RegMapIndex
 	pcdata.To.Type = TYPE_CONST
 	pcdata.To.Offset = oldval
 
@@ -248,7 +246,7 @@ func MarkUnsafePoints(ctxt *Link, p0 *Prog, newprog ProgAlloc, isUnsafePoint fun
 	prev := p0
 	oldval := int64(-1) // entry pcdata
 	for p := prev.Link; p != nil; p, prev = p.Link, p {
-		if p.As == APCDATA && p.From.Offset == objabi.PCDATA_StackMapIndex {
+		if p.As == APCDATA && p.From.Offset == objabi.PCDATA_RegMapIndex {
 			oldval = p.To.Offset
 			continue
 		}

src/runtime/preempt.go

@@ -393,7 +393,7 @@ func isAsyncSafePoint(gp *g, pc, sp, lr uintptr) bool {
 		// use the LR for unwinding, which will be bad.
 		return false
 	}
-	smi := pcdatavalue(f, _PCDATA_StackMapIndex, pc, nil)
+	smi := pcdatavalue(f, _PCDATA_RegMapIndex, pc, nil)
 	if smi == -2 {
 		// Unsafe-point marked by compiler. This includes
 		// atomic sequences (e.g., write barrier) and nosplit