cmd/compile: don't emit stack maps for write barrier calls

These are necessarily deeply non-preemptible, so there's no point in
emitting stack maps for them. We already mark them as unsafe points,
so this only affects the runtime, since user code does not emit stack
maps at unsafe points. SSAGenState.PrepareCall also excludes them when
it's sanity checking call stack maps.

Right now this only drops a handful of unnecessary stack maps from the
runtime, but we're about to start emitting stack maps only at calls
for user code, too. At that point, this will matter much more.

For golang#36365.

Change-Id: Ib3abfedfddc8e724d933a064fa4d573500627990
Reviewed-on: https://go-review.googlesource.com/c/go/+/230542
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/gc/plive.go

@@ -838,7 +838,16 @@ func (lv *Liveness) hasStackMap(v *ssa.Value) bool {
 	// we only need stack maps at call sites. go:nosplit functions
 	// are similar.
 	if compiling_runtime || lv.f.NoSplit {
-		return v.Op.IsCall()
+		if !v.Op.IsCall() {
+			return false
+		}
+		// typedmemclr and typedmemmove are write barriers and
+		// deeply non-preemptible. They are unsafe points and
+		// hence should not have liveness maps.
+		if sym, _ := v.Aux.(*obj.LSym); sym == typedmemclr || sym == typedmemmove {
+			return false
+		}
+		return true
 	}
 
 	switch v.Op {

src/cmd/compile/internal/gc/ssa.go

@@ -6572,9 +6572,7 @@ func (s *SSAGenState) Call(v *ssa.Value) *obj.Prog {
 func (s *SSAGenState) PrepareCall(v *ssa.Value) {
 	idx := s.livenessMap.Get(v)
 	if !idx.StackMapValid() {
-		// typedmemclr and typedmemmove are write barriers and
-		// deeply non-preemptible. They are unsafe points and
-		// hence should not have liveness maps.
+		// See Liveness.hasStackMap.
 		if sym, _ := v.Aux.(*obj.LSym); !(sym == typedmemclr || sym == typedmemmove) {
 			Fatalf("missing stack map index for %v", v.LongString())
 		}