cmd/compile: use raceX instead of raceXrange for types without subcomponents

Change-Id: I9882488e69565dc9da6814fefbdba3621daf74fe
Reviewed-on: https://go-review.googlesource.com/59332
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>

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

@@ -504,13 +504,18 @@ func callinstr(np **Node, init *Nodes, wr int, skip int) bool {
 				name = "msanwrite"
 			}
 			f = mkcall(name, nil, init, uintptraddr(n), nodintconst(w))
-		} else if flag_race && (t.IsStruct() || t.IsArray()) {
+		} else if flag_race && t.NumComponents() > 1 {
+			// for composite objects we have to write every address
+			// because a write might happen to any subobject.
+			// composites with only one element don't have subobjects, though.
 			name := "racereadrange"
 			if wr != 0 {
 				name = "racewriterange"
 			}
 			f = mkcall(name, nil, init, uintptraddr(n), nodintconst(w))
 		} else if flag_race {
+			// for non-composite objects we can write just the start
+			// address, as any write must write the first byte.
 			name := "raceread"
 			if wr != 0 {
 				name = "racewrite"

src/cmd/compile/internal/types/type.go

@@ -1317,6 +1317,23 @@ func (t *Type) SetNumElem(n int64) {
 	at.Bound = n
 }
 
+func (t *Type) NumComponents() int64 {
+	switch t.Etype {
+	case TSTRUCT:
+		if t.IsFuncArgStruct() {
+			Fatalf("NumComponents func arg struct")
+		}
+		var n int64
+		for _, f := range t.FieldSlice() {
+			n += f.Type.NumComponents()
+		}
+		return n
+	case TARRAY:
+		return t.NumElem() * t.Elem().NumComponents()
+	}
+	return 1
+}
+
 // ChanDir returns the direction of a channel type t.
 // The direction will be one of Crecv, Csend, or Cboth.
 func (t *Type) ChanDir() ChanDir {