cmd/gc: enable racewalk of HMUL nodes.

A HMUL node appears in some constant divisions, but
to observe a false negative in race detector the divisor must be
suitably chosen to make sure the only memory access is
done for HMUL.

R=dvyukov
CC=golang-dev
https://golang.org/cl/7935045

src/cmd/gc/racewalk.c

@@ -241,6 +241,7 @@ racewalknode(Node **np, NodeList **init, int wr, int skip)
 	case OXOR:
 	case OSUB:
 	case OMUL:
+	case OHMUL:
 	case OEQ:
 	case ONE:
 	case OLT:
@@ -379,19 +380,18 @@ racewalknode(Node **np, NodeList **init, int wr, int skip)
 	case OPARAM:     // it appears only in fn->exit to copy heap params back
 	case OCLOSUREVAR:// immutable pointer to captured variable
 	case ODOTMETH:   // either part of CALLMETH or CALLPART (lowered to PTRLIT)
-		goto ret;
-
-	// unimplemented
-	case OSLICESTR:
-	case OAPPEND:
-	case ODCL:
+	case OINDREG:    // at this stage, only n(SP) nodes from nodarg
+	case ODCL:       // declarations (without value) cannot be races
 	case ODCLCONST:
 	case ODCLTYPE:
-	case OLITERAL:
 	case OTYPE:
 	case ONONAME:
-	case OINDREG:
-	case OHMUL:
+	case OLITERAL:
+	case OSLICESTR:  // always preceded by bounds checking, avoid double instrumentation.
+		goto ret;
+
+	// unimplemented
+	case OAPPEND:
 		goto ret;
 	}
 

src/pkg/runtime/race/testdata/mop_test.go

@@ -339,11 +339,11 @@ func TestRaceDiv(t *testing.T) {
 }
 
 func TestRaceDivConst(t *testing.T) {
-	var x, y, z int
+	var x, y, z uint32
 	ch := make(chan int, 2)
 
 	go func() {
-		x = y / 3
+		x = y / 3 // involves only a HMUL node
 		ch <- 1
 	}()
 	go func() {