cmd/compile: in escape analysis, use element type for OIND of slice

The escape analysis models the flow of "content" of X with a
level of "indirection" (OIND node) of X. This content can be
pointer dereference, or slice/string element. For the latter
case, the type of the OIND node should be the element type of
the slice/string. This CL fixes this. In particular, this
matters when the element type is pointerless, where the data
flow should not cause any escape.

Fixes #15730.

Change-Id: Iba9f92898681625e7e3ddef76ae65d7cd61c41e0
Reviewed-on: https://go-review.googlesource.com/107597
Reviewed-by: David Chase <drchase@google.com>

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

@@ -1405,11 +1405,13 @@ func (e *EscState) addDereference(n *Node) *Node {
 	e.nodeEscState(ind).Loopdepth = e.nodeEscState(n).Loopdepth
 	ind.Pos = n.Pos
 	t := n.Type
-	if t.IsKind(types.Tptr) {
+	if t.IsKind(types.Tptr) || t.IsSlice() {
 		// This should model our own sloppy use of OIND to encode
-		// decreasing levels of indirection; i.e., "indirecting" an array
-		// might yield the type of an element. To be enhanced...
+		// decreasing levels of indirection; i.e., "indirecting" a slice
+		// yields the type of an element.
 		t = t.Elem()
+	} else if t.IsString() {
+		t = types.Types[TUINT8]
 	}
 	ind.Type = t
 	return ind

test/escape5.go

@@ -194,3 +194,37 @@ func (t *T24730) g() { // ERROR "t does not escape"
 		*z = 2
 	}
 }
+
+// Issue 15730: copy causes unnecessary escape
+
+var sink []byte
+var sink2 []int
+var sink3 []*int
+
+func f15730a(args ...interface{}) { // ERROR "args does not escape"
+	for _, arg := range args {
+		switch a := arg.(type) {
+		case string:
+			copy(sink, a)
+		}
+	}
+}
+
+func f15730b(args ...interface{}) { // ERROR "args does not escape"
+	for _, arg := range args {
+		switch a := arg.(type) {
+		case []int:
+			copy(sink2, a)
+		}
+	}
+}
+
+func f15730c(args ...interface{}) { // ERROR "leaking param content: args"
+	for _, arg := range args {
+		switch a := arg.(type) {
+		case []*int:
+			// copy pointerful data should cause escape
+			copy(sink3, a)
+		}
+	}
+}