[dev.ssa] cmd/compile/internal/ssa: fix string slice types.

Change-Id: I28bc6373bb42d9abf4f179664dbaab8d514a6ab9
Reviewed-on: https://go-review.googlesource.com/14376
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -1722,12 +1722,12 @@ func (s *state) expr(n *Node) *ssa.Value {
 		if n.Right.Left == nil {
 			low = zero
 		} else {
-			low = s.expr(n.Right.Left)
+			low = s.extendIndex(s.expr(n.Right.Left))
 		}
 		if n.Right.Right == nil {
 			high = len
 		} else {
-			high = s.expr(n.Right.Right)
+			high = s.extendIndex(s.expr(n.Right.Right))
 		}
 
 		// Panic if slice indices are not in bounds.

src/cmd/compile/internal/gc/testdata/string_ssa.go

@@ -86,9 +86,39 @@ func testStringSlicePanic() {
 	failed = true
 }
 
+const _Accuracy_name = "BelowExactAbove"
+
+var _Accuracy_index = [...]uint8{0, 5, 10, 15}
+
+func testSmallIndexType_ssa(i int) string {
+	switch { // prevent inlining
+	}
+	return _Accuracy_name[_Accuracy_index[i]:_Accuracy_index[i+1]]
+}
+
+func testSmallIndexType() {
+	tests := []struct {
+		i    int
+		want string
+	}{
+		{0, "Below"},
+		{1, "Exact"},
+		{2, "Above"},
+	}
+
+	for i, t := range tests {
+		if got := testSmallIndexType_ssa(t.i); got != t.want {
+			println("#", i, "got ", got, ", wanted", t.want)
+			failed = true
+		}
+	}
+}
+
 func main() {
 	testStringSlice()
 	testStringSlicePanic()
+	testStructSlice()
+	testSmallIndexType()
 
 	if failed {
 		panic("failed")