cmd/compile: "abc"[1] is not an ideal constant

"abc"[1] is not like 'b', in that -"abc"[1] is uint8 math, not ideal constant math.
Delay the constantification until after ideal constant folding is over.

Fixes #11370.

Change-Id: Iba2fc00ca2455959e7bab8f4b8b4aac14b1f9858
Reviewed-on: https://go-review.googlesource.com/15740
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -1941,6 +1941,12 @@ func (s *state) expr(n *Node) *ssa.Value {
 	case OINDEX:
 		switch {
 		case n.Left.Type.IsString():
+			if n.Bounded && Isconst(n.Left, CTSTR) && Isconst(n.Right, CTINT) {
+				// Replace "abc"[1] with 'b'.
+				// Delayed until now because "abc"[1] is not an ideal constant.
+				// See test/fixedbugs/issue11370.go.
+				return s.newValue0I(ssa.OpConst8, Types[TUINT8], int64(int8(n.Left.Val().U.(string)[n.Right.Int64()])))
+			}
 			a := s.expr(n.Left)
 			i := s.expr(n.Right)
 			i = s.extendIndex(i, panicindex)

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

@@ -1263,18 +1263,8 @@ opswitch:
 			if Debug['m'] != 0 && n.Bounded && !Isconst(n.Right, CTINT) {
 				Warn("index bounds check elided")
 			}
-			if smallintconst(n.Right) {
-				if !n.Bounded {
-					yyerror("index out of bounds")
-				} else {
-					// replace "abc"[1] with 'b'.
-					// delayed until now because "abc"[1] is not
-					// an ideal constant.
-					v := n.Right.Int64()
-
-					Nodconst(n, n.Type, int64(n.Left.Val().U.(string)[v]))
-					n.Typecheck = 1
-				}
+			if smallintconst(n.Right) && !n.Bounded {
+				yyerror("index out of bounds")
 			}
 		}
 

test/fixedbugs/issue11370.go

@@ -0,0 +1,13 @@
+// compile
+
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// issue 11370: cmd/compile: "0"[0] should not be a constant
+
+package p
+
+func main() {
+	println(-"abc"[1] >> 1)
+}