cmd/compile: fix corner case boundedness for oversized shifts

For unsigned x >> k where k exceeds the width of x, the result is zero,
which means that using it as an index doesn't require a bounds
check...but not if the length is zero.

Walk mishandled this. Fix it.

It's a corner case, but the fix is trivial.

Change-Id: Ieb7f2d33563f09e98bb31eefb2d2c01f03064311
Reviewed-on: https://go-review.googlesource.com/c/go/+/778200
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -1015,7 +1015,7 @@ func walkStringHeader(n *ir.StringHeaderExpr, init *ir.Nodes) ir.Node {
 	return n
 }
 
-// return 1 if integer n must be in range [0, max), 0 otherwise.
+// bounded reports whether integer n must be in range [0, max).
 func bounded(n ir.Node, max int64) bool {
 	if n.Type() == nil || !n.Type().IsInteger() {
 		return false
@@ -1073,7 +1073,7 @@ func bounded(n ir.Node, max int64) bool {
 		if !sign && ir.IsSmallIntConst(n.Y) {
 			v := ir.Int64Val(n.Y)
 			if v > int64(bits) {
-				return true
+				return max > 0
 			}
 			bits -= int32(v)
 		}

test/fixedbugs/walk_bounded_overshift_empty_bound.go

@@ -0,0 +1,27 @@
+// run
+
+// Copyright 2026 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.
+
+package main
+
+import "strings"
+
+//go:noinline
+func f(i uint8) byte {
+	return ""[i>>9]
+}
+
+func main() {
+	defer func() {
+		r := recover()
+		if r == nil {
+			panic("missing bounds panic")
+		}
+		if got := r.(error).Error(); !strings.Contains(got, "index out of range") {
+			panic(got)
+		}
+	}()
+	_ = f(7)
+}