cmd/compile: recognize reflect.{Slice,String}Header for -d=checkptr

Avoids false positive pointer arithmetic panic.

Fixes #35027.

Change-Id: Idd008caaab25fcf739327ac50a021b835ef13def
Reviewed-on: https://go-review.googlesource.com/c/go/+/202560
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -3941,6 +3941,10 @@ func walkCheckPtrArithmetic(n *Node, init *Nodes) *Node {
 		return n
 	}
 
+	if n.Left.Op == ODOTPTR && isReflectHeaderDataField(n.Left) {
+		return n
+	}
+
 	// Find original unsafe.Pointer operands involved in this
 	// arithmetic expression.
 	//

test/fixedbugs/issue35027.go

@@ -0,0 +1,23 @@
+// run -gcflags=-d=checkptr
+
+// Copyright 2019 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 (
+	"reflect"
+	"unsafe"
+)
+
+var s []int
+
+func main() {
+	s = []int{42}
+	h := (*reflect.SliceHeader)(unsafe.Pointer(&s))
+	x := *(*int)(unsafe.Pointer(h.Data))
+	if x != 42 {
+		panic(x)
+	}
+}