cmd/compile: update WBLoads during deadcode

When we deadcode-remove a block which is a write barrier test,
remove that block from the list of write barrier test blocks.

Fixes #25516

Change-Id: I1efe732d5476003eab4ad6bf67d0340d7874ff0c
Reviewed-on: https://go-review.googlesource.com/115037
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -252,6 +252,19 @@ func deadcode(f *Func) {
 		b.Values = b.Values[:i]
 	}
 
+	// Remove dead blocks from WBLoads list.
+	i = 0
+	for _, b := range f.WBLoads {
+		if reachable[b.ID] {
+			f.WBLoads[i] = b
+			i++
+		}
+	}
+	for j := i; j < len(f.WBLoads); j++ {
+		f.WBLoads[j] = nil
+	}
+	f.WBLoads = f.WBLoads[:i]
+
 	// Remove unreachable blocks. Return dead blocks to allocator.
 	i = 0
 	for _, b := range f.Blocks {

test/fixedbugs/issue25516.go

@@ -0,0 +1,26 @@
+// compile
+
+// Copyright 2018 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.
+
+// Make sure dead write barriers are handled correctly.
+
+package main
+
+func f(p **int) {
+	// The trick here is to eliminate the block containing the write barrier,
+	// but only after the write barrier branches are inserted.
+	// This requires some delicate code.
+	i := 0
+	var b []bool
+	var s string
+	for true {
+		if b[i] {
+			var a []string
+			s = a[len(s)]
+			i = 0
+		}
+		*p = nil
+	}
+}