cmd/compile: handle degenerate write barrier case

If both branches of a write barrier test go to the same block,
then there's no unsafe points.

This can only happen if the resulting memory state is somehow dead,
which can only occur in degenerate cases, like infinite loops. No
point in cleaning up the useless branch in these situations.

Fixes #26024.

Change-Id: I93a7df9fdf2fc94c6c4b1fe61180dc4fd4a0871f
Reviewed-on: https://go-review.googlesource.com/123655
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Author: randall77

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

@@ -692,6 +692,11 @@ func (lv *Liveness) markUnsafePoints() {
 			lv.f.Fatalf("expected branch at write barrier block %v", wbBlock)
 		}
 		s0, s1 := wbBlock.Succs[0].Block(), wbBlock.Succs[1].Block()
+		if s0 == s1 {
+			// There's no difference between write barrier on and off.
+			// Thus there's no unsafe locations. See issue 26024.
+			continue
+		}
 		if s0.Kind != ssa.BlockPlain || s1.Kind != ssa.BlockPlain {
 			lv.f.Fatalf("expected successors of write barrier block %v to be plain", wbBlock)
 		}

test/fixedbugs/issue26024.go

@@ -0,0 +1,28 @@
+// 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.
+
+package r
+
+// f compiles into code where no stores remain in the two successors
+// of a write barrier block; i.e., they are empty. Pre-fix, this
+// results in an unexpected input to markUnsafePoints, that expects to
+// see a pair of non-empty plain blocks.
+func f() {
+	var i int
+	var s string
+	for len(s) < len(s) {
+		i++
+		s = "a"
+	}
+	var b bool
+	var sa []string
+	for true {
+		sa = []string{""}
+		for b || i == 0 {
+		}
+		b = !b
+		_ = sa
+	}
+}