Update from https://crrev.com/306655

sandbox/linux/seccomp-bpf/codegen_unittest.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "sandbox/linux/seccomp-bpf/codegen.h"
 
 #include <linux/filter.h>
 
 #include <map>
 #include <utility>
@@ skipping 349 matching lines @@
   // Exhaustively test branch offsets near BPF's limits.
   for (size_t jt = 250; jt < 260; ++jt) {
     for (size_t jf = 250; jf < 260; ++jf) {
       nodes.push_back(MakeInstruction(BPF_JMP + BPF_JEQ + BPF_K, 0,
                                       nodes.rbegin()[jt], nodes.rbegin()[jf]));
       RunTest(nodes.back());
     }
   }
 }
 
+TEST_F(ProgramTest, JumpReuse) {
+  // As a code size optimization, we try to reuse jumps when possible
+  // instead of emitting new ones. Here we make sure that optimization
+  // is working as intended.
+  //
+  // NOTE: To simplify testing, we rely on implementation details
+  // about what CodeGen::Node values indicate (i.e., vector indices),
+  // but CodeGen users should treat them as opaque values.
+
+  // Populate with 260 initial instruction nodes.
+  std::vector<CodeGen::Node> nodes;
+  nodes.push_back(MakeInstruction(BPF_RET + BPF_K, 0));
+  for (size_t i = 1; i < 260; ++i) {
+    nodes.push_back(
+        MakeInstruction(BPF_ALU + BPF_ADD + BPF_K, i, nodes.back()));
+  }
+
+  // Branching to nodes[0] and nodes[1] should require 3 new
+  // instructions: two far jumps plus the branch itself.
+  CodeGen::Node one =
+      MakeInstruction(BPF_JMP + BPF_JEQ + BPF_K, 0, nodes[0], nodes[1]);
+  EXPECT_EQ(nodes.back() + 3, one);  // XXX: Implementation detail!
+  RunTest(one);
+
+  // Branching again to the same target nodes should require only one
+  // new instruction, as we can reuse the previous branch's jumps.
+  CodeGen::Node two =
+      MakeInstruction(BPF_JMP + BPF_JEQ + BPF_K, 1, nodes[0], nodes[1]);
+  EXPECT_EQ(one + 1, two);  // XXX: Implementation detail!
+  RunTest(two);
+}
+
 }  // namespace
 }  // namespace sandbox