Prevent clang from optimizing away a malloc in a test.

base/security_unittest.cc

 // Copyright (c) 2013 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 <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
 #include <sys/types.h>
@@ skipping 24 matching lines @@
 #if defined(OS_WIN)
 // This is a permitted size but exhausts memory pretty quickly.
 const size_t kLargePermittedAllocation = 0x7FFFE000;
 
 int OnNoMemory(size_t) {
   _exit(1);
 }
 
 void ExhaustMemoryWithMalloc() {
   for (;;) {
-    void* buf = malloc(kLargePermittedAllocation);
+    // Without the |volatile|, clang optimizes away the allocation.
+    void* volatile buf = malloc(kLargePermittedAllocation);
     if (!buf)
       break;
   }
 }
 
 void ExhaustMemoryWithRealloc() {
   size_t size = kLargePermittedAllocation;
   void* buf = malloc(size);
   if (!buf)
     return;
@@ skipping 310 matching lines @@
   // kRandomMask, so we use it as an additional detection mechanism.
   const uintptr_t kRandomMask = 0x3fffffffffffULL;
   bool impossible_random_address =
       reinterpret_cast<uintptr_t>(ptr.get()) & ~kRandomMask;
   EXPECT_FALSE(impossible_random_address);
 }
 
 #endif  // defined(OS_LINUX) && defined(__x86_64__)
 
 }  // namespace