Fix miscompilation of NULL check.

gpu/command_buffer/client/cmd_buffer_helper.h

 // 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.
 
 // This file contains the command buffer helper class.
 
 #ifndef GPU_COMMAND_BUFFER_CLIENT_CMD_BUFFER_HELPER_H_
 #define GPU_COMMAND_BUFFER_CLIENT_CMD_BUFFER_HELPER_H_
 
 #include <string.h>
@@ skipping 115 matching lines @@
 
     // Allocate space and advance put_.
     CommandBufferEntry* space = &entries_[put_];
     put_ += entries;
     immediate_entry_count_ -= entries;
 
     DCHECK_LE(put_, total_entry_count_);
     return space;
   }
 
+  template <typename T>
+  void ForceNullCheck(T* data) {
+#if defined(OS_WIN) && defined(ARCH_CPU_64_BITS)
+    // 64-bit MSVC's alias analysis was determining that the command buffer
+    // entry couldn't be NULL, so it optimized out the NULL check.
+    // Dereferencing the same datatype through a volatile pointer seems to
+    // prevent that from happening. http://crbug.com/361936
+    if (data)
+      static_cast<volatile T*>(data)->header;
+#endif
+  }
+
   // Typed version of GetSpace. Gets enough room for the given type and returns
   // a reference to it.
   template <typename T>
   T* GetCmdSpace() {
     COMPILE_ASSERT(T::kArgFlags == cmd::kFixed, Cmd_kArgFlags_not_kFixed);
     int32 space_needed = ComputeNumEntries(sizeof(T));
-    void* data = GetSpace(space_needed);
-    return static_cast<T*>(data);
+    T* data = static_cast<T*>(GetSpace(space_needed));
+    ForceNullCheck(data);
+    return data;
   }
 
   // Typed version of GetSpace for immediate commands.
   template <typename T>
   T* GetImmediateCmdSpace(size_t data_space) {
     COMPILE_ASSERT(T::kArgFlags == cmd::kAtLeastN, Cmd_kArgFlags_not_kAtLeastN);
     int32 space_needed = ComputeNumEntries(sizeof(T) + data_space);
-    void* data = GetSpace(space_needed);
-    return static_cast<T*>(data);
+    T* data = static_cast<T*>(GetSpace(space_needed));
+    ForceNullCheck(data);
+    return data;
   }
 
   // Typed version of GetSpace for immediate commands.
   template <typename T>
   T* GetImmediateCmdSpaceTotalSize(size_t total_space) {
     COMPILE_ASSERT(T::kArgFlags == cmd::kAtLeastN, Cmd_kArgFlags_not_kAtLeastN);
     int32 space_needed = ComputeNumEntries(total_space);
-    void* data = GetSpace(space_needed);
-    return static_cast<T*>(data);
+    T* data = static_cast<T*>(GetSpace(space_needed));
+    ForceNullCheck(data);
+    return data;
   }
 
   int32 last_token_read() const {
     return command_buffer_->GetLastToken();
   }
 
   int32 get_offset() const {
     return command_buffer_->GetLastState().get_offset;
   }
 
@@ skipping 145 matching lines @@
   // Can be used to track when prior commands have been flushed.
   uint32 flush_generation_;
 
   friend class CommandBufferHelperTest;
   DISALLOW_COPY_AND_ASSIGN(CommandBufferHelper);
 };
 
 }  // namespace gpu
 
 #endif  // GPU_COMMAND_BUFFER_CLIENT_CMD_BUFFER_HELPER_H_