Improve performance of the malloc shim layer

base/allocator/allocator_shim.cc

 // Copyright 2016 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 "base/allocator/allocator_shim.h"
 
 #include <errno.h>
 #include <unistd.h>
 
 #include <new>
@@ skipping 54 matching lines @@
   }
   if (!nh)
     return false;
   (*nh)();
   // Assume the new_handler will abort if it fails. Exception are disabled and
   // we don't support the case of a new_handler throwing std::bad_balloc.
   return true;
 }
 
 inline const allocator::AllocatorDispatch* GetChainHead() {
+  // TODO(primiano): Just use NoBarrier_Load once crbug.com/593344 is fixed.

[Nico, 2016/03/10 22:50:00]

(filed crbug.com/593874 today and made 3344 blocked on it)

[Primiano Tucci (use gerrit), 2016/03/11 00:11:08]

Seen that. Thanks.

+  // Unfortunately due to that bug NoBarrier_Load() is mistakenly fully
+  // barriered on Linux+Clang, and that causes visible perf regressons.
   return reinterpret_cast<const allocator::AllocatorDispatch*>(
-      subtle::NoBarrier_Load(&g_chain_head));
+#if defined(OS_LINUX) && defined(__clang__)

[Nico, 2016/03/10 22:50:00]

Do we want to check for x86 too?

[Primiano Tucci (use gerrit), 2016/03/11 00:11:08]

Especially for x86, which is where I saw the bug :)

[Nico, 2016/03/11 15:42:04]

I mean on ARM fences are pretty different, so I'm not sure we want the volatile
cast there. (And no idea about mips.)

+      *static_cast<const volatile subtle::AtomicWord*>(&g_chain_head)
+#else
+      subtle::NoBarrier_Load(&g_chain_head)
+#endif
+  );
 }
 
 }  // namespace
 
 namespace base {
 namespace allocator {
 
 void SetCallNewHandlerOnMallocFailure(bool value) {
   g_call_new_handler_on_malloc_failure = value;
 }
@@ skipping 150 matching lines @@
 // Cross-checks.
 
 #if defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
 #error The allocator shim should not be compiled when building for memory tools.
 #endif
 
 #if (defined(__GNUC__) && defined(__EXCEPTIONS)) || \
     (defined(_HAS_EXCEPTIONS) && _HAS_EXCEPTIONS)
 #error This code cannot be used when exceptions are turned on.
 #endif