allocator: add use_experimental_allocator_shim build flag

third_party/tcmalloc/chromium/src/libc_override.h

 // Copyright (c) 2011, Google Inc.
 // All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
@@ skipping 41 matching lines @@
 #define TCMALLOC_LIBC_OVERRIDE_INL_H_
 
 #include <config.h>
 #ifdef HAVE_FEATURES_H
 #include <features.h>   // for __GLIBC__
 #endif
 #include <gperftools/tcmalloc.h>
 
 static void ReplaceSystemAlloc();  // defined in the .h files below
 
+#if defined(TCMALLOC_DONT_REPLACE_SYSTEM_ALLOC)
+
+static void ReplaceSystemAlloc() {}

[Will Harris, 2016/02/08 19:23:55]

add some comment here. it's a bit subtle what this is doing.

[Primiano Tucci (use gerrit), 2016/02/09 11:38:53]

Done.

+
 // For windows, there are two ways to get tcmalloc.  If we're
 // patching, then src/windows/patch_function.cc will do the necessary
 // overriding here.  Otherwise, we doing the 'redefine' trick, where
 // we remove malloc/new/etc from mscvcrt.dll, and just need to define
 // them now.
-#if defined(_WIN32) && defined(WIN32_DO_PATCHING)
+#elif defined(_WIN32) && defined(WIN32_DO_PATCHING)
 void PatchWindowsFunctions();   // in src/windows/patch_function.cc
 static void ReplaceSystemAlloc() { PatchWindowsFunctions(); }
 
 #elif defined(_WIN32) && !defined(WIN32_DO_PATCHING)
 // "libc_override_redefine.h" is included in the original gperftools.  But,
 // we define allocator functions in Chromium's base/allocator/allocator_shim.cc
 // on Windows.  We don't include libc_override_redefine.h here.
 // ReplaceSystemAlloc() is defined here instead.
 static void ReplaceSystemAlloc() { }
 
 #elif defined(__APPLE__)
 #include "libc_override_osx.h"
 
 #elif defined(__GLIBC__)
 #include "libc_override_glibc.h"
 
 // Not all gcc systems necessarily support weak symbols, but all the
 // ones I know of do, so for now just assume they all do.
 #elif defined(__GNUC__)
 #include "libc_override_gcc_and_weak.h"
 
 #else
 #error Need to add support for your libc/OS here
 
 #endif
 
 #endif  // TCMALLOC_LIBC_OVERRIDE_INL_H_