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diff --git a/memory/build/replace_malloc.h b/memory/build/replace_malloc.h
new file mode 100644
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+++ b/memory/build/replace_malloc.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef replace_malloc_h
+#define replace_malloc_h
+
+// The replace_malloc facility allows an external library to replace or
+// supplement the jemalloc implementation.
+//
+// The external library may be hooked by setting one of the following
+// environment variables to the library path:
+//   - LD_PRELOAD on Linux,
+//   - DYLD_INSERT_LIBRARIES on OSX,
+//   - MOZ_REPLACE_MALLOC_LIB on Windows and Android.
+//
+// An initialization function is called before any malloc replacement
+// function, and has the following declaration:
+//
+//   void replace_init(malloc_table_t*, ReplaceMallocBridge**)
+//
+// The malloc_table_t pointer given to that function is a table containing
+// pointers to the original allocator implementation, so that replacement
+// functions can call them back if they need to. The initialization function
+// needs to alter that table to replace the function it wants to replace.
+// If it needs the original implementation, it thus needs a copy of the
+// original table.
+//
+// The ReplaceMallocBridge* pointer is an outparam that allows the
+// replace_init function to return a pointer to its ReplaceMallocBridge
+// (see replace_malloc_bridge.h).
+//
+// The functions to be implemented in the external library are of the form:
+//
+//   void* replace_malloc(size_t size)
+//   {
+//     // Fiddle with the size if necessary.
+//     // orig->malloc doesn't have to be called if the external library
+//     // provides its own allocator, but in this case it will have to
+//     // implement all functions.
+//     void *ptr = orig->malloc(size);
+//     // Do whatever you want with the ptr.
+//     return ptr;
+//   }
+//
+// where "orig" is a pointer to a copy of the table replace_init got.
+//
+// See malloc_decls.h for a list of functions that can be replaced this
+// way. The implementations are all in the form:
+//   return_type replace_name(arguments [,...])
+//
+// They don't all need to be provided.
+//
+// Building a replace-malloc library is like rocket science. It can end up
+// with things blowing up, especially when trying to use complex types, and
+// even more especially when these types come from XPCOM or other parts of the
+// Mozilla codebase.
+// It is recommended to add the following to a replace-malloc implementation's
+// moz.build:
+//   DISABLE_STL_WRAPPING = True # Avoid STL wrapping
+//
+// If your replace-malloc implementation lives under memory/replace, these
+// are taken care of by memory/replace/defs.mk.
+
+#ifdef replace_malloc_bridge_h
+#  error Do not include replace_malloc_bridge.h before replace_malloc.h. \
+  In fact, you only need the latter.
+#endif
+
+#define REPLACE_MALLOC_IMPL
+
+#include "replace_malloc_bridge.h"
+
+// Implementing a replace-malloc library is incompatible with using mozalloc.
+#define MOZ_NO_MOZALLOC 1
+
+#include "mozilla/MacroArgs.h"
+#include "mozilla/Types.h"
+
+MOZ_BEGIN_EXTERN_C
+
+// MOZ_REPLACE_WEAK is only defined in mozjemalloc.cpp. Normally including
+// this header will add function definitions.
+#ifndef MOZ_REPLACE_WEAK
+#  define MOZ_REPLACE_WEAK
+#endif
+
+// When building a replace-malloc library for static linking, we want
+// each to have a different name for their "public" functions.
+// The build system defines MOZ_REPLACE_MALLOC_PREFIX in that case.
+#ifdef MOZ_REPLACE_MALLOC_PREFIX
+#  define replace_init MOZ_CONCAT(MOZ_REPLACE_MALLOC_PREFIX, _init)
+#  define MOZ_REPLACE_PUBLIC
+#else
+#  define MOZ_REPLACE_PUBLIC MOZ_EXPORT
+#endif
+
+struct ReplaceMallocBridge;
+typedef void (*jemalloc_init_func)(malloc_table_t*,
+                                   struct ReplaceMallocBridge**);
+
+// Replace-malloc library initialization function. See top of this file
+MOZ_REPLACE_PUBLIC void replace_init(
+    malloc_table_t*, struct ReplaceMallocBridge**) MOZ_REPLACE_WEAK;
+
+// ensure this is visible and libxul/etc reference it with a weak ref
+MFBT_API void jemalloc_replace_dynamic(jemalloc_init_func);
+
+MOZ_END_EXTERN_C
+
+#endif  // replace_malloc_h