Index: lib/asan/CMakeLists.txt
===================================================================
--- lib/asan/CMakeLists.txt
+++ lib/asan/CMakeLists.txt
@@ -26,7 +26,8 @@
   asan_suppressions.cc
   asan_thread.cc
   asan_win.cc
-  asan_win_coverage_sections.cc)
+  asan_win_coverage_sections.cc
+  asan_win_weak_interception.cc)
 
 set(ASAN_CXX_SOURCES
   asan_new_delete.cc)
Index: lib/asan/asan_win_weak_interception.cc
===================================================================
--- /dev/null
+++ lib/asan/asan_win_weak_interception.cc
@@ -0,0 +1,92 @@
+//===-- asan_win_weak_interception.cc -------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// This module should be statically linked to the Asan dll library in order to
+// delegate the calls of weak functions to the implementation in the main
+// executable when possible. (Based on: asan_win_dll_thunk.cc)
+//===----------------------------------------------------------------------===//
+
+#include "sanitizer_common/sanitizer_platform.h"
+#if SANITIZER_WINDOWS && ASAN_DYNAMIC
+#include "asan_interface_internal.h"
+#include "interception/interception.h"
+#include "sanitizer_common/sanitizer_allocator_interface.h"
+#include "sanitizer_common/sanitizer_interface_internal.h"
+#include "sanitizer_common/sanitizer_platform_interceptors.h"
+
+#ifdef _M_IX86
+#define WINAPI __stdcall
+#else
+#define WINAPI
+#endif
+
+// Interception helper functions and macros.
+extern "C" {
+void *WINAPI GetModuleHandleA(const char *module_name);
+void *WINAPI GetProcAddress(void *module, const char *proc_name);
+void abort();
+}
+
+using __sanitizer::uptr;
+using __sanitizer::u32;
+
+// Try to get a pointer to real_function in the main module and override
+// dll_function with that pointer. If the function isn't found, nothing changes.
+static void interceptWhenPossible(uptr dll_function,
+                                  const char *real_function) {
+  uptr real = __interception::InternalGetProcAddress(
+    (void *)GetModuleHandleA(0), real_function);
+  if (real && !__interception::OverrideFunction((uptr)dll_function, real, 0))
+    abort();
+}
+
+// Override default functions with implementation provided by users.
+#define INTERCEPT_WEAK(Name)                                                   \
+  interceptWhenPossible((uptr) Name, #Name);
+
+static int asan_weak_init() {
+  // We need to intercept weak functions and delegate the calls to the
+  // implementation in the main executable when non-defult implementations are
+  // provided.
+  INTERCEPT_WEAK(__sanitizer_malloc_hook)
+  INTERCEPT_WEAK(__sanitizer_free_hook)
+  INTERCEPT_WEAK(__sanitizer_print_memory_profile)
+  INTERCEPT_WEAK(__sanitizer_sandbox_on_notify)
+  INTERCEPT_WEAK(__sanitizer_report_error_summary)
+  INTERCEPT_WEAK(__sanitizer_cov_trace_cmp)
+  INTERCEPT_WEAK(__sanitizer_cov_trace_cmp1)
+  INTERCEPT_WEAK(__sanitizer_cov_trace_cmp2)
+  INTERCEPT_WEAK(__sanitizer_cov_trace_cmp4)
+  INTERCEPT_WEAK(__sanitizer_cov_trace_cmp8)
+  INTERCEPT_WEAK(__sanitizer_cov_trace_switch)
+  INTERCEPT_WEAK(__sanitizer_cov_trace_div4)
+  INTERCEPT_WEAK(__sanitizer_cov_trace_div8)
+  INTERCEPT_WEAK(__sanitizer_cov_trace_gep)
+  INTERCEPT_WEAK(__sanitizer_cov_trace_pc_indir)
+  INTERCEPT_WEAK(__sanitizer_cov_trace_pc_guard)
+  INTERCEPT_WEAK(__sanitizer_cov_trace_pc_guard_init)
+  INTERCEPT_WEAK(__asan_default_options)
+  INTERCEPT_WEAK(__asan_on_error)
+  // In DLLs, the callbacks are expected to return 0,
+  // otherwise CRT initialization fails.
+  return 0;
+}
+
+#pragma section(".CRT$XIB", long, read)  // NOLINT
+__declspec(allocate(".CRT$XIB")) int (*__asan_weak_preinit)() = asan_weak_init;
+
+static void WINAPI asan_weak_thread_init(void *mod, unsigned long reason,
+                                        void *reserved) {
+  if (reason == /*DLL_PROCESS_ATTACH=*/1) asan_weak_init();
+}
+
+#pragma section(".CRT$XLAB", long, read)  // NOLINT
+__declspec(allocate(".CRT$XLAB")) void (WINAPI *__asan_weak_tls_init)(void *,
+    unsigned long, void *) = asan_weak_thread_init;
+
+#endif // SANITIZER_WINDOWS && ASAN_DYNAMIC