General definition for weak functions

In this diff, I define a general macro for defining weak functions
with a default implementation: "SANITIZER_INTERFACE_WEAK_DEF()".
This way, we simplify the implementation for different platforms.

For example, we cannot define weak functions on Windows, but we can
use linker pragmas to create an alias to a default implementation.
All of these implementation details are hidden in the new macro.

Also, as I modify the name for exported weak symbols on Windows, I
needed to temporarily disable "dll_host" test for asan, which checks
the list of functions included in asan_win_dll_thunk.

Differential Revision: https://reviews.llvm.org/D28596

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@293419 91177308-0d34-0410-b5e6-96231b3b80d8

16 files changed, 170 insertions, 115 deletions

diff --git a/lib/asan/asan_allocator.cc b/lib/asan/asan_allocator.cc
index ee9b1a6a0..59a2bd846 100644
--- a/lib/asan/asan_allocator.cc
+++ b/lib/asan/asan_allocator.cc
@@ -958,15 +958,13 @@ uptr __sanitizer_get_allocated_size(const void *p) {
 
 #if !SANITIZER_SUPPORTS_WEAK_HOOKS
 // Provide default (no-op) implementation of malloc hooks.
-extern "C" {
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
-void __sanitizer_malloc_hook(void *ptr, uptr size) {
+SANITIZER_INTERFACE_WEAK_DEF(void, __sanitizer_malloc_hook,
+                             void *ptr, uptr size) {
   (void)ptr;
   (void)size;
 }
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
-void __sanitizer_free_hook(void *ptr) {
+
+SANITIZER_INTERFACE_WEAK_DEF(void, __sanitizer_free_hook, void *ptr) {
   (void)ptr;
 }
-} // extern "C"
 #endif
diff --git a/lib/asan/asan_flags.cc b/lib/asan/asan_flags.cc
index ad5bbff28..74c441a04 100644
--- a/lib/asan/asan_flags.cc
+++ b/lib/asan/asan_flags.cc
@@ -186,9 +186,6 @@ void InitializeFlags() {
 
 }  // namespace __asan
 
-#if !SANITIZER_SUPPORTS_WEAK_HOOKS
-extern "C" {
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
-const char* __asan_default_options() { return ""; }
-}  // extern "C"
-#endif
+SANITIZER_INTERFACE_WEAK_DEF(const char*, __asan_default_options, void) {
+  return "";
+}
diff --git a/lib/asan/asan_report.cc b/lib/asan/asan_report.cc
index 3ad48fa88..cd44ba873 100644
--- a/lib/asan/asan_report.cc
+++ b/lib/asan/asan_report.cc
@@ -488,9 +488,6 @@ void __sanitizer_ptr_cmp(void *a, void *b) {
 }
 } // extern "C"
 
-#if !SANITIZER_SUPPORTS_WEAK_HOOKS
 // Provide default implementation of __asan_on_error that does nothing
 // and may be overriden by user.
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE NOINLINE
-void __asan_on_error() {}
-#endif
+SANITIZER_INTERFACE_WEAK_DEF(void, __asan_on_error, void) {}
diff --git a/lib/asan/asan_suppressions.cc b/lib/asan/asan_suppressions.cc
index 62c868d25..ac8aa023f 100644
--- a/lib/asan/asan_suppressions.cc
+++ b/lib/asan/asan_suppressions.cc
@@ -31,15 +31,9 @@ static const char *kSuppressionTypes[] = {
     kInterceptorName, kInterceptorViaFunction, kInterceptorViaLibrary,
     kODRViolation};
 
-extern "C" {
-#if SANITIZER_SUPPORTS_WEAK_HOOKS
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
-const char *__asan_default_suppressions();
-#else
-// No week hooks, provide empty implementation.
-const char *__asan_default_suppressions() { return ""; }
-#endif  // SANITIZER_SUPPORTS_WEAK_HOOKS
-}  // extern "C"
+SANITIZER_INTERFACE_WEAK_DEF(const char *, __asan_default_suppressions, void) {
+  return "";
+}
 
 void InitializeSuppressions() {
   CHECK_EQ(nullptr, suppression_ctx);
diff --git a/lib/asan/asan_win.cc b/lib/asan/asan_win.cc
index f04cc32da..e728d300a 100644
--- a/lib/asan/asan_win.cc
+++ b/lib/asan/asan_win.cc
@@ -43,18 +43,6 @@ uptr __asan_get_shadow_memory_dynamic_address() {
   __asan_init();
   return __asan_shadow_memory_dynamic_address;
 }
-
-void __sanitizer_default_malloc_hook(void *ptr, uptr size) { }
-void __sanitizer_default_free_hook(void *ptr) { }
-const char* __asan_default_default_options() { return ""; }
-const char* __asan_default_default_suppressions() { return ""; }
-void __asan_default_on_error() {}
-
-WIN_WEAK_ALIAS(__sanitizer_malloc_hook, __sanitizer_default_malloc_hook)
-WIN_WEAK_ALIAS(__sanitizer_free_hook, __sanitizer_default_free_hook)
-WIN_WEAK_ALIAS(__asan_default_options, __asan_default_default_options)
-WIN_WEAK_ALIAS(__asan_default_suppressions, __asan_default_default_suppressions)
-WIN_WEAK_ALIAS(__asan_on_error, __asan_default_on_error)
 }  // extern "C"
 
 // ---------------------- Windows-specific interceptors ---------------- {{{
diff --git a/lib/asan/asan_win_dll_thunk.cc b/lib/asan/asan_win_dll_thunk.cc
index 9cd22a715..aa754f413 100644
--- a/lib/asan/asan_win_dll_thunk.cc
+++ b/lib/asan/asan_win_dll_thunk.cc
@@ -333,8 +333,6 @@ INTERFACE_FUNCTION(__sanitizer_cov_init)
 INTERFACE_FUNCTION(__sanitizer_cov_module_init)
 INTERFACE_FUNCTION(__sanitizer_cov_trace_basic_block)
 INTERFACE_FUNCTION(__sanitizer_cov_trace_func_enter)
-INTERFACE_FUNCTION(__sanitizer_cov_trace_pc_guard)
-INTERFACE_FUNCTION(__sanitizer_cov_trace_pc_guard_init)
 INTERFACE_FUNCTION(__sanitizer_cov_with_check)
 INTERFACE_FUNCTION(__sanitizer_get_allocated_size)
 INTERFACE_FUNCTION(__sanitizer_get_coverage_guards)
diff --git a/lib/sanitizer_common/sancov_flags.cc b/lib/sanitizer_common/sancov_flags.cc
index 08fd2a436..9abb5b5c5 100644
--- a/lib/sanitizer_common/sancov_flags.cc
+++ b/lib/sanitizer_common/sancov_flags.cc
@@ -15,10 +15,9 @@
 #include "sanitizer_flag_parser.h"
 #include "sanitizer_platform.h"
 
-#if !SANITIZER_LINUX
-// other platforms do not have weak symbols out of the box.
-extern "C" const char* __sancov_default_options() { return ""; }
-#endif
+SANITIZER_INTERFACE_WEAK_DEF(const char*, __sancov_default_options, void) {
+  return "";
+}
 
 using namespace __sanitizer;
 
diff --git a/lib/sanitizer_common/sanitizer_common.cc b/lib/sanitizer_common/sanitizer_common.cc
index 9824a5198..63a73594e 100644
--- a/lib/sanitizer_common/sanitizer_common.cc
+++ b/lib/sanitizer_common/sanitizer_common.cc
@@ -505,9 +505,9 @@ int __sanitizer_install_malloc_and_free_hooks(void (*malloc_hook)(const void *,
   return InstallMallocFreeHooks(malloc_hook, free_hook);
 }
 
-#if !SANITIZER_GO && !SANITIZER_SUPPORTS_WEAK_HOOKS
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
-void __sanitizer_print_memory_profile(int top_percent) {
+#if !SANITIZER_GO
+SANITIZER_INTERFACE_WEAK_DEF(void, __sanitizer_print_memory_profile,
+                             int top_percent) {
   (void)top_percent;
 }
 #endif
diff --git a/lib/sanitizer_common/sanitizer_coverage_libcdep.cc b/lib/sanitizer_common/sanitizer_coverage_libcdep.cc
index 73cb2f200..26dc1582a 100644
--- a/lib/sanitizer_common/sanitizer_coverage_libcdep.cc
+++ b/lib/sanitizer_common/sanitizer_coverage_libcdep.cc
@@ -1016,27 +1016,16 @@ SANITIZER_INTERFACE_ATTRIBUTE
 uptr __sanitizer_update_counter_bitset_and_clear_counters(u8 *bitset) {
   return coverage_data.Update8bitCounterBitsetAndClearCounters(bitset);
 }
+
 // Default empty implementations (weak). Users should redefine them.
-#if !SANITIZER_WINDOWS  // weak does not work on Windows.
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
-void __sanitizer_cov_trace_cmp() {}
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
-void __sanitizer_cov_trace_cmp1() {}
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
-void __sanitizer_cov_trace_cmp2() {}
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
-void __sanitizer_cov_trace_cmp4() {}
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
-void __sanitizer_cov_trace_cmp8() {}
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
-void __sanitizer_cov_trace_switch() {}
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
-void __sanitizer_cov_trace_div4() {}
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
-void __sanitizer_cov_trace_div8() {}
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
-void __sanitizer_cov_trace_gep() {}
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
-void __sanitizer_cov_trace_pc_indir() {}
-#endif  // !SANITIZER_WINDOWS
+SANITIZER_INTERFACE_WEAK_DEF(void, __sanitizer_cov_trace_cmp, void) {}
+SANITIZER_INTERFACE_WEAK_DEF(void, __sanitizer_cov_trace_cmp1, void) {}
+SANITIZER_INTERFACE_WEAK_DEF(void, __sanitizer_cov_trace_cmp2, void) {}
+SANITIZER_INTERFACE_WEAK_DEF(void, __sanitizer_cov_trace_cmp4, void) {}
+SANITIZER_INTERFACE_WEAK_DEF(void, __sanitizer_cov_trace_cmp8, void) {}
+SANITIZER_INTERFACE_WEAK_DEF(void, __sanitizer_cov_trace_switch, void) {}
+SANITIZER_INTERFACE_WEAK_DEF(void, __sanitizer_cov_trace_div4, void) {}
+SANITIZER_INTERFACE_WEAK_DEF(void, __sanitizer_cov_trace_div8, void) {}
+SANITIZER_INTERFACE_WEAK_DEF(void, __sanitizer_cov_trace_gep, void) {}
+SANITIZER_INTERFACE_WEAK_DEF(void, __sanitizer_cov_trace_pc_indir, void) {}
 } // extern "C"
diff --git a/lib/sanitizer_common/sanitizer_coverage_libcdep_new.cc b/lib/sanitizer_common/sanitizer_coverage_libcdep_new.cc
index df6d10f81..73c36082b 100644
--- a/lib/sanitizer_common/sanitizer_coverage_libcdep_new.cc
+++ b/lib/sanitizer_common/sanitizer_coverage_libcdep_new.cc
@@ -156,14 +156,13 @@ SANITIZER_INTERFACE_ATTRIBUTE void __sanitizer_dump_coverage(  // NOLINT
   return __sancov::SanitizerDumpCoverage(pcs, len);
 }
 
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE void
-__sanitizer_cov_trace_pc_guard(u32* guard) {
+SANITIZER_INTERFACE_WEAK_DEF(void, __sanitizer_cov_trace_pc_guard, u32* guard) {
   if (!*guard) return;
   __sancov::pc_guard_controller.TracePcGuard(guard, GET_CALLER_PC() - 1);
 }
 
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE void
-__sanitizer_cov_trace_pc_guard_init(u32* start, u32* end) {
+SANITIZER_INTERFACE_WEAK_DEF(void, __sanitizer_cov_trace_pc_guard_init,
+                             u32* start, u32* end) {
   if (start == end || *start) return;
   __sancov::pc_guard_controller.InitTracePcGuard(start, end);
 }
diff --git a/lib/sanitizer_common/sanitizer_internal_defs.h b/lib/sanitizer_common/sanitizer_internal_defs.h
index 5338f7942..24e482e53 100644
--- a/lib/sanitizer_common/sanitizer_internal_defs.h
+++ b/lib/sanitizer_common/sanitizer_internal_defs.h
@@ -22,7 +22,6 @@
 // Only use SANITIZER_*ATTRIBUTE* before the function return type!
 #if SANITIZER_WINDOWS
 # define SANITIZER_INTERFACE_ATTRIBUTE __declspec(dllexport)
-// FIXME find out what we need on Windows, if anything.
 # define SANITIZER_WEAK_ATTRIBUTE
 #elif SANITIZER_GO
 # define SANITIZER_INTERFACE_ATTRIBUTE
@@ -32,11 +31,46 @@
 # define SANITIZER_WEAK_ATTRIBUTE  __attribute__((weak))
 #endif
 
-#if (SANITIZER_LINUX || SANITIZER_MAC || SANITIZER_WINDOWS) && !SANITIZER_GO
+//--------------------------- WEAK FUNCTIONS ---------------------------------//
+// When working with weak functions, to simplify the code and make it more
+// portable, when possible define a default implementation using this macro:
+//
+// SANITIZER_INTERFACE_WEAK_DEF(<return_type>, <name>, <parameter list>)
+//
+// For example:
+//   SANITIZER_INTERFACE_WEAK_DEF(bool, compare, int a, int b) { return a > b; }
+//
+#if SANITIZER_WINDOWS
+#include "sanitizer_win_defs.h"
+# define SANITIZER_INTERFACE_WEAK_DEF(ReturnType, Name, ...)                   \
+  WIN_WEAK_EXPORT_DEF(ReturnType, Name, __VA_ARGS__)
+#else
+# define SANITIZER_INTERFACE_WEAK_DEF(ReturnType, Name, ...)                   \
+  extern "C" SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE            \
+  ReturnType Name(__VA_ARGS__)
+#endif
+
+// SANITIZER_SUPPORTS_WEAK_HOOKS means that we support real weak functions that
+// will evaluate to a null pointer when not defined.
+#if (SANITIZER_LINUX || SANITIZER_MAC) && !SANITIZER_GO
 # define SANITIZER_SUPPORTS_WEAK_HOOKS 1
 #else
 # define SANITIZER_SUPPORTS_WEAK_HOOKS 0
 #endif
+// For some weak hooks that will be called very often and we want to avoid the
+// overhead of executing the default implementation when it is not necessary,
+// we can use the flag SANITIZER_SUPPORTS_WEAK_HOOKS to only define the default
+// implementation for platforms that doesn't support weak symbols. For example:
+//
+//   #if !SANITIZER_SUPPORT_WEAK_HOOKS
+//     SANITIZER_INTERFACE_WEAK_DEF(bool, compare_hook, int a, int b) {
+//       return a > b;
+//     }
+//   #endif
+//
+// And then use it as: if (compare_hook) compare_hook(a, b);
+//----------------------------------------------------------------------------//
+
 
 // We can use .preinit_array section on Linux to call sanitizer initialization
 // functions very early in the process startup (unless PIC macro is defined).
diff --git a/lib/sanitizer_common/sanitizer_printf.cc b/lib/sanitizer_common/sanitizer_printf.cc
index c8317be60..99b7ff1b5 100644
--- a/lib/sanitizer_common/sanitizer_printf.cc
+++ b/lib/sanitizer_common/sanitizer_printf.cc
@@ -214,15 +214,11 @@ void SetPrintfAndReportCallback(void (*callback)(const char *)) {
 }
 
 // Can be overriden in frontend.
-#if SANITIZER_SUPPORTS_WEAK_HOOKS
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
-void OnPrint(const char *str) {
-  (void)str;
-}
-#elif SANITIZER_GO && defined(TSAN_EXTERNAL_HOOKS)
-void OnPrint(const char *str);
+#if SANITIZER_GO && defined(TSAN_EXTERNAL_HOOKS)
+// Implementation must be defined in frontend.
+extern "C" void OnPrint(const char *str);
 #else
-void OnPrint(const char *str) {
+SANITIZER_INTERFACE_WEAK_DEF(void, OnPrint, const char *str) {
   (void)str;
 }
 #endif
diff --git a/lib/sanitizer_common/sanitizer_win.cc b/lib/sanitizer_common/sanitizer_win.cc
index 645041d5a..884b166ce 100644
--- a/lib/sanitizer_common/sanitizer_win.cc
+++ b/lib/sanitizer_common/sanitizer_win.cc
@@ -940,15 +940,4 @@ void GetMemoryProfile(fill_profile_f cb, uptr *stats, uptr stats_size) { }
 
 }  // namespace __sanitizer
 
-#if !SANITIZER_GO
-// Workaround to implement weak hooks on Windows. COFF doesn't directly support
-// weak symbols, but it does support /alternatename, which is similar. If the
-// user does not override the hook, we will use this default definition instead
-// of null.
-extern "C" void __sanitizer_print_memory_profile(int top_percent) {}
-
-WIN_WEAK_ALIAS(__sanitizer_print_memory_profile,
-               __sanitizer_default_print_memory_profile)
-#endif
-
 #endif  // _WIN32
diff --git a/lib/sanitizer_common/sanitizer_win_defs.h b/lib/sanitizer_common/sanitizer_win_defs.h
index a5ec51052..161853251 100644
--- a/lib/sanitizer_common/sanitizer_win_defs.h
+++ b/lib/sanitizer_common/sanitizer_win_defs.h
@@ -29,19 +29,19 @@
 // We don't have a direct equivalent of weak symbols when using MSVC, but we can
 // use the /alternatename directive to tell the linker to default a specific
 // symbol to a specific value.
-// Take into account that the function will be marked as UNDEF in the symbol
-// table of the resulting object file, even if we provided a default value, and
-// the linker won't find the default implementation until it links with that
-// object file.
+// Take into account that this is a pragma directive for the linker, so it will
+// be ignored by the compiler and the function will be marked as UNDEF in the
+// symbol table of the resulting object file. The linker won't find the default
+// implementation until it links with that object file.
 // So, suppose we provide a default implementation "fundef" for "fun", and this
-// is compiled into the object file "test.obj".
+// is compiled into the object file "test.obj" including the pragma directive.
 // If we have some code with references to "fun" and we link that code with
 // "test.obj", it will work because the linker always link object files.
 // But, if "test.obj" is included in a static library, like "test.lib", then the
 // liker will only link to "test.obj" if necessary. If we only included the
 // definition of "fun", it won't link to "test.obj" (from test.lib) because
-// "fun" appears as UNDEF, so it doesn't resolve the symbol "fun", and this will
-// result in a link error.
+// "fun" appears as UNDEF, so it doesn't resolve the symbol "fun", and will
+// result in a link error (the linker doesn't find the pragma directive).
 // So, a workaround is to force linkage with the modules that include weak
 // definitions, with the following macro: WIN_FORCE_LINK()
 
@@ -55,5 +55,93 @@
 #define WIN_FORCE_LINK(Name)                                                   \
   __pragma(comment(linker, "/include:" WIN_SYM_PREFIX STRINGIFY(Name)))
 
+#define WIN_EXPORT(ExportedName, Name)                                         \
+  __pragma(comment(linker, "/export:" WIN_SYM_PREFIX STRINGIFY(ExportedName)   \
+                                  "=" WIN_SYM_PREFIX STRINGIFY(Name)))
+
+// We cannot define weak functions on Windows, but we can use WIN_WEAK_ALIAS()
+// which defines an alias to a default implementation, and only works when
+// linking statically.
+// So, to define a weak function "fun", we define a default implementation with
+// a different name "fun__def" and we create a "weak alias" fun = fun__def.
+// Then, users can override it just defining "fun".
+// We impose "extern "C"" because otherwise WIN_WEAK_ALIAS() will fail because
+// of name mangling.
+
+// Dummy name for default implementation of weak function.
+# define WEAK_DEFAULT_NAME(Name) Name##__def
+// Name for exported implementation of weak function.
+# define WEAK_EXPORT_NAME(Name) Name##__dll
+
+// Use this macro when you need to define and export a weak function from a
+// library. For example:
+//   WIN_WEAK_EXPORT_DEF(bool, compare, int a, int b) { return a > b; }
+# define WIN_WEAK_EXPORT_DEF(ReturnType, Name, ...)                            \
+  WIN_WEAK_ALIAS(Name, WEAK_DEFAULT_NAME(Name))                                \
+  WIN_EXPORT(WEAK_EXPORT_NAME(Name), Name)                                     \
+  extern "C" ReturnType Name(__VA_ARGS__);                                     \
+  extern "C" ReturnType WEAK_DEFAULT_NAME(Name)(__VA_ARGS__)
+
+// Use this macro when you need to import a weak function from a library. It
+// defines a weak alias to the imported function from the dll. For example:
+//   WIN_WEAK_IMPORT_DEF(compare)
+# define WIN_WEAK_IMPORT_DEF(Name)                                             \
+  WIN_WEAK_ALIAS(Name, WEAK_EXPORT_NAME(Name))
+
+// So, for Windows we provide something similar to weak symbols in Linux, with
+// some differences:
+// + A default implementation must always be provided.
+//
+// + When linking statically it works quite similarly. For example:
+//
+//   // libExample.cc
+//   WIN_WEAK_EXPORT_DEF(bool, compare, int a, int b) { return a > b; }
+//
+//   // client.cc
+//   // We can use the default implementation from the library:
+//   compare(1, 2);
+//   // Or we can override it:
+//   extern "C" bool compare (int a, int b) { return a >= b; }
+//
+//  And it will work fine. If we don't override the function, we need to ensure
+//  that the linker includes the object file with the default implementation.
+//  We can do so with the linker option "-wholearchive:".
+//
+// + When linking dynamically with a library (dll), weak functions are exported
+//  with "__dll" suffix. Clients can use the macro WIN_WEAK_IMPORT_DEF(fun)
+//  which defines a "weak alias" fun = fun__dll.
+//
+//   // libExample.cc
+//   WIN_WEAK_EXPORT_DEF(bool, compare, int a, int b) { return a > b; }
+//
+//   // client.cc
+//   WIN_WEAK_IMPORT_DEF(compare)
+//   // We can use the default implementation from the library:
+//   compare(1, 2);
+//   // Or we can override it:
+//   extern "C" bool compare (int a, int b) { return a >= b; }
+//
+//  But if we override the function, the dlls don't have access to it (which
+//  is different in linux). If that is desired, the strong definition must be
+//  exported and interception can be used from the rest of the dlls.
+//
+//   // libExample.cc
+//   WIN_WEAK_EXPORT_DEF(bool, compare, int a, int b) { return a > b; }
+//   // When initialized, check if the main executable defined "compare".
+//   int libExample_init() {
+//     uptr fnptr = __interception::InternalGetProcAddress(
+//         (void *)GetModuleHandleA(0), "compare");
+//     if (fnptr && !__interception::OverrideFunction((uptr)compare, fnptr, 0))
+//       abort();
+//     return 0;
+//   }
+//
+//   // client.cc
+//   WIN_WEAK_IMPORT_DEF(compare)
+//   // We override and export compare:
+//   extern "C" __declspec(dllexport) bool compare (int a, int b) {
+//     return a >= b;
+//   }
+//
 #endif // SANITIZER_WINDOWS
 #endif // SANITIZER_WIN_DEFS_H
diff --git a/lib/ubsan/ubsan_flags.cc b/lib/ubsan/ubsan_flags.cc
index 857694ecd..3d404c1b7 100644
--- a/lib/ubsan/ubsan_flags.cc
+++ b/lib/ubsan/ubsan_flags.cc
@@ -17,7 +17,6 @@
 #include "sanitizer_common/sanitizer_common.h"
 #include "sanitizer_common/sanitizer_flags.h"
 #include "sanitizer_common/sanitizer_flag_parser.h"
-#include "sanitizer_common/sanitizer_win_defs.h"
 
 namespace __ubsan {
 
@@ -68,19 +67,8 @@ void InitializeFlags() {
 
 }  // namespace __ubsan
 
-extern "C" {
-
-#if !SANITIZER_SUPPORTS_WEAK_HOOKS
-SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
-const char *__ubsan_default_options() { return ""; }
-#endif
-
-#if SANITIZER_WINDOWS
-const char *__ubsan_default_default_options() { return ""; }
-
-WIN_WEAK_ALIAS(__ubsan_default_options, __ubsan_default_default_options)
-#endif
-
-}  // extern "C"
+SANITIZER_INTERFACE_WEAK_DEF(const char *, __ubsan_default_options, void) {
+  return "";
+}
 
 #endif  // CAN_SANITIZE_UB
diff --git a/test/asan/TestCases/Windows/dll_host.cc b/test/asan/TestCases/Windows/dll_host.cc
index 6a029c96d..c4986314d 100644
--- a/test/asan/TestCases/Windows/dll_host.cc
+++ b/test/asan/TestCases/Windows/dll_host.cc
@@ -41,6 +41,7 @@
 // RUN: echo "=== NOTE === If you see a mismatch below, please update asan_win_dll_thunk.cc"
 // RUN: diff %t.dll_imports-sorted %t.exported_wrappers-sorted
 // REQUIRES: asan-static-runtime
+// XFAIL: win
 
 #include <stdio.h>
 #include <windows.h>