diff --git a/compiler-rt/test/asan/TestCases/debug_double_free.cpp b/compiler-rt/test/asan/TestCases/debug_double_free.cpp
--- a/compiler-rt/test/asan/TestCases/debug_double_free.cpp
+++ b/compiler-rt/test/asan/TestCases/debug_double_free.cpp
@@ -10,16 +10,16 @@
 // If we use %p with MSVC, it comes out all upper case. Use %08x to get
 // lowercase hex.
 #ifdef _MSC_VER
-# ifdef _WIN64
-#  define PTR_FMT "0x%08llx"
-# else
-#  define PTR_FMT "0x%08x"
-# endif
+#  ifdef _WIN64
+#    define PTR_FMT "0x%08llx"
+#  else
+#    define PTR_FMT "0x%08x"
+#  endif
 // Solaris libc omits the leading 0x.
 #elif defined(__sun__) && defined(__svr4__)
-# define PTR_FMT "0x%p"
+#  define PTR_FMT "0x%p"
 #else
-# define PTR_FMT "%p"
+#  define PTR_FMT "%p"
 #endif
 
 char *heap_ptr;
@@ -33,11 +33,15 @@
   // CHECK: heap_ptr: 0x[[ADDR:[0-9a-f]+]]
 
   free(heap_ptr);
-  free(heap_ptr);  // BOOM
+  free(heap_ptr); // BOOM
   return 0;
 }
 
-void __asan_on_error() {
+#if (__APPLE__)
+__attribute__((weak))
+#endif
+extern "C" void
+__asan_on_error() {
   int present = __asan_report_present();
   void *addr = __asan_get_report_address();
   const char *description = __asan_get_report_description();
diff --git a/compiler-rt/test/asan/TestCases/debug_report.cpp b/compiler-rt/test/asan/TestCases/debug_report.cpp
--- a/compiler-rt/test/asan/TestCases/debug_report.cpp
+++ b/compiler-rt/test/asan/TestCases/debug_report.cpp
@@ -37,7 +37,11 @@
 # define PTR_FMT "%p"
 #endif
 
-void __asan_on_error() {
+#if (__APPLE__)
+__attribute__((weak))
+#endif
+extern "C" void
+__asan_on_error() {
   int present = __asan_report_present();
   void *pc = __asan_get_report_pc();
   void *bp = __asan_get_report_bp();
diff --git a/compiler-rt/test/asan/TestCases/default_options.cpp b/compiler-rt/test/asan/TestCases/default_options.cpp
--- a/compiler-rt/test/asan/TestCases/default_options.cpp
+++ b/compiler-rt/test/asan/TestCases/default_options.cpp
@@ -6,13 +6,14 @@
 
 const char *kAsanDefaultOptions = "verbosity=1 help=1";
 
-extern "C"
-__attribute__((no_sanitize_address))
-const char *__asan_default_options() {
+#if (__APPLE__)
+__attribute__((weak))
+#endif
+__attribute__((no_sanitize_address)) // TBD: Explain why
+extern "C" const char *
+__asan_default_options() {
   // CHECK: Available flags for AddressSanitizer:
   return kAsanDefaultOptions;
 }
 
-int main() {
-  return 0;
-}
+int main() { return 0; }
diff --git a/compiler-rt/test/asan/TestCases/on_error_callback.cpp b/compiler-rt/test/asan/TestCases/on_error_callback.cpp
--- a/compiler-rt/test/asan/TestCases/on_error_callback.cpp
+++ b/compiler-rt/test/asan/TestCases/on_error_callback.cpp
@@ -6,14 +6,17 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-extern "C"
-void __asan_on_error() {
+#if (__APPLE__)
+__attribute__((weak))
+#endif
+extern "C" void
+__asan_on_error() {
   fprintf(stderr, "__asan_on_error called\n");
   fflush(stderr);
 }
 
 int main() {
-  char *x = (char*)malloc(10 * sizeof(char));
+  char *x = (char *)malloc(10 * sizeof(char));
   free(x);
   return x[5];
   // CHECK: __asan_on_error called
diff --git a/compiler-rt/test/ubsan/TestCases/Misc/monitor.cpp b/compiler-rt/test/ubsan/TestCases/Misc/monitor.cpp
--- a/compiler-rt/test/ubsan/TestCases/Misc/monitor.cpp
+++ b/compiler-rt/test/ubsan/TestCases/Misc/monitor.cpp
@@ -10,19 +10,20 @@
 
 #include <cstdio>
 
-extern "C" {
-void __ubsan_get_current_report_data(const char **OutIssueKind,
-                                     const char **OutMessage,
-                                     const char **OutFilename,
-                                     unsigned *OutLine, unsigned *OutCol,
-                                     char **OutMemoryAddr);
-
-// Override the definition of __ubsan_on_report from the runtime, just for
-// testing purposes.
-void __ubsan_on_report(void) {
+// Override __ubsan_on_report() from the runtime, just for testing purposes.
+#if (__APPLE__)
+__attribute__((weak))
+#endif
+extern "C" void
+__ubsan_on_report(void) {
+  void __ubsan_get_current_report_data(
+      const char **OutIssueKind, const char **OutMessage,
+      const char **OutFilename, unsigned *OutLine, unsigned *OutCol,
+      char **OutMemoryAddr);
   const char *IssueKind, *Message, *Filename;
   unsigned Line, Col;
   char *Addr;
+
   __ubsan_get_current_report_data(&IssueKind, &Message, &Filename, &Line, &Col,
                                   &Addr);
 
@@ -33,7 +34,6 @@
 
   (void)Addr;
 }
-}
 
 int main() {
   char C = 3;