Index: lib/asan/asan_fake_stack.cc
===================================================================
--- lib/asan/asan_fake_stack.cc
+++ lib/asan/asan_fake_stack.cc
@@ -27,8 +27,10 @@
   CHECK_EQ(SHADOW_SCALE, 3);  // This code expects SHADOW_SCALE=3.
   u64 *shadow = reinterpret_cast<u64*>(MemToShadow(ptr));
   if (class_id <= 6) {
-    for (uptr i = 0; i < (1U << class_id); i++)
+    for (uptr i = 0; i < (1U << class_id); i++) {
       shadow[i] = magic;
+      break_optimization(0);  // Make sure this does not become memset.
+    }
   } else {
     // The size class is too big, it's cheaper to poison only size bytes.
     PoisonShadow(ptr, size, static_cast<u8>(magic));
Index: lib/sanitizer_common/sanitizer_common.h
===================================================================
--- lib/sanitizer_common/sanitizer_common.h
+++ lib/sanitizer_common/sanitizer_common.h
@@ -552,6 +552,20 @@
 INLINE void GetExtraActivationFlags(char *buf, uptr size) { *buf = '\0'; }
 INLINE void SanitizerInitializeUnwinder() {}
 #endif
+
+// Make the compiler think that something is going on there.
+// Use this inside a loop that looks like memset/memcpy/etc to prevent the
+// compiler from recognising it and turning it into an actual call to
+// memset/memcpy/etc.
+static inline void break_optimization(void *arg) {
+#if _MSC_VER
+  // FIXME: make sure this is actually enough.
+  __asm;
+#else
+  __asm__ __volatile__("" : : "r" (arg) : "memory");
+#endif
+}
+
 }  // namespace __sanitizer
 
 inline void *operator new(__sanitizer::operator_new_size_type size,
Index: lib/sanitizer_common/sanitizer_libc.cc
===================================================================
--- lib/sanitizer_common/sanitizer_libc.cc
+++ lib/sanitizer_common/sanitizer_libc.cc
@@ -16,16 +16,6 @@
 
 namespace __sanitizer {
 
-// Make the compiler think that something is going on there.
-static inline void break_optimization(void *arg) {
-#if _MSC_VER
-  // FIXME: make sure this is actually enough.
-  __asm;
-#else
-  __asm__ __volatile__("" : : "r" (arg) : "memory");
-#endif
-}
-
 s64 internal_atoll(const char *nptr) {
   return internal_simple_strtoll(nptr, (char**)0, 10);
 }
Index: lib/sanitizer_common/tests/sanitizer_test_utils.h
===================================================================
--- lib/sanitizer_common/tests/sanitizer_test_utils.h
+++ lib/sanitizer_common/tests/sanitizer_test_utils.h
@@ -64,13 +64,6 @@
 #  define SANITIZER_WORDSIZE 32
 #endif
 
-// Make the compiler thinks that something is going on there.
-inline void break_optimization(void *arg) {
-#if !defined(_WIN32) || defined(__clang__)
-  __asm__ __volatile__("" : : "r" (arg) : "memory");
-#endif
-}
-
 // This function returns its parameter but in such a way that compiler
 // can not prove it.
 template<class T>