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;
+      SanitizerBreakOptimization(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
@@ -535,6 +535,20 @@
 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 SanitizerBreakOptimization(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);
 }
@@ -78,7 +68,7 @@
   CHECK_EQ((reinterpret_cast<uptr>(s) | n) & 15, 0);
   for (S16 *p = reinterpret_cast<S16*>(s), *end = p + n / 16; p < end; p++) {
     p->a = p->b = 0;
-    break_optimization(0);  // Make sure this does not become memset.
+    SanitizerBreakOptimization(0);  // Make sure this does not become memset.
   }
 }