diff --git a/compiler-rt/lib/msan/msan.cpp b/compiler-rt/lib/msan/msan.cpp
--- a/compiler-rt/lib/msan/msan.cpp
+++ b/compiler-rt/lib/msan/msan.cpp
@@ -527,6 +527,9 @@
 sptr __msan_test_shadow(const void *x, uptr size) {
   if (!MEM_IS_APP(x)) return -1;
   unsigned char *s = (unsigned char *)MEM_TO_SHADOW((uptr)x);
+  if (__sanitizer::mem_is_zero((const char *)s, size))
+    return -1;
+  // Slow path: loop through again to find the location.
   for (uptr i = 0; i < size; ++i)
     if (s[i])
       return i;