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
@@ -527,9 +527,13 @@
 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);
-  for (uptr i = 0; i < size; ++i)
-    if (s[i])
-      return i;
+  bool poisoned = !__sanitizer::mem_is_zero((const char *)s, size);
+  if (poisoned) {
+    // Slow path: loop through again to find the location.
+    for (uptr i = 0; i < size; ++i)
+      if (s[i])
+        return i;
+  }
   return -1;
 }