diff --git a/compiler-rt/lib/scudo/standalone/string_utils.h b/compiler-rt/lib/scudo/standalone/string_utils.h
--- a/compiler-rt/lib/scudo/standalone/string_utils.h
+++ b/compiler-rt/lib/scudo/standalone/string_utils.h
@@ -18,7 +18,11 @@
 
 class ScopedString {
 public:
-  explicit ScopedString() { String.push_back('\0'); }
+  explicit ScopedString() { init(); }
+  explicit ScopedString(uptr Size) {
+    String.reserve(Size + 1);
+    init();
+  }
   uptr length() { return String.size() - 1; }
   const char *data() { return String.data(); }
   void clear() {
@@ -30,6 +34,7 @@
   void output() const { outputRaw(String.data()); }
 
 private:
+  void init() { String.push_back('\0'); }
   Vector<char> String;
 };
 
diff --git a/compiler-rt/lib/scudo/standalone/tests/strings_test.cpp b/compiler-rt/lib/scudo/standalone/tests/strings_test.cpp
--- a/compiler-rt/lib/scudo/standalone/tests/strings_test.cpp
+++ b/compiler-rt/lib/scudo/standalone/tests/strings_test.cpp
@@ -43,9 +43,10 @@
 }
 
 TEST(ScudoStringsTest, ClearLarge) {
-  scudo::ScopedString Str;
+  const char appendString[] = "123";
+  scudo::ScopedString Str(sizeof(appendString) * 10000);
   for (int i = 0; i < 10000; ++i)
-    Str.append("123");
+    Str.append(appendString);
   Str.clear();
   EXPECT_EQ(0ul, Str.length());
   EXPECT_EQ('\0', *Str.data());
@@ -75,7 +76,7 @@
   // Use a ScopedString that spans a page, and attempt to write past the end
   // of it with variations of append. The expectation is for nothing to crash.
   const scudo::uptr PageSize = scudo::getPageSizeCached();
-  scudo::ScopedString Str;
+  scudo::ScopedString Str(2 * PageSize);
   Str.clear();
   fillString(Str, 2 * PageSize);
   Str.clear();