diff --git a/FuzzerUtil.cpp b/FuzzerUtil.cpp
index 7aa84a1..87180d1 100644
--- a/FuzzerUtil.cpp
+++ b/FuzzerUtil.cpp
@@ -151,32 +151,36 @@
   return true;
 }
 
+// Code duplicated (and tested) in llvm/include/llvm/Support/Base64.h
 std::string Base64(const Unit &U) {
   static const char Table[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
                               "abcdefghijklmnopqrstuvwxyz"
                               "0123456789+/";
-  std::string Res;
-  size_t i;
-  for (i = 0; i + 2 < U.size(); i += 3) {
-    uint32_t x = (U[i] << 16) + (U[i + 1] << 8) + U[i + 2];
-    Res += Table[(x >> 18) & 63];
-    Res += Table[(x >> 12) & 63];
-    Res += Table[(x >> 6) & 63];
-    Res += Table[x & 63];
+  std::string Buffer;
+  Buffer.resize(((U.size() + 2) / 3) * 4);
+
+  size_t i = 0, j = 0;
+  for (size_t n = U.size() / 3 * 3; i < n; i += 3, j += 4) {
+    uint32_t x = (U[i] << 16) | (U[i + 1] << 8) | U[i + 2];
+    Buffer[j + 0] = Table[(x >> 18) & 63];
+    Buffer[j + 1] = Table[(x >> 12) & 63];
+    Buffer[j + 2] = Table[(x >> 6) & 63];
+    Buffer[j + 3] = Table[x & 63];
   }
   if (i + 1 == U.size()) {
     uint32_t x = (U[i] << 16);
-    Res += Table[(x >> 18) & 63];
-    Res += Table[(x >> 12) & 63];
-    Res += "==";
+    Buffer[j + 0] = Table[(x >> 18) & 63];
+    Buffer[j + 1] = Table[(x >> 12) & 63];
+    Buffer[j + 2] = '=';
+    Buffer[j + 3] = '=';
   } else if (i + 2 == U.size()) {
-    uint32_t x = (U[i] << 16) + (U[i + 1] << 8);
-    Res += Table[(x >> 18) & 63];
-    Res += Table[(x >> 12) & 63];
-    Res += Table[(x >> 6) & 63];
-    Res += "=";
+    uint32_t x = (U[i] << 16) | (U[i + 1] << 8);
+    Buffer[j + 0] = Table[(x >> 18) & 63];
+    Buffer[j + 1] = Table[(x >> 12) & 63];
+    Buffer[j + 2] = Table[(x >> 6) & 63];
+    Buffer[j + 3] = '=';
   }
-  return Res;
+  return Buffer;
 }
 
 static std::mutex SymbolizeMutex;