diff --git a/llvm/lib/FileCheck/FileCheck.cpp b/llvm/lib/FileCheck/FileCheck.cpp
--- a/llvm/lib/FileCheck/FileCheck.cpp
+++ b/llvm/lib/FileCheck/FileCheck.cpp
@@ -23,6 +23,7 @@
 #include <cstdint>
 #include <list>
 #include <tuple>
+#include <unordered_set>
 #include <utility>
 
 using namespace llvm;
@@ -1825,7 +1826,10 @@
   // found.
   unsigned LineNumber = 1;
 
-  bool FoundUsedCheckPrefix = false;
+  StringSet<> PrefixesNotFound;
+  for (auto CP : Req.CheckPrefixes)
+    PrefixesNotFound.insert(CP);
+
   while (1) {
     Check::FileCheckType CheckTy;
 
@@ -1837,7 +1841,7 @@
     if (UsedPrefix.empty())
       break;
     if (CheckTy != Check::CheckComment)
-      FoundUsedCheckPrefix = true;
+      PrefixesNotFound.erase(UsedPrefix);
 
     assert(UsedPrefix.data() == Buffer.data() &&
            "Failed to move Buffer's start forward, or pointed prefix outside "
@@ -1930,14 +1934,16 @@
 
   // When there are no used prefixes we report an error except in the case that
   // no prefix is specified explicitly but -implicit-check-not is specified.
-  if (!FoundUsedCheckPrefix &&
+  if (!PrefixesNotFound.empty() &&
       (ImplicitNegativeChecks.empty() || !Req.IsDefaultCheckPrefix)) {
     errs() << "error: no check strings found with prefix"
            << (Req.CheckPrefixes.size() > 1 ? "es " : " ");
-    for (size_t I = 0, E = Req.CheckPrefixes.size(); I != E; ++I) {
-      if (I != 0)
+    bool First = true;
+    for (auto P : PrefixesNotFound.keys()) {
+      if (!First)
         errs() << ", ";
-      errs() << "\'" << Req.CheckPrefixes[I] << ":'";
+      errs() << "\'" << P << ":'";
+      First = false;
     }
     errs() << '\n';
     return true;