Index: llvm/lib/Support/FileCheck.cpp
===================================================================
--- llvm/lib/Support/FileCheck.cpp
+++ llvm/lib/Support/FileCheck.cpp
@@ -1874,33 +1874,37 @@
   return StartPos;
 }
 
-// A check prefix must contain only alphanumeric, hyphens and underscores.
-static bool ValidateCheckPrefix(StringRef CheckPrefix) {
-  static const Regex Validator("^[a-zA-Z0-9_-]*$");
-  return Validator.match(CheckPrefix);
-}
-
-bool FileCheck::ValidateCheckPrefixes() {
-  StringSet<> PrefixSet;
-
-  for (StringRef Prefix : Req.CheckPrefixes) {
-    // Reject empty prefixes.
-    if (Prefix.empty())
+static bool ValidatePrefixes(StringSet<> &UniquePrefixes,
+                             ArrayRef<StringRef> SuppliedPrefixes) {
+  for (StringRef Prefix : SuppliedPrefixes) {
+    if (Prefix.empty()) {
+      errs() << "error: supplied check prefix must not be the empty string\n";
       return false;
-
-    if (!PrefixSet.insert(Prefix).second)
+    }
+    static const Regex Validator("^[a-zA-Z0-9_-]*$");
+    if (!Validator.match(Prefix)) {
+      errs() << "error: supplied check prefix must start with a letter and "
+             << "contain only alphanumeric characters, hyphens, and "
+             << "underscores: '" << Prefix << "'\n";
       return false;
-
-    if (!ValidateCheckPrefix(Prefix))
+    }
+    if (!UniquePrefixes.insert(Prefix).second) {
+      errs() << "error: supplied check prefix must be unique among check "
+             << "prefixes: '" << Prefix << "'\n";
       return false;
+    }
   }
+  return true;
+}
 
+bool FileCheck::ValidateCheckPrefixes() {
+  StringSet<> UniquePrefixes;
+  if (!ValidatePrefixes(UniquePrefixes, Req.CheckPrefixes))
+    return false;
   return true;
 }
 
 Regex FileCheck::buildCheckPrefixRegex() {
-  // I don't think there's a way to specify an initial value for cl::list,
-  // so if nothing was specified, add the default
   if (Req.CheckPrefixes.empty()) {
     Req.CheckPrefixes.push_back("CHECK");
     Req.IsDefaultCheckPrefix = true;
Index: llvm/test/FileCheck/validate-check-prefix.txt
===================================================================
--- llvm/test/FileCheck/validate-check-prefix.txt
+++ llvm/test/FileCheck/validate-check-prefix.txt
@@ -1,10 +1,13 @@
 // RUN: %ProtectFileCheckOutput not FileCheck -check-prefix=A! -input-file %s %s 2>&1 | FileCheck -check-prefix=BAD_PREFIX %s
 // RUN: FileCheck -check-prefix=A1a-B_c -input-file %s %s
-// RUN: %ProtectFileCheckOutput not FileCheck -check-prefix=REPEAT -check-prefix=REPEAT -input-file %s %s 2>&1 | FileCheck -check-prefix=BAD_PREFIX %s
+// RUN: %ProtectFileCheckOutput not FileCheck -check-prefix=REPEAT -check-prefix=REPEAT -input-file %s %s 2>&1 | FileCheck -check-prefix=DUPLICATE_PREFIX %s
 // RUN: %ProtectFileCheckOutput not FileCheck -check-prefix=VALID -check-prefix=A! -input-file %s %s 2>&1 | FileCheck -check-prefix=BAD_PREFIX %s
-// RUN: %ProtectFileCheckOutput not FileCheck -check-prefix= -input-file %s %s 2>&1 | FileCheck -check-prefix=BAD_PREFIX %s
+// RUN: %ProtectFileCheckOutput not FileCheck -check-prefix= -input-file %s %s 2>&1 | FileCheck -check-prefix=EMPTY_PREFIX %s
 foobar
 ; A1a-B_c: foobar
 
-; BAD_PREFIX: Supplied check-prefix is invalid! Prefixes must be
-  unique and start with a letter and contain only alphanumeric characters, hyphens and underscores
+; BAD_PREFIX: supplied check prefix must start with a letter and contain only alphanumeric characters, hyphens, and underscores: 'A!'
+
+; DUPLICATE_PREFIX: error: supplied check prefix must be unique among check prefixes: 'REPEAT'
+
+; EMPTY_PREFIX: error: supplied check prefix must not be the empty string
Index: llvm/utils/FileCheck/FileCheck.cpp
===================================================================
--- llvm/utils/FileCheck/FileCheck.cpp
+++ llvm/utils/FileCheck/FileCheck.cpp
@@ -601,12 +601,8 @@
     Req.Verbose = true;
 
   FileCheck FC(Req);
-  if (!FC.ValidateCheckPrefixes()) {
-    errs() << "Supplied check-prefix is invalid! Prefixes must be unique and "
-              "start with a letter and contain only alphanumeric characters, "
-              "hyphens and underscores\n";
+  if (!FC.ValidateCheckPrefixes())
     return 2;
-  }
 
   Regex PrefixRE = FC.buildCheckPrefixRegex();
   std::string REError;