diff --git a/llvm/lib/AsmParser/LLParser.cpp b/llvm/lib/AsmParser/LLParser.cpp
--- a/llvm/lib/AsmParser/LLParser.cpp
+++ b/llvm/lib/AsmParser/LLParser.cpp
@@ -3330,6 +3330,12 @@
   } else {
     FwdVal = new Argument(Ty, Name);
   }
+  if (FwdVal->getName() != Name) {
+    P.error(Loc, "name is too long which can result in name collisions, "
+                 "consider making the name shorter or "
+                 "increasing -non-global-value-max-name-size");
+    return nullptr;
+  }
 
   ForwardRefVals[Name] = std::make_pair(FwdVal, Loc);
   return FwdVal;
diff --git a/llvm/test/Assembler/non-global-value-max-name-size.ll b/llvm/test/Assembler/non-global-value-max-name-size.ll
--- a/llvm/test/Assembler/non-global-value-max-name-size.ll
+++ b/llvm/test/Assembler/non-global-value-max-name-size.ll
@@ -1,10 +1,15 @@
-; RUN: opt < %s -S -non-global-value-max-name-size=4
-; Test that local value name lookup works if the name is capped
+; RUN: opt < %s -S -non-global-value-max-name-size=5
+; RUN: not opt < %s -S -non-global-value-max-name-size=4 2>&1 | FileCheck %s
+
+; CHECK: name is too long
 
 define void @f() {
 bb0:
   br label %testz
 
 testz:
+  br label %testa
+
+testa:
   br label %testz
 }