diff --git a/llvm/lib/IR/Mangler.cpp b/llvm/lib/IR/Mangler.cpp
index 7adcc59..be3086c 100644
--- a/llvm/lib/IR/Mangler.cpp
+++ b/llvm/lib/IR/Mangler.cpp
@@ -44,6 +44,9 @@
     return;
   }
 
+  if (DL.doNotMangleLeadingQuestionMark() && Name[0] == '?')
+    Prefix = '\0';
+
   if (PrefixTy == Private)
     OS << DL.getPrivateGlobalPrefix();
   else if (PrefixTy == LinkerPrivate)
@@ -135,8 +138,13 @@
   // Mangle functions with Microsoft calling conventions specially.  Only do
   // this mangling for x86_64 vectorcall and 32-bit x86.
   const Function *MSFunc = dyn_cast<Function>(GV);
-  if (Name.startswith("\01"))
-    MSFunc = nullptr; // Don't mangle when \01 is present.
+
+  // Don't add byte count suffixes when '\01' or '?' are in the first
+  // character.
+  if (Name.startswith("\01") ||
+      (DL.doNotMangleLeadingQuestionMark() && Name.startswith("?")))
+    MSFunc = nullptr;
+
   CallingConv::ID CC =
       MSFunc ? MSFunc->getCallingConv() : (unsigned)CallingConv::C;
   if (!DL.hasMicrosoftFastStdCallMangling() &&