diff --git a/llvm/lib/IR/Mangler.cpp b/llvm/lib/IR/Mangler.cpp
--- a/llvm/lib/IR/Mangler.cpp
+++ b/llvm/lib/IR/Mangler.cpp
@@ -144,7 +144,7 @@
 
   // 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);
+  const Function *MSFunc = dyn_cast<Function>(GV->getAliaseeObject());
 
   // Don't add byte count suffixes when '\01' or '?' are in the first
   // character.
diff --git a/llvm/test/CodeGen/X86/fastcall-correct-mangling.ll b/llvm/test/CodeGen/X86/fastcall-correct-mangling.ll
--- a/llvm/test/CodeGen/X86/fastcall-correct-mangling.ll
+++ b/llvm/test/CodeGen/X86/fastcall-correct-mangling.ll
@@ -31,3 +31,7 @@
 ; CHECK64-LABEL: {{^}}.LdontCrash:
   ret void
 }
+
+@alias = alias void(i64, i8, i8, i16), void(i64, i8, i8, i16)* @func
+; CHECK32-LABEL: {{^}}.set @alias@20, @func@20
+; CHECK64-LABEL: {{^}}.set alias, func