diff --git a/llvm/test/CodeGen/X86/lit.local.cfg b/llvm/test/CodeGen/X86/lit.local.cfg
--- a/llvm/test/CodeGen/X86/lit.local.cfg
+++ b/llvm/test/CodeGen/X86/lit.local.cfg
@@ -1,2 +1,7 @@
+from lit.llvm.subst import ToolSubst
+
 if not 'X86' in config.root.targets:
     config.unsupported = True
+
+fc = ToolSubst('FileCheck', unresolved='fatal')
+config.substitutions.insert(0, (fc.regex, 'FileCheck --allow-unused-prefixes'))