Index: llvm/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.h
===================================================================
--- llvm/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.h
+++ llvm/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.h
@@ -125,6 +125,10 @@
 
 } // End llvm namespace
 
+// Some Unix systems define an FSCALE macro in sys/param.h. This macro somehow
+// gets pulled into this header when -fmodules is enabled, preventing us from
+// textually including X86GenInstrInfo.inc.
+#undef FSCALE
 
 // Defines symbolic names for X86 registers.  This defines a mapping from
 // register name to register number.