diff --git a/lib/Target/X86/X86CodeEmitter.cpp b/lib/Target/X86/X86CodeEmitter.cpp
index e08688b..e5bef57 100644
--- a/lib/Target/X86/X86CodeEmitter.cpp
+++ b/lib/Target/X86/X86CodeEmitter.cpp
@@ -84,6 +84,7 @@
                                intptr_t PCAdj = 0);
 
     void emitRegModRMByte(unsigned ModRMReg, unsigned RegOpcodeField);
+    void emitRegModRMByte(unsigned RegOpcodeField);
     void emitSIBByte(unsigned SS, unsigned Index, unsigned Base);
     void emitConstant(uint64_t Val, unsigned Size);
 
@@ -231,6 +232,10 @@
   MCE.emitByte(ModRMByte(3, RegOpcodeFld, getX86RegNum(ModRMReg)));
 }
 
+void Emitter::emitRegModRMByte(unsigned RegOpcodeFld) {
+  MCE.emitByte(ModRMByte(3, RegOpcodeFld, 0));
+}
+
 void Emitter::emitSIBByte(unsigned SS, unsigned Index, unsigned Base) {
   // SIB byte is in the same format as the ModRMByte...
   MCE.emitByte(ModRMByte(SS, Index, Base));
@@ -631,10 +636,16 @@
   case X86II::MRM0r: case X86II::MRM1r:
   case X86II::MRM2r: case X86II::MRM3r:
   case X86II::MRM4r: case X86II::MRM5r:
-  case X86II::MRM6r: case X86II::MRM7r:
+  case X86II::MRM6r: case X86II::MRM7r: {
     MCE.emitByte(BaseOpcode);
-    emitRegModRMByte(MI.getOperand(CurOp++).getReg(),
-                     (Desc->TSFlags & X86II::FormMask)-X86II::MRM0r);
+
+    // Special handling of lfence and mfence. 
+    if (Desc->getOpcode() == X86::LFENCE ||
+        Desc->getOpcode() == X86::MFENCE)
+      emitRegModRMByte((Desc->TSFlags & X86II::FormMask)-X86II::MRM0r);
+    else
+      emitRegModRMByte(MI.getOperand(CurOp++).getReg(),
+                       (Desc->TSFlags & X86II::FormMask)-X86II::MRM0r);
 
     if (CurOp != NumOps) {
       const MachineOperand &MO1 = MI.getOperand(CurOp++);
@@ -660,6 +671,7 @@
       }
     }
     break;
+  }
 
   case X86II::MRM0m: case X86II::MRM1m:
   case X86II::MRM2m: case X86II::MRM3m:
diff --git a/lib/Target/X86/X86InstrSSE.td b/lib/Target/X86/X86InstrSSE.td
index 0a5089d..426bddc 100644
--- a/lib/Target/X86/X86InstrSSE.td
+++ b/lib/Target/X86/X86InstrSSE.td
@@ -2239,9 +2239,9 @@
               TB, Requires<[HasSSE2]>;
 
 // Load, store, and memory fence
-def LFENCE : I<0xAE, MRM5m, (outs), (ins),
+def LFENCE : I<0xAE, MRM5r, (outs), (ins),
                "lfence", [(int_x86_sse2_lfence)]>, TB, Requires<[HasSSE2]>;
-def MFENCE : I<0xAE, MRM6m, (outs), (ins),
+def MFENCE : I<0xAE, MRM6r, (outs), (ins),
                "mfence", [(int_x86_sse2_mfence)]>, TB, Requires<[HasSSE2]>;
 
 //TODO: custom lower this so as to never even generate the noop