diff --git a/lib/Target/X86/X86CodeEmitter.cpp b/lib/Target/X86/X86CodeEmitter.cpp
index 63bf18d..efd64e0 100644
--- a/lib/Target/X86/X86CodeEmitter.cpp
+++ b/lib/Target/X86/X86CodeEmitter.cpp
@@ -671,13 +671,26 @@
   case X86II::MRM6r: case X86II::MRM7r: {
     MCE.emitByte(BaseOpcode);
 
-    // Special handling of lfence and mfence. 
+    // Special handling of lfence, mfence, monitor, and mwait.
     if (Desc->getOpcode() == X86::LFENCE ||
-        Desc->getOpcode() == X86::MFENCE)
+        Desc->getOpcode() == X86::MFENCE ||
+        Desc->getOpcode() == X86::MONITOR ||
+        Desc->getOpcode() == X86::MWAIT) {
       emitRegModRMByte((Desc->TSFlags & X86II::FormMask)-X86II::MRM0r);
-    else
+
+      switch (Desc->getOpcode()) {
+      default: break;
+      case X86::MONITOR:
+        MCE.emitByte(0xC8);
+        break;
+      case X86::MWAIT:
+        MCE.emitByte(0xC9);
+        break;
+      }
+    } else {
       emitRegModRMByte(MI.getOperand(CurOp++).getReg(),
                        (Desc->TSFlags & X86II::FormMask)-X86II::MRM0r);
+    }
 
     if (CurOp != NumOps) {
       const MachineOperand &MO1 = MI.getOperand(CurOp++);
diff --git a/lib/Target/X86/X86InstrInfo.cpp b/lib/Target/X86/X86InstrInfo.cpp
index 5fca9c7..2cd3733 100644
--- a/lib/Target/X86/X86InstrInfo.cpp
+++ b/lib/Target/X86/X86InstrInfo.cpp
@@ -2885,7 +2885,7 @@
   // Emit the lock opcode prefix as needed.
   if (Desc->TSFlags & X86II::LOCK) ++FinalSize;
 
-  // Emit segment overrid opcode prefix as needed.
+  // Emit segment override opcode prefix as needed.
   switch (Desc->TSFlags & X86II::SegOvrMask) {
   case X86II::FS:
   case X86II::GS:
@@ -2943,7 +2943,7 @@
   case X86II::T8:  // 0F 38
     ++FinalSize;
     break;
-  case X86II::TA:    // 0F 3A
+  case X86II::TA:  // 0F 3A
     ++FinalSize;
     break;
   }
@@ -3087,11 +3087,15 @@
   case X86II::MRM4r: case X86II::MRM5r:
   case X86II::MRM6r: case X86II::MRM7r:
     ++FinalSize;
-    // Special handling of lfence and mfence. 
     if (Desc->getOpcode() == X86::LFENCE ||
-        Desc->getOpcode() == X86::MFENCE)
+        Desc->getOpcode() == X86::MFENCE) {
+      // Special handling of lfence and mfence;
       FinalSize += sizeRegModRMByte();
-    else {
+    } else if (Desc->getOpcode() == X86::MONITOR ||
+               Desc->getOpcode() == X86::MWAIT) {
+      // Special handling of monitor and mwait.
+      FinalSize += sizeRegModRMByte() + 1; // +1 for the opcode.
+    } else {
       ++CurOp;
       FinalSize += sizeRegModRMByte();
     }
diff --git a/lib/Target/X86/X86InstrSSE.td b/lib/Target/X86/X86InstrSSE.td
index 83eae18..1fafa46 100644
--- a/lib/Target/X86/X86InstrSSE.td
+++ b/lib/Target/X86/X86InstrSSE.td
@@ -2504,9 +2504,9 @@
 }
 
 // Thread synchronization
-def MONITOR : I<0xC8, RawFrm, (outs), (ins), "monitor",
+def MONITOR : I<0x01, MRM1r, (outs), (ins), "monitor",
                 [(int_x86_sse3_monitor EAX, ECX, EDX)]>,TB, Requires<[HasSSE3]>;
-def MWAIT   : I<0xC9, RawFrm, (outs), (ins), "mwait",
+def MWAIT   : I<0x01, MRM1r, (outs), (ins), "mwait",
                 [(int_x86_sse3_mwait ECX, EAX)]>, TB, Requires<[HasSSE3]>;
 
 // vector_shuffle v1, <undef> <1, 1, 3, 3>