Index: llvm/trunk/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp
===================================================================
--- llvm/trunk/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp
+++ llvm/trunk/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp
@@ -590,6 +590,8 @@
                                            int MemOperand, const MCInst &MI,
                                            const MCInstrDesc &Desc,
                                            raw_ostream &OS) const {
+  assert(!(TSFlags & X86II::LOCK) && "Can't have LOCK VEX.");
+
   uint64_t Encoding = TSFlags & X86II::EncodingMask;
   bool HasEVEX_K = TSFlags & X86II::EVEX_K;
   bool HasVEX_4V = TSFlags & X86II::VEX_4V;
@@ -1109,6 +1111,10 @@
                                                          : X86II::OpSize16))
     EmitByte(0x66, CurByte, OS);
 
+  // Emit the LOCK opcode prefix.
+  if (TSFlags & X86II::LOCK)
+    EmitByte(0xF0, CurByte, OS);
+
   switch (TSFlags & X86II::OpPrefixMask) {
   case X86II::PD:   // 66
     EmitByte(0x66, CurByte, OS);
@@ -1182,10 +1188,6 @@
   int MemoryOperand = X86II::getMemoryOperandNo(TSFlags, Opcode);
   if (MemoryOperand != -1) MemoryOperand += CurOp;
 
-  // Emit the lock opcode prefix as needed.
-  if (TSFlags & X86II::LOCK)
-    EmitByte(0xF0, CurByte, OS);
-
   // Emit segment override opcode prefix as needed.
   if (MemoryOperand >= 0)
     EmitSegmentOverridePrefix(CurByte, MemoryOperand+X86::AddrSegmentReg,
Index: llvm/trunk/test/CodeGen/X86/atomic16.ll
===================================================================
--- llvm/trunk/test/CodeGen/X86/atomic16.ll
+++ llvm/trunk/test/CodeGen/X86/atomic16.ll
@@ -15,17 +15,17 @@
 ; X32:       incw
   %t2 = atomicrmw add  i16* @sc16, i16 3 acquire
 ; X64:       lock
-; X64:       addw $3, {{.*}} # encoding: [0xf0,0x66
+; X64:       addw $3, {{.*}} # encoding: [0x66,0xf0
 ; X32:       lock
 ; X32:       addw $3
   %t3 = atomicrmw add  i16* @sc16, i16 5 acquire
 ; X64:       lock
-; X64:       xaddw {{.*}} # encoding: [0xf0,0x66
+; X64:       xaddw {{.*}} # encoding: [0x66,0xf0
 ; X32:       lock
 ; X32:       xaddw
   %t4 = atomicrmw add  i16* @sc16, i16 %t3 acquire
 ; X64:       lock
-; X64:       addw {{.*}} # encoding: [0xf0,0x66
+; X64:       addw {{.*}} # encoding: [0x66,0xf0
 ; X32:       lock
 ; X32:       addw
   ret void
@@ -43,17 +43,17 @@
 ; X32:       decw
   %t2 = atomicrmw sub  i16* @sc16, i16 3 acquire
 ; X64:       lock
-; X64:       subw $3, {{.*}} # encoding: [0xf0,0x66
+; X64:       subw $3, {{.*}} # encoding: [0x66,0xf0
 ; X32:       lock
 ; X32:       subw $3
   %t3 = atomicrmw sub  i16* @sc16, i16 5 acquire
 ; X64:       lock
-; X64:       xaddw {{.*}} # encoding: [0xf0,0x66
+; X64:       xaddw {{.*}} # encoding: [0x66,0xf0
 ; X32:       lock
 ; X32:       xaddw
   %t4 = atomicrmw sub  i16* @sc16, i16 %t3 acquire
 ; X64:       lock
-; X64:       subw {{.*}} # encoding: [0xf0,0x66
+; X64:       subw {{.*}} # encoding: [0x66,0xf0
 ; X32:       lock
 ; X32:       subw
   ret void
@@ -66,7 +66,7 @@
 ; X32-LABEL:   atomic_fetch_and16
   %t1 = atomicrmw and  i16* @sc16, i16 3 acquire
 ; X64:       lock
-; X64:       andw $3, {{.*}} # encoding: [0xf0,0x66
+; X64:       andw $3, {{.*}} # encoding: [0x66,0xf0
 ; X32:       lock
 ; X32:       andw $3
   %t2 = atomicrmw and  i16* @sc16, i16 5 acquire
@@ -78,7 +78,7 @@
 ; X32:       cmpxchgw
   %t3 = atomicrmw and  i16* @sc16, i16 %t2 acquire
 ; X64:       lock
-; X64:       andw {{.*}} # encoding: [0xf0,0x66
+; X64:       andw {{.*}} # encoding: [0x66,0xf0
 ; X32:       lock
 ; X32:       andw
   ret void
@@ -91,7 +91,7 @@
 ; X32-LABEL:   atomic_fetch_or16
   %t1 = atomicrmw or   i16* @sc16, i16 3 acquire
 ; X64:       lock
-; X64:       orw $3, {{.*}} # encoding: [0xf0,0x66
+; X64:       orw $3, {{.*}} # encoding: [0x66,0xf0
 ; X32:       lock
 ; X32:       orw $3
   %t2 = atomicrmw or   i16* @sc16, i16 5 acquire
@@ -103,7 +103,7 @@
 ; X32:       cmpxchgw
   %t3 = atomicrmw or   i16* @sc16, i16 %t2 acquire
 ; X64:       lock
-; X64:       orw {{.*}} # encoding: [0xf0,0x66
+; X64:       orw {{.*}} # encoding: [0x66,0xf0
 ; X32:       lock
 ; X32:       orw
   ret void
@@ -116,7 +116,7 @@
 ; X32-LABEL:   atomic_fetch_xor16
   %t1 = atomicrmw xor  i16* @sc16, i16 3 acquire
 ; X64:       lock
-; X64:       xorw $3, {{.*}} # encoding: [0xf0,0x66
+; X64:       xorw $3, {{.*}} # encoding: [0x66,0xf0
 ; X32:       lock
 ; X32:       xorw $3
   %t2 = atomicrmw xor  i16* @sc16, i16 5 acquire
@@ -128,7 +128,7 @@
 ; X32:       cmpxchgw
   %t3 = atomicrmw xor  i16* @sc16, i16 %t2 acquire
 ; X64:       lock
-; X64:       xorw {{.*}} # encoding: [0xf0,0x66
+; X64:       xorw {{.*}} # encoding: [0x66,0xf0
 ; X32:       lock
 ; X32:       xorw
   ret void