diff --git a/include/llvm/Target/Target.td b/include/llvm/Target/Target.td
index d4a1850..5a8707b 100644
--- a/include/llvm/Target/Target.td
+++ b/include/llvm/Target/Target.td
@@ -221,6 +221,11 @@
   bit mayHaveSideEffects = 0;
   bit neverHasSideEffects = 0;
 
+  // Is this instruction a "real" instruction (with a distinct machine
+  // encoding), or is it a pseudo instruction used for codegen modeling
+  // purposes.
+  bit isCodeGenOnly = 0;
+
   InstrItinClass Itinerary = NoItinerary;// Execution steps used for scheduling.
 
   string Constraints = "";  // OperandConstraint, e.g. $src = $dst.
@@ -386,7 +391,8 @@
   bit isLittleEndianEncoding = 0;
 }
 
-// Standard Instructions.
+// Standard Pseudo Instructions.
+let isCodeGenOnly = 1 in {
 def PHI : Instruction {
   let OutOperandList = (ops);
   let InOperandList = (ops variable_ops);
@@ -466,6 +472,7 @@
   let neverHasSideEffects = 1;
   let isAsCheapAsAMove = 1;
 }
+}
 
 //===----------------------------------------------------------------------===//
 // AsmParser - This class can be implemented by targets that wish to implement 
diff --git a/lib/Target/X86/X86InstrInfo.td b/lib/Target/X86/X86InstrInfo.td
index fbfde7d..8c80811 100644
--- a/lib/Target/X86/X86InstrInfo.td
+++ b/lib/Target/X86/X86InstrInfo.td
@@ -3166,7 +3166,8 @@
 
 // Alias instructions that map movr0 to xor.
 // FIXME: remove when we can teach regalloc that xor reg, reg is ok.
-let Defs = [EFLAGS], isReMaterializable = 1, isAsCheapAsAMove = 1 in {
+let Defs = [EFLAGS], isReMaterializable = 1, isAsCheapAsAMove = 1,
+    isCodeGenOnly = 1 in {
 def MOV8r0   : I<0x30, MRMInitReg, (outs GR8 :$dst), (ins),
                  "xor{b}\t$dst, $dst",
                  [(set GR8:$dst, 0)]>;
diff --git a/lib/Target/X86/X86InstrMMX.td b/lib/Target/X86/X86InstrMMX.td
index dfdf700..ce76b4e 100644
--- a/lib/Target/X86/X86InstrMMX.td
+++ b/lib/Target/X86/X86InstrMMX.td
@@ -516,7 +516,7 @@
 //===----------------------------------------------------------------------===//
 
 // Alias instructions that map zero vector to pxor.
-let isReMaterializable = 1 in {
+let isReMaterializable = 1, isCodeGenOnly = 1 in {
   def MMX_V_SET0       : MMXI<0xEF, MRMInitReg, (outs VR64:$dst), (ins),
                               "pxor\t$dst, $dst",
                               [(set VR64:$dst, (v2i32 immAllZerosV))]>;
diff --git a/lib/Target/X86/X86InstrSSE.td b/lib/Target/X86/X86InstrSSE.td
index 4547ad2..f596527 100644
--- a/lib/Target/X86/X86InstrSSE.td
+++ b/lib/Target/X86/X86InstrSSE.td
@@ -472,7 +472,7 @@
 // that start with 'Fs'.
 
 // Alias instructions that map fld0 to pxor for sse.
-let isReMaterializable = 1, isAsCheapAsAMove = 1 in
+let isReMaterializable = 1, isAsCheapAsAMove = 1, isCodeGenOnly = 1 in
 def FsFLD0SS : I<0xEF, MRMInitReg, (outs FR32:$dst), (ins),
                  "pxor\t$dst, $dst", [(set FR32:$dst, fp32imm0)]>,
                Requires<[HasSSE1]>, TB, OpSize;
@@ -992,7 +992,8 @@
 // Alias instructions that map zero vector to pxor / xorp* for sse.
 // We set canFoldAsLoad because this can be converted to a constant-pool
 // load of an all-zeros value if folding it would be beneficial.
-let isReMaterializable = 1, isAsCheapAsAMove = 1, canFoldAsLoad = 1 in
+let isReMaterializable = 1, isAsCheapAsAMove = 1, canFoldAsLoad = 1,
+    isCodeGenOnly = 1 in
 def V_SET0 : PSI<0x57, MRMInitReg, (outs VR128:$dst), (ins),
                  "xorps\t$dst, $dst",
                  [(set VR128:$dst, (v4i32 immAllZerosV))]>;
@@ -1208,7 +1209,7 @@
 // that start with 'Fs'.
 
 // Alias instructions that map fld0 to pxor for sse.
-let isReMaterializable = 1, isAsCheapAsAMove = 1 in
+let isReMaterializable = 1, isAsCheapAsAMove = 1, isCodeGenOnly = 1 in
 def FsFLD0SD : I<0xEF, MRMInitReg, (outs FR64:$dst), (ins),
                  "pxor\t$dst, $dst", [(set FR64:$dst, fpimm0)]>,
                Requires<[HasSSE2]>, TB, OpSize;
@@ -2245,7 +2246,8 @@
 // Alias instructions that map zero vector to pxor / xorp* for sse.
 // We set canFoldAsLoad because this can be converted to a constant-pool
 // load of an all-ones value if folding it would be beneficial.
-let isReMaterializable = 1, isAsCheapAsAMove = 1, canFoldAsLoad = 1 in
+let isReMaterializable = 1, isAsCheapAsAMove = 1, canFoldAsLoad = 1,
+    isCodeGenOnly = 1 in
   def V_SETALLONES : PDI<0x76, MRMInitReg, (outs VR128:$dst), (ins),
                          "pcmpeqd\t$dst, $dst",
                          [(set VR128:$dst, (v4i32 immAllOnesV))]>;
diff --git a/utils/TableGen/AsmMatcherEmitter.cpp b/utils/TableGen/AsmMatcherEmitter.cpp
index 4dbd24c..122f124 100644
--- a/utils/TableGen/AsmMatcherEmitter.cpp
+++ b/utils/TableGen/AsmMatcherEmitter.cpp
@@ -210,18 +210,17 @@
 static bool IsAssemblerInstruction(const StringRef &Name,
                                    const CodeGenInstruction &CGI, 
                                    const SmallVectorImpl<StringRef> &Tokens) {
-  // Ignore psuedo ops.
+  // Ignore "codegen only" instructions.
+  if (CGI.TheDef->getValueAsBit("isCodeGenOnly"))
+    return false;
+
+  // Ignore pseudo ops.
   //
-  // FIXME: This is a hack.
+  // FIXME: This is a hack; can we convert these instructions to set the
+  // "codegen only" bit instead?
   if (const RecordVal *Form = CGI.TheDef->getValue("Form"))
     if (Form->getValue()->getAsString() == "Pseudo")
       return false;
-  
-  // Ignore "PHI" node.
-  //
-  // FIXME: This is also a hack.
-  if (Name == "PHI")
-    return false;
 
   // Ignore "Int_*" and "*_Int" instructions, which are internal aliases.
   //
@@ -245,11 +244,8 @@
   //
   // FIXME: Is this true?
   //
-  // Also, we ignore instructions which reference the operand multiple times;
-  // this implies a constraint we would not currently honor. These are
-  // currently always fake instructions for simplifying codegen.
-  //
-  // FIXME: Encode this assumption in the .td, so we can error out here.
+  // Also, check for instructions which reference the operand multiple times;
+  // this implies a constraint we would not honor.
   std::set<std::string> OperandNames;
   for (unsigned i = 1, e = Tokens.size(); i < e; ++i) {
     if (Tokens[i][0] == '$' && 
@@ -258,18 +254,15 @@
       DEBUG({
           errs() << "warning: '" << Name << "': "
                  << "ignoring instruction; operand with attribute '" 
-                 << Tokens[i] << "', \n";
+                 << Tokens[i] << "'\n";
         });
       return false;
     }
 
     if (Tokens[i][0] == '$' && !OperandNames.insert(Tokens[i]).second) {
-      DEBUG({
-          errs() << "warning: '" << Name << "': "
-                 << "ignoring instruction; tied operand '" 
-                 << Tokens[i] << "'\n";
-        });
-      return false;
+      std::string Err = "'" + Name.str() + "': " +
+        "invalid assembler instruction; tied operand '" + Tokens[i].str() + "'";
+      throw TGError(CGI.TheDef->getLoc(), Err);
     }
   }