Index: llvm/trunk/include/llvm/MC/MCExpr.h
===================================================================
--- llvm/trunk/include/llvm/MC/MCExpr.h
+++ llvm/trunk/include/llvm/MC/MCExpr.h
@@ -589,6 +589,8 @@
   virtual bool evaluateAsRelocatableImpl(MCValue &Res,
                                          const MCAsmLayout *Layout,
                                          const MCFixup *Fixup) const = 0;
+  // allow Target Expressions to be checked for equality
+  virtual bool isEqualTo(const MCExpr *x) const { return false; }
   // This should be set when assigned expressions are not valid ".set"
   // expressions, e.g. registers, and must be inlined.
   virtual bool inlineAssignedExpr() const { return false; }
Index: llvm/trunk/include/llvm/MC/MCParser/MCAsmParserUtils.h
===================================================================
--- llvm/trunk/include/llvm/MC/MCParser/MCAsmParserUtils.h
+++ llvm/trunk/include/llvm/MC/MCParser/MCAsmParserUtils.h
@@ -25,7 +25,7 @@
 /// On success, returns false and sets the Symbol and Value output parameters.
 bool parseAssignmentExpression(StringRef Name, bool allow_redef,
                                MCAsmParser &Parser, MCSymbol *&Symbol,
-                               const MCExpr *&Value, bool AllowExtendedExpr = false);
+                               const MCExpr *&Value);
 
 } // namespace MCParserUtils
 
Index: llvm/trunk/include/llvm/MC/MCParser/MCTargetAsmParser.h
===================================================================
--- llvm/trunk/include/llvm/MC/MCParser/MCTargetAsmParser.h
+++ llvm/trunk/include/llvm/MC/MCParser/MCTargetAsmParser.h
@@ -372,9 +372,9 @@
     SemaCallback = Callback;
   }
 
-  // Target-specific parsing of assembler-level variable assignment.
-  virtual bool parseAssignmentExpression(const MCExpr *&Res, SMLoc &EndLoc) {
-    return getParser().parseExpression(Res, EndLoc);
+  // Target-specific parsing of expression.
+  virtual bool parsePrimaryExpr(const MCExpr *&Res, SMLoc &EndLoc) {
+    return getParser().parsePrimaryExpr(Res, EndLoc);
   }
 
   virtual bool ParseRegister(unsigned &RegNo, SMLoc &StartLoc,
Index: llvm/trunk/lib/MC/MCExpr.cpp
===================================================================
--- llvm/trunk/lib/MC/MCExpr.cpp
+++ llvm/trunk/lib/MC/MCExpr.cpp
@@ -751,8 +751,22 @@
     if (!ABE->getLHS()->evaluateAsRelocatableImpl(LHSValue, Asm, Layout, Fixup,
                                                   Addrs, InSet) ||
         !ABE->getRHS()->evaluateAsRelocatableImpl(RHSValue, Asm, Layout, Fixup,
-                                                  Addrs, InSet))
+                                                  Addrs, InSet)) {
+      // Check if both are Target Expressions, see if we can compare them.
+      if (const MCTargetExpr *L = dyn_cast<MCTargetExpr>(ABE->getLHS()))
+        if (const MCTargetExpr *R = cast<MCTargetExpr>(ABE->getRHS())) {
+          switch (ABE->getOpcode()) {
+          case MCBinaryExpr::EQ:
+            Res = MCValue::get((L->isEqualTo(R)) ? -1 : 0);
+            return true;
+          case MCBinaryExpr::NE:
+            Res = MCValue::get((R->isEqualTo(R)) ? 0 : -1);
+            return true;
+          default: {}
+          }
+        }
       return false;
+    }
 
     // We only support a few operations on non-constant expressions, handle
     // those first.
Index: llvm/trunk/lib/MC/MCParser/AsmParser.cpp
===================================================================
--- llvm/trunk/lib/MC/MCParser/AsmParser.cpp
+++ llvm/trunk/lib/MC/MCParser/AsmParser.cpp
@@ -337,7 +337,7 @@
   StringRef parseStringToComma();
 
   bool parseAssignment(StringRef Name, bool allow_redef,
-                       bool NoDeadStrip = false, bool AllowExtendedExpr = false);
+                       bool NoDeadStrip = false);
 
   unsigned getBinOpPrecedence(AsmToken::TokenKind K,
                               MCBinaryExpr::Opcode &Kind);
@@ -1363,7 +1363,8 @@
 bool AsmParser::parseExpression(const MCExpr *&Res, SMLoc &EndLoc) {
   // Parse the expression.
   Res = nullptr;
-  if (parsePrimaryExpr(Res, EndLoc) || parseBinOpRHS(1, Res, EndLoc))
+  if (getTargetParser().parsePrimaryExpr(Res, EndLoc) ||
+      parseBinOpRHS(1, Res, EndLoc))
     return true;
 
   // As a special case, we support 'a op b @ modifier' by rewriting the
@@ -1617,7 +1618,7 @@
 
     // Eat the next primary expression.
     const MCExpr *RHS;
-    if (parsePrimaryExpr(RHS, EndLoc))
+    if (getTargetParser().parsePrimaryExpr(RHS, EndLoc))
       return true;
 
     // If BinOp binds less tightly with RHS than the operator after RHS, let
@@ -1826,7 +1827,7 @@
     // identifier '=' ... -> assignment statement
     Lex();
 
-    return parseAssignment(IDVal, true, /*NoDeadStrip*/ false, /*AllowExtendedExpr*/true);
+    return parseAssignment(IDVal, true);
 
   default: // Normal instruction or directive.
     break;
@@ -2766,11 +2767,11 @@
 }
 
 bool AsmParser::parseAssignment(StringRef Name, bool allow_redef,
-                                bool NoDeadStrip, bool AllowExtendedExpr) {
+                                bool NoDeadStrip) {
   MCSymbol *Sym;
   const MCExpr *Value;
   if (MCParserUtils::parseAssignmentExpression(Name, allow_redef, *this, Sym,
-                                               Value, AllowExtendedExpr))
+                                               Value))
     return true;
 
   if (!Sym) {
@@ -5839,17 +5840,13 @@
 
 bool parseAssignmentExpression(StringRef Name, bool allow_redef,
                                MCAsmParser &Parser, MCSymbol *&Sym,
-                               const MCExpr *&Value, bool AllowExtendedExpr) {
+                               const MCExpr *&Value) {
 
   // FIXME: Use better location, we should use proper tokens.
   SMLoc EqualLoc = Parser.getTok().getLoc();
   SMLoc EndLoc;
-  if (AllowExtendedExpr) {
-    if (Parser.getTargetParser().parseAssignmentExpression(Value, EndLoc)) {
-      return Parser.TokError("missing expression");
-    }
-  } else if (Parser.parseExpression(Value, EndLoc))
-      return Parser.TokError("missing expression");
+  if (Parser.parseExpression(Value))
+    return Parser.TokError("missing expression");
 
   // Note: we don't count b as used in "a = b". This is to allow
   // a = b
Index: llvm/trunk/lib/Target/X86/AsmParser/X86AsmParser.cpp
===================================================================
--- llvm/trunk/lib/Target/X86/AsmParser/X86AsmParser.cpp
+++ llvm/trunk/lib/Target/X86/AsmParser/X86AsmParser.cpp
@@ -955,7 +955,7 @@
 
   void SetFrameRegister(unsigned RegNo) override;
 
-  bool parseAssignmentExpression(const MCExpr *&Res, SMLoc &EndLoc) override;
+  bool parsePrimaryExpr(const MCExpr *&Res, SMLoc &EndLoc) override;
 
   bool ParseInstruction(ParseInstructionInfo &Info, StringRef Name,
                         SMLoc NameLoc, OperandVector &Operands) override;
@@ -2260,15 +2260,17 @@
   return X86Operand::CreateMem(getPointerWidth(), Disp, MemStart, MemEnd);
 }
 
-// Parse either a standard expression or a register.
-bool X86AsmParser::parseAssignmentExpression(const MCExpr *&Res,
-                                             SMLoc &EndLoc) {
+// Parse either a standard primary expression or a register.
+bool X86AsmParser::parsePrimaryExpr(const MCExpr *&Res, SMLoc &EndLoc) {
   MCAsmParser &Parser = getParser();
-  if (Parser.parseExpression(Res, EndLoc)) {
+  if (Parser.parsePrimaryExpr(Res, EndLoc)) {
     SMLoc StartLoc = Parser.getTok().getLoc();
     // Normal Expression parse fails, check if it could be a register.
     unsigned RegNo;
-    if (Parser.getTargetParser().ParseRegister(RegNo, StartLoc, EndLoc))
+    bool TryRegParse =
+        getTok().is(AsmToken::Percent) ||
+        (isParsingIntelSyntax() && getTok().is(AsmToken::Identifier));
+    if (!TryRegParse || ParseRegister(RegNo, StartLoc, EndLoc))
       return true;
     // Clear previous parse error and return correct expression.
     Parser.clearPendingErrors();
Index: llvm/trunk/lib/Target/X86/MCTargetDesc/X86MCExpr.h
===================================================================
--- llvm/trunk/lib/Target/X86/MCTargetDesc/X86MCExpr.h
+++ llvm/trunk/lib/Target/X86/MCTargetDesc/X86MCExpr.h
@@ -48,7 +48,7 @@
   /// @}
 
   void printImpl(raw_ostream &OS, const MCAsmInfo *MAI) const override {
-    if (MAI->getAssemblerDialect() == 0)
+    if (!MAI || MAI->getAssemblerDialect() == 0)
       OS << '%';
     OS << X86ATTInstPrinter::getRegisterName(RegNo);
   }
@@ -59,6 +59,11 @@
   }
   // Register values should be inlined as they are not valid .set expressions.
   bool inlineAssignedExpr() const override { return true; }
+  bool isEqualTo(const MCExpr *X) const override {
+    if (auto *E = dyn_cast<X86MCExpr>(X))
+      return getRegNo() == E->getRegNo();
+    return false;
+  }
   void visitUsedExpr(MCStreamer &Streamer) const override{};
   MCFragment *findAssociatedFragment() const override { return nullptr; }
 
Index: llvm/trunk/test/MC/X86/pr37425.s
===================================================================
--- llvm/trunk/test/MC/X86/pr37425.s
+++ llvm/trunk/test/MC/X86/pr37425.s
@@ -1,5 +1,4 @@
-// RUN: llvm-mc -triple x86_64-unknown-unknown -defsym=ERR=0 %s -o -      | FileCheck %s
-// RUN: not llvm-mc -triple x86_64-unknown-unknown -defsym=ERR=1 %s -o - 2>&1 | FileCheck --check-prefix=ERR %s
+// RUN: llvm-mc -triple x86_64-unknown-unknown %s -o -      | FileCheck %s
 	
 // CHECK-NOT: .set var_xdata
 var_xdata = %rcx
@@ -7,10 +6,15 @@
 // CHECK: xorq %rcx, %rcx
 xorq var_xdata, var_xdata
 
-.if (ERR==1)
-// ERR: [[@LINE+2]]:15: error: unknown token in expression in '.set' directive
-// ERR: [[@LINE+1]]:15: error: missing expression in '.set' directive
-.set err_var, %rcx
-.endif	
+// CHECK: .data
+// CHECK-NEXT: .byte 1	
+.data 
+.if var_xdata == %rax
+  .byte 0
+.elseif var_xdata == %rcx
+  .byte 1
+.else
+  .byte 2	
+.endif