diff --git a/llvm/include/llvm/MC/MCSymbol.h b/llvm/include/llvm/MC/MCSymbol.h
index 2ee7dd44cff1..1b432c2b0a84 100644
--- a/llvm/include/llvm/MC/MCSymbol.h
+++ b/llvm/include/llvm/MC/MCSymbol.h
@@ -52,10 +52,15 @@ namespace llvm {
     /// "Lfoo" or ".foo".
     unsigned IsTemporary : 1;
 
+    /// IsUsedInExpr - True if this symbol has been used in an expression and
+    /// cannot be redefined.
+    unsigned IsUsedInExpr : 1;
+
   private:  // MCContext creates and uniques these.
     friend class MCContext;
     MCSymbol(StringRef name, bool isTemporary)
-      : Name(name), Section(0), Value(0), IsTemporary(isTemporary) {}
+      : Name(name), Section(0), Value(0),
+        IsTemporary(isTemporary), IsUsedInExpr(false) {}
 
     MCSymbol(const MCSymbol&);       // DO NOT IMPLEMENT
     void operator=(const MCSymbol&); // DO NOT IMPLEMENT
@@ -63,13 +68,15 @@ namespace llvm {
     /// getName - Get the symbol name.
     StringRef getName() const { return Name; }
 
-    /// @name Symbol Type
+    /// @name Accessors
     /// @{
 
     /// isTemporary - Check if this is an assembler temporary symbol.
-    bool isTemporary() const {
-      return IsTemporary;
-    }
+    bool isTemporary() const { return IsTemporary; }
+
+    /// isUsedInExpr - Check if this is an assembler temporary symbol.
+    bool isUsedInExpr() const { return IsUsedInExpr; }
+    void setUsedInExpr(bool Value) { IsUsedInExpr = Value; }
 
     /// @}
     /// @name Associated Sections
diff --git a/llvm/lib/MC/MCParser/AsmParser.cpp b/llvm/lib/MC/MCParser/AsmParser.cpp
index e5af357de34d..0df839b84d9b 100644
--- a/llvm/lib/MC/MCParser/AsmParser.cpp
+++ b/llvm/lib/MC/MCParser/AsmParser.cpp
@@ -189,6 +189,9 @@ bool AsmParser::ParsePrimaryExpr(const MCExpr *&Res, SMLoc &EndLoc) {
     std::pair<StringRef, StringRef> Split = getTok().getIdentifier().split('@');
     MCSymbol *Sym = CreateSymbol(Split.first);
 
+    // Mark the symbol as used in an expression.
+    Sym->setUsedInExpr(true);
+
     // Lookup the symbol variant if used.
     MCSymbolRefExpr::VariantKind Variant = MCSymbolRefExpr::VK_None;
     if (Split.first.size() != getTok().getIdentifier().size())
@@ -788,7 +791,9 @@ bool AsmParser::ParseAssignment(const StringRef &Name) {
     //
     // FIXME: Diagnostics. Note the location of the definition as a label.
     // FIXME: Diagnose assignment to protected identifier (e.g., register name).
-    if (!Sym->isUndefined() && !Sym->isAbsolute())
+    if (Sym->isUndefined() && !Sym->isUsedInExpr())
+      ; // Allow redefinitions of undefined symbols only used in directives.
+    else if (!Sym->isUndefined() && !Sym->isAbsolute())
       return Error(EqualLoc, "redefinition of '" + Name + "'");
     else if (!Sym->isVariable())
       return Error(EqualLoc, "invalid assignment to '" + Name + "'");
@@ -800,6 +805,8 @@ bool AsmParser::ParseAssignment(const StringRef &Name) {
 
   // FIXME: Handle '.'.
 
+  Sym->setUsedInExpr(true);
+
   // Do the assignment.
   Out.EmitAssignment(Sym, Value);
 
diff --git a/llvm/test/MC/AsmParser/assignment.s b/llvm/test/MC/AsmParser/assignment.s
index 882fae8bae64..73ce8600db0b 100644
--- a/llvm/test/MC/AsmParser/assignment.s
+++ b/llvm/test/MC/AsmParser/assignment.s
@@ -2,6 +2,10 @@
 
 # CHECK: TEST0:
 # CHECK: a = 0
-TEST0:  
+TEST0:
         a = 0
-        
+
+# CHECK: .globl	_f1
+# CHECK: _f1 = 0
+        .globl _f1
+        _f1 = 0