diff --git a/llvm/lib/MC/MCParser/MasmParser.cpp b/llvm/lib/MC/MCParser/MasmParser.cpp
--- a/llvm/lib/MC/MCParser/MasmParser.cpp
+++ b/llvm/lib/MC/MCParser/MasmParser.cpp
@@ -383,8 +383,10 @@
 
   /// maps assembly-time variable names to variables.
   struct Variable {
+    enum RedefinableKind { NOT_REDEFINABLE, WARN_ON_REDEFINITION, REDEFINABLE };
+
     StringRef Name;
-    bool Redefinable = true;
+    RedefinableKind Redefinable = REDEFINABLE;
     bool IsText = false;
     int64_t NumericValue = 0;
     std::string TextValue;
@@ -865,7 +867,7 @@
 
   // "=", "equ", "textequ"
   bool parseDirectiveEquate(StringRef IDVal, StringRef Name,
-                            DirectiveKind DirKind);
+                            DirectiveKind DirKind, SMLoc NameLoc);
 
   bool parseDirectiveOrg(); // ".org"
   bool parseDirectiveAlign();  // "align"
@@ -2479,7 +2481,7 @@
   case DK_EQU:
   case DK_TEXTEQU:
     Lex();
-    return parseDirectiveEquate(nextVal, IDVal, DirKind);
+    return parseDirectiveEquate(nextVal, IDVal, DirKind, IDLoc);
   case DK_BYTE:
   case DK_SBYTE:
   case DK_DB:
@@ -3271,14 +3273,20 @@
 ///    | name "equ" text-list
 ///    | name "textequ" text-list
 bool MasmParser::parseDirectiveEquate(StringRef IDVal, StringRef Name,
-                                      DirectiveKind DirKind) {
+                                      DirectiveKind DirKind, SMLoc NameLoc) {
   Variable &Var = Variables[Name.lower()];
   if (Var.Name.empty()) {
     Var.Name = Name;
-  } else if (!Var.Redefinable) {
+  } else if (Var.Redefinable == Variable::NOT_REDEFINABLE) {
     return TokError("invalid variable redefinition");
+  } else if (Var.Redefinable == Variable::WARN_ON_REDEFINITION &&
+             Warning(NameLoc, "redefining '" + Name +
+                                  "', already defined on the command line")) {
+    return true;
   }
-  Var.Redefinable = (DirKind != DK_EQU);
+
+  if (DirKind == DK_EQU)
+    Var.Redefinable = Variable::NOT_REDEFINABLE;
 
   SMLoc StartLoc = Lexer.getLoc();
   if (DirKind == DK_EQU || DirKind == DK_TEXTEQU) {
@@ -3310,7 +3318,7 @@
   if (parseExpression(Expr, EndLoc))
     return addErrorSuffix(" in '" + Twine(IDVal) + "' directive");
   MCSymbol *Sym = getContext().getOrCreateSymbol(Var.Name);
-  Sym->setRedefinable(Var.Redefinable);
+  Sym->setRedefinable(Var.Redefinable != Variable::NOT_REDEFINABLE);
   Sym->setVariableValue(Expr);
   Sym->setExternal(false);
 
@@ -6925,10 +6933,14 @@
   Variable &Var = Variables[Name.lower()];
   if (Var.Name.empty()) {
     Var.Name = Name;
-  } else if (!Var.Redefinable) {
-    return TokError("invalid variable redefinition");
+  } else if (Var.Redefinable == Variable::NOT_REDEFINABLE) {
+    return Error(SMLoc(), "invalid variable redefinition");
+  } else if (Var.Redefinable == Variable::WARN_ON_REDEFINITION &&
+             Warning(SMLoc(), "redefining '" + Name +
+                                  "', already defined on the command line")) {
+    return true;
   }
-  Var.Redefinable = true;
+  Var.Redefinable = Variable::WARN_ON_REDEFINITION;
   Var.IsText = true;
   Var.TextValue = Value.str();
   return false;
diff --git a/llvm/test/tools/llvm-ml/command_line_defines.asm b/llvm/test/tools/llvm-ml/command_line_defines.asm
--- a/llvm/test/tools/llvm-ml/command_line_defines.asm
+++ b/llvm/test/tools/llvm-ml/command_line_defines.asm
@@ -1,4 +1,4 @@
-; RUN: llvm-ml -filetype=s %s /Fo - /DT1=test1 /D T2=test2 | FileCheck %s
+; RUN: llvm-ml -filetype=s %s /Fo - /DT1=test1 /D T2=test2 /Dtest5=def | FileCheck %s
 
 .code
 
@@ -35,4 +35,7 @@
 ; CHECK: xor ebx, ebx
 ; CHECK: ret
 
+t5:
+test5 textequ <redef>
+
 end