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;
@@ -867,7 +869,7 @@
 
   // "=", "equ", "textequ"
   bool parseDirectiveEquate(StringRef IDVal, StringRef Name,
-                            DirectiveKind DirKind);
+                            DirectiveKind DirKind, SMLoc NameLoc);
 
   bool parseDirectiveOrg(); // ".org"
   bool parseDirectiveAlign();  // "align"
@@ -2522,7 +2524,7 @@
   case DK_EQU:
   case DK_TEXTEQU:
     Lex();
-    return parseDirectiveEquate(nextVal, IDVal, DirKind);
+    return parseDirectiveEquate(nextVal, IDVal, DirKind, IDLoc);
   case DK_BYTE:
     if (afterNextTok.is(AsmToken::Identifier) &&
         afterNextTok.getString().equals_lower("ptr")) {
@@ -3343,7 +3345,7 @@
 ///    | name "equ" text-list
 ///    | name "textequ" text-list (redefinability unspecified)
 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;
@@ -3367,11 +3369,23 @@
       if (parseOptionalToken(AsmToken::Comma) && parseMany(parseItem))
         return addErrorSuffix(" in '" + Twine(IDVal) + "' directive");
 
-      if (!Var.Redefinable && (!Var.IsText || Var.TextValue != Value))
-        return Error(getTok().getLoc(), "invalid variable redefinition");
+      if (!Var.IsText || Var.TextValue != Value) {
+        switch (Var.Redefinable) {
+        case Variable::NOT_REDEFINABLE:
+          return Error(getTok().getLoc(), "invalid variable redefinition");
+        case Variable::WARN_ON_REDEFINITION:
+          if (Warning(NameLoc, "redefining '" + Name +
+                                   "', already defined on the command line")) {
+            return true;
+          }
+          break;
+        default:
+          break;
+        }
+      }
       Var.IsText = true;
       Var.TextValue = Value;
-      Var.Redefinable = true;
+      Var.Redefinable = Variable::REDEFINABLE;
 
       return false;
     }
@@ -3390,19 +3404,44 @@
     // Not an absolute expression; define as a text replacement.
     StringRef ExprAsString = StringRef(
         StartLoc.getPointer(), EndLoc.getPointer() - StartLoc.getPointer());
-    if (!Var.Redefinable && (!Var.IsText && Var.TextValue != ExprAsString))
-      return Error(getTok().getLoc(), "invalid variable redefinition");
+    if (!Var.IsText || Var.TextValue != ExprAsString) {
+      switch (Var.Redefinable) {
+      case Variable::NOT_REDEFINABLE:
+        return Error(getTok().getLoc(), "invalid variable redefinition");
+      case Variable::WARN_ON_REDEFINITION:
+        if (Warning(NameLoc, "redefining '" + Name +
+                                 "', already defined on the command line")) {
+          return true;
+        }
+        break;
+      default:
+        break;
+      }
+    }
     Var.IsText = true;
     Var.TextValue = ExprAsString.str();
   } else {
-    if (!Var.Redefinable && (Var.IsText || Var.NumericValue != Value))
-      return Error(getTok().getLoc(), "invalid variable redefinition");
+    if (Var.IsText || Var.NumericValue != Value) {
+      switch (Var.Redefinable) {
+      case Variable::NOT_REDEFINABLE:
+        return Error(getTok().getLoc(), "invalid variable redefinition");
+      case Variable::WARN_ON_REDEFINITION:
+        if (Warning(NameLoc, "redefining '" + Name +
+                                 "', already defined on the command line")) {
+          return true;
+        }
+        break;
+      default:
+        break;
+      }
+    }
     Var.NumericValue = Value;
   }
-  Var.Redefinable = (DirKind == DK_ASSIGN);
+  Var.Redefinable = (DirKind == DK_ASSIGN) ? Variable::REDEFINABLE
+                                           : Variable::NOT_REDEFINABLE;
 
   MCSymbol *Sym = getContext().getOrCreateSymbol(Var.Name);
-  Sym->setRedefinable(Var.Redefinable);
+  Sym->setRedefinable(Var.Redefinable != Variable::NOT_REDEFINABLE);
   Sym->setVariableValue(Expr);
   Sym->setExternal(false);
 
@@ -7008,10 +7047,14 @@
   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 +35,20 @@
 ; CHECK: xor ebx, ebx
 ; CHECK: ret
 
+% t5_original BYTE "&test5"
+; CHECK-LABEL: t5_original:
+; CHECK-NEXT: .byte 100
+; CHECK-NEXT: .byte 101
+; CHECK-NEXT: .byte 102
+
+test5 textequ <redef>
+
+% t5_changed BYTE "&test5"
+; CHECK-LABEL: t5_changed:
+; CHECK-NEXT: .byte 114
+; CHECK-NEXT: .byte 101
+; CHECK-NEXT: .byte 100
+; CHECK-NEXT: .byte 101
+; CHECK-NEXT: .byte 102
+
 end
diff --git a/llvm/test/tools/llvm-ml/command_line_defines_errors.asm b/llvm/test/tools/llvm-ml/command_line_defines_errors.asm
new file mode 100644
--- /dev/null
+++ b/llvm/test/tools/llvm-ml/command_line_defines_errors.asm
@@ -0,0 +1,8 @@
+; RUN: llvm-ml -filetype=s %s /Fo - /Dtest1=def 2>&1 | FileCheck %s --implicit-check-not=error: --implicit-check-not=warning:
+
+.code
+
+; CHECK: :[[# @LINE + 1]]:1: warning: redefining 'test1', already defined on the command line
+test1 textequ <redef>
+
+end