Index: cfe/trunk/include/clang/Driver/CC1Options.td
===================================================================
--- cfe/trunk/include/clang/Driver/CC1Options.td
+++ cfe/trunk/include/clang/Driver/CC1Options.td
@@ -743,4 +743,6 @@
 def dwarf_debug_producer : Separate<["-"], "dwarf-debug-producer">,
   HelpText<"The string to embed in the Dwarf debug AT_producer record.">;
 
+def defsym : Separate<["-"], "defsym">,
+  HelpText<"Define a value for a symbol">;
 } // let Flags = [CC1AsOption]
Index: cfe/trunk/lib/Driver/Tools.cpp
===================================================================
--- cfe/trunk/lib/Driver/Tools.cpp
+++ cfe/trunk/lib/Driver/Tools.cpp
@@ -3115,6 +3115,9 @@
       } else if (Value.startswith("-mcpu") || Value.startswith("-mfpu") ||
                  Value.startswith("-mhwdiv") || Value.startswith("-march")) {
         // Do nothing, we'll validate it later.
+      } else if (Value == "-defsym") {
+          CmdArgs.push_back(Value.data());
+          TakeNextArg = true;
       } else {
         D.Diag(diag::err_drv_unsupported_option_argument)
             << A->getOption().getName() << Value;
Index: cfe/trunk/test/Driver/defsym.s
===================================================================
--- cfe/trunk/test/Driver/defsym.s
+++ cfe/trunk/test/Driver/defsym.s
@@ -0,0 +1,22 @@
+// RUN: %clang -### -c -integrated-as %s \
+// RUN: -Wa,-defsym,abc=5 -Wa,-defsym,xyz=0xa \
+// RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM1
+
+// RUN: %clang -### -c -no-integrated-as %s \
+// RUN: -Wa,-defsym,abc=5 -Wa,-defsym,xyz=0xa \
+// RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM1
+
+// CHECK-DEFSYM1: "-defsym"
+// CHECK-DEFSYM1: "abc=5"
+// CHECK-DEFSYM1: "-defsym"
+// CHECK-DEFSYM1: "xyz=0xa"
+
+// RUN: not %clang -c -integrated-as %s \
+// RUN: -Wa,-defsym,abc= \
+// RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-ERR1
+// CHECK-DEFSYM-ERR1: error: defsym must be of the form: sym=value: abc=
+
+// RUN: not %clang -c -integrated-as %s \
+// RUN: -Wa,-defsym,abc=1a2b3c \
+// RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-ERR2
+// CHECK-DEFSYM-ERR2: error: Value is not an integer: 1a2b3c
Index: cfe/trunk/tools/driver/cc1as_main.cpp
===================================================================
--- cfe/trunk/tools/driver/cc1as_main.cpp
+++ cfe/trunk/tools/driver/cc1as_main.cpp
@@ -77,6 +77,9 @@
   /// be a list of strings starting with '+' or '-'.
   std::vector<std::string> Features;
 
+  /// The list of symbol definitions.
+  std::vector<std::string> SymbolDefs;
+
   /// @}
   /// @name Language Options
   /// @{
@@ -252,6 +255,7 @@
   Opts.RelocationModel = Args.getLastArgValue(OPT_mrelocation_model, "pic");
   Opts.IncrementalLinkerCompatible =
       Args.hasArg(OPT_mincremental_linker_compatible);
+  Opts.SymbolDefs = Args.getAllArgValues(OPT_defsym);
 
   return Success;
 }
@@ -420,6 +424,14 @@
   if (!TAP)
     Failed = Diags.Report(diag::err_target_unknown_triple) << Opts.Triple;
 
+  // Set values for symbols, if any.
+  for (auto &S : Opts.SymbolDefs) {
+    if (Ctx.setSymbolValue(Parser->getStreamer(), S)) {
+      Failed = true;
+      break;
+    }
+  }
+
   if (!Failed) {
     Parser->setTargetParser(*TAP.get());
     Failed = Parser->Run(Opts.NoInitialTextSection);