Index: include/clang/Driver/CC1Options.td
===================================================================
--- include/clang/Driver/CC1Options.td
+++ include/clang/Driver/CC1Options.td
@@ -769,6 +769,4 @@
 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: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -543,6 +543,8 @@
 def dM : Flag<["-"], "dM">, Group<d_Group>, Flags<[CC1Option]>,
   HelpText<"Print macro definitions in -E mode instead of normal output">;
 def dead__strip : Flag<["-"], "dead_strip">;
+def defsym : Separate<["-"], "defsym">, Group<d_Group>, Flags<[CC1Option, CC1AsOption]>,
+  HelpText<"Define an assembly-time variable with the given name and value">;
 def dependency_file : Separate<["-"], "dependency-file">, Flags<[CC1Option]>,
   HelpText<"Filename (or -) to write dependency output to">;
 def dependency_dot : Separate<["-"], "dependency-dot">, Flags<[CC1Option]>,
Index: lib/Driver/ToolChains/Clang.cpp
===================================================================
--- lib/Driver/ToolChains/Clang.cpp
+++ lib/Driver/ToolChains/Clang.cpp
@@ -113,6 +113,24 @@
   }
 }
 
+// Validate the value to a -defsym option
+static bool validateDefsym(const char *S, const Driver &D) {
+  auto Pair = StringRef(S).split('=');
+  auto Sym = Pair.first;
+  auto SVal = Pair.second;
+
+  if (Sym.empty() || SVal.empty()) {
+    D.Diag(diag::err_drv_defsym_invalid_format) << S;
+    return false;
+  }
+  int64_t IVal;
+  if (SVal.getAsInteger(0, IVal)) {
+    D.Diag(diag::err_drv_defsym_invalid_symval) << SVal;
+    return false;
+  }
+  return true;
+}
+
 /// Apply \a Work on the current tool chain \a RegularToolChain and any other
 /// offloading tool chain that is associated with the current action \a JA.
 static void
@@ -1728,6 +1746,21 @@
   // arg after parsing the '-I' arg.
   bool TakeNextArg = false;
 
+  // Handle the top-level implementation of the integrated assembler
+  // -defsym option
+  for (const Arg *A : Args.filtered(options::OPT_defsym)) {
+    A->claim();
+    if (A->getNumValues() != 1) {
+      D.Diag(diag::err_drv_defsym_invalid_format) << A->getAsString(Args);
+      break;
+    }
+    const char *S = A->getValue(0);
+    if (validateDefsym(S, D)) {
+      CmdArgs.push_back("-defsym");
+      CmdArgs.push_back(S);
+    }
+  }
+
   // When using an integrated assembler, translate -Wa, and -Xassembler
   // options.
   bool CompressDebugSections = false;
@@ -1843,21 +1876,10 @@
             break;
           }
           const char *S = A->getValue(1);
-          auto Pair = StringRef(S).split('=');
-          auto Sym = Pair.first;
-          auto SVal = Pair.second;
-
-          if (Sym.empty() || SVal.empty()) {
-            D.Diag(diag::err_drv_defsym_invalid_format) << S;
-            break;
+          if (validateDefsym(S, D)) {
+            CmdArgs.push_back(Value.data());
+            TakeNextArg = true;
           }
-          int64_t IVal;
-          if (SVal.getAsInteger(0, IVal)) {
-            D.Diag(diag::err_drv_defsym_invalid_symval) << SVal;
-            break;
-          }
-          CmdArgs.push_back(Value.data());
-          TakeNextArg = true;
       } else {
         D.Diag(diag::err_drv_unsupported_option_argument)
             << A->getOption().getName() << Value;
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -2500,6 +2500,12 @@
     Success = false;
   }
 
+  // Issue warning for integrated assembler -defsym option
+  for (const Arg *A : Args.filtered(options::OPT_defsym)) {
+    A->claim();
+    Diags.Report(diag::warn_drv_unused_argument) << A->getAsString(Args);
+  }
+
   // Issue errors on unknown arguments.
   for (const Arg *A : Args.filtered(OPT_UNKNOWN)) {
     Diags.Report(diag::err_drv_unknown_argument) << A->getAsString(Args);
Index: test/Driver/defsym.c
===================================================================
--- /dev/null
+++ test/Driver/defsym.c
@@ -0,0 +1,13 @@
+// RUN: not %clang -c -o /dev/null %s \
+// RUN: -defsym \
+// RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-ERR
+// CHECK-DEFSYM-ERR: error: argument to '-defsym' is missing (expected 1 value)
+
+// RUN: %clang -c -o /dev/null %s \
+// RUN: -defsym bar=1 \
+// RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-WARN
+// CHECK-DEFSYM-WARN: warning: argument unused during compilation: '-defsym bar=1'
+
+int foo(void) {
+	return 0;
+}
Index: test/Driver/defsym.s
===================================================================
--- test/Driver/defsym.s
+++ test/Driver/defsym.s
@@ -1,37 +1,73 @@
 // 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 -integrated-as %s \
+// RUN: -defsym abc=5 -defsym xyz=0xa \
+// RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM1
 
 // RUN: %clang -### -c -no-integrated-as -target x86_64-unknown-unknown %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 -target x86_64-unknown-unknown %s \
+// RUN: -defsym abc=5 -defsym xyz=0xa \
+// RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-UNUSED
 
 // CHECK-DEFSYM1: "-defsym"
 // CHECK-DEFSYM1: "abc=5"
 // CHECK-DEFSYM1: "-defsym"
 // CHECK-DEFSYM1: "xyz=0xa"
+// CHECK-UNUSED:  warning: argument unused during compilation: '-defsym abc=5' [-Wunused-command-line-argument]
+// CHECK-UNUSED:  warning: argument unused during compilation: '-defsym xyz=0xa' [-Wunused-command-line-argument]
 
 // RUN: not %clang -c -integrated-as -o /dev/null %s \
 // RUN: -Wa,-defsym,abc= \
 // RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-ERR1
+// RUN: not %clang -c -integrated-as -o /dev/null %s \
+// RUN: -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 -o /dev/null %s \
-// RUN: -Wa,-defsym,=123 \
+// RUN: -Wa,-defsym,abc \
 // RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-ERR2
-// CHECK-DEFSYM-ERR2: error: defsym must be of the form: sym=value: =123
+// RUN: not %clang -c -integrated-as -o /dev/null %s \
+// RUN: -defsym abc \
+// RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-ERR2
+// CHECK-DEFSYM-ERR2: error: defsym must be of the form: sym=value: abc
 
 // RUN: not %clang -c -integrated-as -o /dev/null %s \
-// RUN: -Wa,-defsym,abc=1a2b3c \
+// RUN: -Wa,-defsym,=123 \
+// RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-ERR3
+// RUN: not %clang -c -integrated-as -o /dev/null %s \
+// RUN: -defsym =123 \
 // RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-ERR3
-// CHECK-DEFSYM-ERR3: error: Value is not an integer: 1a2b3c
+// CHECK-DEFSYM-ERR3: error: defsym must be of the form: sym=value: =123
 
 // RUN: not %clang -c -integrated-as -o /dev/null %s \
-// RUN: -Wa,-defsym \
+// RUN: -Wa,-defsym,123 \
+// RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-ERR4
+// RUN: not %clang -c -integrated-as -o /dev/null %s \
+// RUN: -defsym 123 \
 // RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-ERR4
+// CHECK-DEFSYM-ERR4: error: defsym must be of the form: sym=value: 123
 
 // RUN: not %clang -c -integrated-as -o /dev/null %s \
+// RUN: -Wa,-defsym,abc=1a2b3c \
+// RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-ERR5
+// RUN: not %clang -c -integrated-as -o /dev/null %s \
+// RUN: -defsym abc=1a2b3c \
+// RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-ERR5
+// CHECK-DEFSYM-ERR5: error: Value is not an integer: 1a2b3c
+
+// RUN: not %clang -c -integrated-as -o /dev/null %s \
+// RUN: -Wa,-defsym \
+// RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-ERR6
+// RUN: not %clang -c -integrated-as -o /dev/null %s \
 // RUN: -Wa,-defsym, \
-// RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-ERR4
+// RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-ERR6
+// CHECK-DEFSYM-ERR6: error: defsym must be of the form: sym=value: -defsym
 
-// CHECK-DEFSYM-ERR4: error: defsym must be of the form: sym=value: -defsym
+// RUN: not %clang -c -integrated-as -o /dev/null %s \
+// RUN: -defsym \
+// RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-ERR7
+// CHECK-DEFSYM-ERR7: error: argument to '-defsym' is missing (expected 1 value)
\ No newline at end of file