Index: include/clang/Basic/DiagnosticDriverKinds.td
===================================================================
--- include/clang/Basic/DiagnosticDriverKinds.td
+++ include/clang/Basic/DiagnosticDriverKinds.td
@@ -277,4 +277,6 @@
   InGroup<InvalidOrNonExistentDirectory>;
 
 def err_drv_unsupported_linker : Error<"unsupported value '%0' for -linker option">;
+def err_drv_defsym_invalid_format : Error<"defsym must be of the form: sym=value: %0">;
+def err_drv_defsym_invalid_symval : Error<"Value is not an integer: %0">;
 }
Index: test/Driver/defsym.s
===================================================================
--- test/Driver/defsym.s
+++ test/Driver/defsym.s
@@ -17,6 +17,11 @@
 // 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,abc=1a2b3c \
+// RUN: -Wa,-defsym,=123 \
 // RUN: 2>&1 | FileCheck %s --check-prefix=CHECK-DEFSYM-ERR2
-// CHECK-DEFSYM-ERR2: error: Value is not an integer: 1a2b3c
+// CHECK-DEFSYM-ERR2: 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-ERR3
+// CHECK-DEFSYM-ERR3: error: Value is not an integer: 1a2b3c
Index: tools/driver/cc1as_main.cpp
===================================================================
--- tools/driver/cc1as_main.cpp
+++ tools/driver/cc1as_main.cpp
@@ -426,10 +426,23 @@
 
   // Set values for symbols, if any.
   for (auto &S : Opts.SymbolDefs) {
-    if (Ctx.setSymbolValue(Parser->getStreamer(), S)) {
+    auto Pair = StringRef(S).split('=');
+    auto Sym = Pair.first;
+    auto Val = Pair.second;
+
+    if (Sym.empty() || Val.empty()) {
+      Diags.Report(diag::err_drv_defsym_invalid_format) << S;
       Failed = true;
       break;
     }
+    int64_t Value;
+    if (Val.getAsInteger(0, Value)) {
+      Diags.Report(diag::err_drv_defsym_invalid_symval) << Val;
+      Failed = true;
+      break;
+    }
+
+    Ctx.setSymbolValue(Parser->getStreamer(), Sym, Value);
   }
 
   if (!Failed) {