diff --git a/clang/lib/Basic/Targets/ARM.cpp b/clang/lib/Basic/Targets/ARM.cpp
--- a/clang/lib/Basic/Targets/ARM.cpp
+++ b/clang/lib/Basic/Targets/ARM.cpp
@@ -705,8 +705,11 @@
   // For bare-metal none-eabi.
   if (getTriple().getOS() == llvm::Triple::UnknownOS &&
       (getTriple().getEnvironment() == llvm::Triple::EABI ||
-       getTriple().getEnvironment() == llvm::Triple::EABIHF))
+       getTriple().getEnvironment() == llvm::Triple::EABIHF)) {
     Builder.defineMacro("__ELF__");
+    if (Opts.CPlusPlus)
+      Builder.defineMacro("_GNU_SOURCE");
+  }
 
   // Target properties.
   Builder.defineMacro("__REGISTER_PREFIX__", "");
diff --git a/clang/test/Driver/arm-baremetal-defines.cpp b/clang/test/Driver/arm-baremetal-defines.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Driver/arm-baremetal-defines.cpp
@@ -0,0 +1,11 @@
+// RUN: %clangxx --target=arm-none-eabi -march=armv7-m %s -emit-llvm -S -c -o - 2>&1 | FileCheck %s
+
+// CHECK: __ELF__defined
+#ifdef __ELF__
+void __ELF__defined() {}
+#endif
+
+// CHECK: _GNU_SOURCEdefined
+#ifdef _GNU_SOURCE
+void _GNU_SOURCEdefined() {}
+#endif