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/Preprocessor/init-arm.c b/clang/test/Preprocessor/init-arm.c
--- a/clang/test/Preprocessor/init-arm.c
+++ b/clang/test/Preprocessor/init-arm.c
@@ -1450,3 +1450,8 @@
 
 // THUMB-MINGW:#define __ARM_DWARF_EH__ 1
 
+// RUN: %clang_cc1 -E -dM -ffreestanding -triple=thumbv6m-none-unknown-eabi < /dev/null | FileCheck -match-full-lines -check-prefix Thumbv6m-elf %s
+// Thumbv6m-elf: #define __ELF__ 1
+
+// RUN: %clang_cc1 -x c++ -E -dM -ffreestanding -triple=thumbv6m-none-unknown-eabi < /dev/null | FileCheck -match-full-lines -check-prefix Thumbv6m-cxx %s
+// Thumbv6m-cxx: #define _GNU_SOURCE 1