commit 2497d5aa7716a664c4f73df1980b026c906c7522
Author: Manoj Gupta <manojgupta@google.com>
Date:   Tue Oct 25 12:29:03 2022 -0700

    Define _GNU_SOURCE for arm baremetal in C++ mode.
    
    This matches other C++ drivers e.g. Linux that define
    _GNU_SOURCE. This lets clang compiler more code by default
    without explicitly passing _GNU_SOURCE on command line.
    
    Reviewed By: MaskRay
    
    Differential Revision: https://reviews.llvm.org/D136712

diff --git a/clang/lib/Basic/Targets/ARM.cpp b/clang/lib/Basic/Targets/ARM.cpp
index f2db186aac4c..c38849058e13 100644
--- a/clang/lib/Basic/Targets/ARM.cpp
+++ b/clang/lib/Basic/Targets/ARM.cpp
@@ -705,8 +705,11 @@ void ARMTargetInfo::getTargetDefines(const LangOptions &Opts,
   // 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
index e317ffa67393..a55d0d63a79b 100644
--- 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