[clang] [clang] Set _GNU_SOURCE for AArch64 bare-metal targets (PR #225071)

[Hussam Alhassan via cfe-commits]

https://github.com/hussam-alhassan created https://github.com/llvm/llvm-project/pull/225071

Set the _GNU_SOURCE macro when compiling for aarch64-none-elf in C++ mode to enable libc++ headers to use GNU interfaces. This matches ARM.cpp when compiling or arm-none-eabi.

>From 4ab286bb15ad315cbd7f505c7aa01263323075f6 Mon Sep 17 00:00:00 2001
From: Hussam Alhassan <hussam.alhassan at arm.com>
Date: Fri, 18 Sep 2026 12:42:25 +0100
Subject: [PATCH] [clang] Set _GNU_SOURCE for AArch64 bare-metal targets

Set the _GNU_SOURCE macro when compiling for aarch64-none-elf in C++ mode to
enable libc++ headers to use GNU interfaces
---
 clang/lib/Basic/Targets/AArch64.cpp    | 11 +++++++++--
 clang/test/Preprocessor/init-aarch64.c |  3 +++
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Basic/Targets/AArch64.cpp b/clang/lib/Basic/Targets/AArch64.cpp
index a4841514be35e..9aac476a078f5 100644
--- a/clang/lib/Basic/Targets/AArch64.cpp
+++ b/clang/lib/Basic/Targets/AArch64.cpp
@@ -402,8 +402,9 @@ void AArch64TargetInfo::getTargetDefinesARMV97A(const LangOptions &Opts,
 
 void AArch64TargetInfo::getTargetDefines(const LangOptions &Opts,
                                          MacroBuilder &Builder) const {
+  const llvm::Triple &T = getTriple();
   // Target identification.
-  if (getTriple().isWindowsArm64EC()) {
+  if (T.isWindowsArm64EC()) {
     // Define the same set of macros as would be defined on x86_64 to ensure that
     // ARM64EC datatype layouts match those of x86_64 compiled code
     Builder.defineMacro("__amd64__");
@@ -415,7 +416,13 @@ void AArch64TargetInfo::getTargetDefines(const LangOptions &Opts,
     Builder.defineMacro("__aarch64__");
   }
 
-  if (getTriple().isLFI())
+  // For bare-metal none-elf.
+  if (T.getOS() == llvm::Triple::UnknownOS && T.getEnvironmentName() == "elf" &&
+      Opts.CPlusPlus) {
+    Builder.defineMacro("_GNU_SOURCE");
+  }
+
+  if (T.isLFI())
     Builder.defineMacro("__LFI__");
 
   // Inline assembly supports AArch64 flag outputs.
diff --git a/clang/test/Preprocessor/init-aarch64.c b/clang/test/Preprocessor/init-aarch64.c
index 44b92ea2331b0..51aabb40242c2 100644
--- a/clang/test/Preprocessor/init-aarch64.c
+++ b/clang/test/Preprocessor/init-aarch64.c
@@ -1170,3 +1170,6 @@
 // CMODEL_TINY: #define __AARCH64_CMODEL_TINY__ 1
 // CMODEL_SMALL: #define __AARCH64_CMODEL_SMALL__ 1
 // CMODEL_LARGE: #define __AARCH64_CMODEL_LARGE__ 1
+
+// RUN: %clang_cc1 -x c++ -E -dM -ffreestanding -triple=aarch64-none-elf < /dev/null | FileCheck -match-full-lines -check-prefix AARCH64-NONE-ELF-CXX %s
+// AARCH64-NONE-ELF-CXX: #define _GNU_SOURCE 1