[clang] [Clang][Hexagon] Define _GNU_SOURCE when using picolibc in C++ mode (PR #201599)

[via cfe-commits]

https://github.com/quic-k updated https://github.com/llvm/llvm-project/pull/201599

>From c2a820dfacbb67e6ac23e2b92297c4a182fe4777 Mon Sep 17 00:00:00 2001
From: Kushal Pal <kushpal at qti.qualcomm.com>
Date: Thu, 4 Jun 2026 20:00:00 +0530
Subject: [PATCH] [Clang][Hexagon] Predefine _GNU_SOURCE for C++ compilations

C++ programs using both picolibc and libcxx headers on Hexagon required
manually passing -D_GNU_SOURCE on every compilation. Picolibc guards
GNU-extension locale functions (e.g. strtold_l) behind __GNU_VISIBLE,
which is only enabled when _GNU_SOURCE is defined.

Predefine _GNU_SOURCE in C++ mode for H2 and QURT Hexagon targets,
following the convention of other targets.

Signed-off-by: Kushal Pal <kushpal at qti.qualcomm.com>
---
 clang/lib/Basic/Targets/Hexagon.cpp          |  3 +++
 clang/lib/Basic/Targets/OSTargets.h          |  4 ++++
 clang/test/Preprocessor/hexagon-predefines.c | 16 ++++++++++++++++
 3 files changed, 23 insertions(+)

diff --git a/clang/lib/Basic/Targets/Hexagon.cpp b/clang/lib/Basic/Targets/Hexagon.cpp
index 9bf34e67a03fd..615114f0fd1ea 100644
--- a/clang/lib/Basic/Targets/Hexagon.cpp
+++ b/clang/lib/Basic/Targets/Hexagon.cpp
@@ -116,6 +116,9 @@ void HexagonTargetInfo::getTargetDefines(const LangOptions &Opts,
   Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_2");
   Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4");
   Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_8");
+
+  if (Opts.CPlusPlus && getTriple().getOS() == llvm::Triple::UnknownOS)
+    Builder.defineMacro("_GNU_SOURCE");
 }
 
 bool HexagonTargetInfo::initFeatureMap(
diff --git a/clang/lib/Basic/Targets/OSTargets.h b/clang/lib/Basic/Targets/OSTargets.h
index 943373c20af32..9461680df8bdb 100644
--- a/clang/lib/Basic/Targets/OSTargets.h
+++ b/clang/lib/Basic/Targets/OSTargets.h
@@ -1092,6 +1092,8 @@ class LLVM_LIBRARY_VISIBILITY QURTTargetInfo : public OSTargetInfo<Target> {
   void getOSDefines(const LangOptions &Opts, const llvm::Triple &Triple,
                     MacroBuilder &Builder) const override {
     Builder.defineMacro("__qurt__");
+    if (Opts.CPlusPlus)
+      Builder.defineMacro("_GNU_SOURCE");
   }
 
 public:
@@ -1105,6 +1107,8 @@ class LLVM_LIBRARY_VISIBILITY H2TargetInfo : public OSTargetInfo<Target> {
   void getOSDefines(const LangOptions &Opts, const llvm::Triple &Triple,
                     MacroBuilder &Builder) const override {
     Builder.defineMacro("__h2__");
+    if (Opts.CPlusPlus)
+      Builder.defineMacro("_GNU_SOURCE");
   }
 
 public:
diff --git a/clang/test/Preprocessor/hexagon-predefines.c b/clang/test/Preprocessor/hexagon-predefines.c
index cb3e9492ea07e..f115e6e0a9926 100644
--- a/clang/test/Preprocessor/hexagon-predefines.c
+++ b/clang/test/Preprocessor/hexagon-predefines.c
@@ -261,3 +261,19 @@
 // CHECK-H2: #define __h2__ 1
 // CHECK-H2: #define __hexagon__ 1
 // CHECK-H2-NOT: #define __linux__
+
+// RUN: %clang_cc1 -E -dM -triple hexagon-unknown-elf -x c++ %s | FileCheck \
+// RUN: %s -check-prefix CHECK-CXX-GNU
+// CHECK-CXX-GNU: #define _GNU_SOURCE 1
+
+// RUN: %clang_cc1 -E -dM -triple hexagon-unknown-h2 -x c++ %s | FileCheck \
+// RUN: %s -check-prefix CHECK-H2-CXX-GNU
+// CHECK-H2-CXX-GNU: #define _GNU_SOURCE 1
+
+// RUN: %clang_cc1 -E -dM -triple hexagon-unknown-qurt -x c++ %s | FileCheck \
+// RUN: %s -check-prefix CHECK-QURT-CXX-GNU
+// CHECK-QURT-CXX-GNU: #define _GNU_SOURCE 1
+
+// RUN: %clang_cc1 -E -dM -triple hexagon-unknown-elf %s | FileCheck \
+// RUN: %s -check-prefix CHECK-C-GNU
+// CHECK-C-GNU-NOT: #define _GNU_SOURCE