Define _GNU_SOURCE for arm baremetal in C++ mode.

[manojgupta]

Summary

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.

[tomhughes]

clang/lib/Basic/Targets/ARM.cpp

710–711

I think this should apply to both C and C++

[manojgupta]

Removed c++ limitation.

[manojgupta]

I am not sure who is a good reviewer for this. Starting with @MaskRay and @efriedma

[MaskRay]

Is this an upstream GCC behavior or a behavior from some gcc-none-eabi distributions?
GCC's C++ mode defines _GNU_SOURCE historically due to a libstdc++ limitation https://gcc.gnu.org/onlinedocs/libstdc++/faq.html#faq.predefined and nowadays "the ship has sailed" and many targets do the same for compatibility.
For C, most targets don't force defining _GNU_SOURCE.

[manojgupta]

@tomhughes has more details on this but if we do not define it in clang itself, we'll need to define it for every package manually at build time since the usage goes deep inside newlib headers.

[abidh]

In D136712#3892986, @manojgupta wrote:

@tomhughes has more details on this but if we do not define it in clang itself, we'll need to define it for every package manually at build time since the usage goes deep inside newlib headers.

I remember facing a similar issue while I was trying to build a baremetal llvm toolchain for riscv. I think I defined it in libcxx config for newlib.

[tomhughes]

More context: https://issuetracker.google.com/254916723

I take back my earlier comment about it applying for both C and C++. Checking locally I see the behavior @MaskRay describes:

_GNU_SOURCE not defined:

gcc -E -dD -o empty empty.c

_GNU_SOURCE defined:

g++ -E -dD -o empty empty.c

I'm just looking for compatibility with gcc/g++ behavior.

[MaskRay]

The _GNU_SOURCE definition needs to be guarded by if (Opts.CPlusPlus).

clang/test/Driver/arm-baremetal-defines.cpp

1 ↗

(On Diff #470611)

Move this to test/Preprocessor/init-arm.c

[manojgupta]

restore back to C++ only

[manojgupta]

Restore back to C++ only.

[MaskRay]

clang/test/Driver/arm-baremetal-defines.cpp

1 ↗

(On Diff #470611)

test/Preprocessor/init-* tests use %clang_cc1 -triple xxx -E -dM %s

[MaskRay]

LG with a test nit.

[manojgupta]

Updated test.

clang/lib/Basic/Targets/ARM.cpp

 void ARMTargetInfo::getTargetDefines(const LangOptions &Opts,
                                      MacroBuilder &Builder) const {
   // Target identification.
   Builder.defineMacro("__arm");
   Builder.defineMacro("__arm__");
   // 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__", "");
 
   // Unfortunately, __ARM_ARCH_7K__ is now more of an ABI descriptor. The CPU
   // happens to be Cortex-A7 though, so it should still get __ARM_ARCH_7A__.
   if (getTriple().isWatchABI())
     Builder.defineMacro("__ARM_ARCH_7K__", "2");

clang/test/Preprocessor/init-arm.c

 // RUN: %clang_cc1 -E -dM -ffreestanding -triple=thumbebv7 < /dev/null | FileCheck -match-full-lines -check-prefix Thumbebv7 %s
 // Thumbebv7: #define __THUMB_INTERWORK__ 1
 // Thumbebv7: #define __thumb2__ 1
 
 // RUN: %clang_cc1 -E -dM -ffreestanding -triple=thumbv7-pc-windows-gnu -exception-model=dwarf %s -o - | FileCheck -match-full-lines -check-prefix THUMB-MINGW %s
 
 // 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