[Driver] Silence stdlib warning when linking C on FreeBSD

[dankm]

Similar to the Gnu toolchain, ignore uses of '-stdlib=libc++' when linking C code. CMake insists on adding it to the command line when linking C, and a bunch of other build systems do similarly.

[llvmbot]

@llvm/pr-subscribers-clang-driver

@llvm/pr-subscribers-clang

Changes

Similar to the Gnu toolchain, ignore uses of '-stdlib=libc++' when linking C code. CMake insists on adding it to the command line when linking C, and a bunch of other build systems to similarly.

---

Full diff: https://github.com/llvm/llvm-project/pull/68011.diff

1 Files Affected:

• (modified) clang/lib/Driver/ToolChains/FreeBSD.cpp (+3)

diff --git a/clang/lib/Driver/ToolChains/FreeBSD.cpp b/clang/lib/Driver/ToolChains/FreeBSD.cpp
index ff4d94c56f0d0ff..4d998e884f8ad42 100644
--- a/clang/lib/Driver/ToolChains/FreeBSD.cpp
+++ b/clang/lib/Driver/ToolChains/FreeBSD.cpp
@@ -356,6 +356,9 @@ void freebsd::Linker::ConstructJob(Compilation &C, const JobAction &JA,
 
   ToolChain.addProfileRTLibs(Args, CmdArgs);
 
+  // Silence warnings when linking C code with a C++ '-stdlib' argument.
+  Args.ClaimAllArgs(options::OPT_stdlib_EQ);
+
   const char *Exec = Args.MakeArgString(getToolChain().GetLinkerPath());
   C.addCommand(std::make_unique<Command>(JA, *this,
                                          ResponseFileSupport::AtFileCurCP(),

[dankm]

This fixes this inconsistency:

Linux:

$ clang -o test -stdlib=libc++ test.c
$ ./test
Hello world

FreeBSD:

$ clang -o test -stdlib=libc++ test.c
clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
$ ./test
Hello world

[egorzhdan]

The Darwin toolchain also does this. This change LGTM, although I'm not an expert in FreeBSD.

[dankm]

The Darwin toolchain also does this. This change LGTM, although I'm not an expert in FreeBSD.

Thanks. I think I'd like some FreeBSD folks to take a look. I wonder if it's rude to @ mention a few... maybe I will.

The ones I can think of immediately are @DimitryAndric, @emaste, and @jrtc27.

[emaste]

Yeah, I don't see any reason there'd be a problem with this. It seems a bit silly for build systems to pass this in, but if they do there's no value in reporting it.

[DimitryAndric]

Does this only affect cases where one passes -stdlib=libc++? What happens if somebody passes -stdlib=libstdc++?

[dankm]

Does this only affect cases where one passes -stdlib=libc++? What happens if somebody passes -stdlib=libstdc++?

It handles both cases. It simply claims all the -stdlib= arguments. This is the same behaviour as the Gnu driver used by Linux, but seems to diverge from Darwin.

[dankm]

You do bring up an interesting point, though. When linking C code with this change, for both Gnu and FreeBSD, -stdlib=libbad is ignored for C and raises an error for C++.

Also for reference, the toolchains that ignore the stdlib argument for C are: baremetal, Darwin, Fuchsia, Gnu, Hexagon, MipsLinux, NaCL, OHOS, and WebAssembly. I was mistaken above about Darwin.

[emaste]

It handles both cases. It simply claims all the -stdlib= arguments.

If I understand correctly this means in effect -stdlib= becomes a don't-care when linking C, and anyhow it already had no effect.

[dankm]

If I understand correctly this means in effect -stdlib= becomes a don't-care when linking C, and anyhow it already had no effect.

That's correct. It was previously a harmless warning, and is now (with this change) completely ignored.

Edit: shouldn't say harmless, -Werror turns this into an error:

$ clang15 -Werror --stdlib=libc++ -o test test.c
clang-15: error: argument unused during compilation: '--stdlib=libc++' [-Werror,-Wunused-command-line-argument]

[dankm]

If this looks good to people, is anyone able to merge it? I cannot do it myself.