unsupported option '-mwindows' for target 'aarch64-w64-mingw32'

[brechtsanders]

When doing a native build of NSPR 4.35 on Windows on Arm using LLVM 17.0.0 RC1 I got the error unsupported option '-mwindows' for target 'aarch64-w64-mingw32'.

However when building for MinGW-w64 the -mwindows is normally supported (at least I know it is for GCC).

Full output:

make -C config export
make[1]: Entering directory '/r/winlibs_llvm17rc1/11064.nspr/nspr-4.35/nspr/config'
/d/Prog/winlibs_llvm17rc1/llvm-mingw-20230730-ucrt-aarch64/bin/clang.exe -mwindows -o now.o -c     -Wno-int-conversion -pthread -g   -UNDEBUG -DDEBUG_brecht -DPACKAGE_NAME=\"\" -DPACKAGE_TARNAME=\"\" -DPACKAGE_VERSION=\"\" -DPACKAGE_STRING=\"\" -DPACKAGE_BUGREPORT=\"\" -DPACKAGE_URL=\"\" -DDEBUG=1 -DXP_PC=1 -DWIN32=1 -DWINNT=1 -D_ARM64_=1 -DHAVE_STRERROR=1 -DFORCE_PR_LOG   now.c
clang-17: error: unsupported option '-mwindows' for target 'aarch64-w64-mingw32'
make[1]: *** [../config/rules.mk:450: now.o] Error 1
make[1]: Leaving directory '/r/winlibs_llvm17rc1/11064.nspr/nspr-4.35/nspr/config'
make: *** [config/rules.mk:163: export] Error 2

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

[alvinhochun]

I am guessing this is because -mwindows/-mconsole are only used when it needs to invoke the linker:

llvm-project/clang/lib/Driver/ToolChains/MinGW.cpp

Lines 141 to 150 in f108c6c

	Arg *SubsysArg =
	Args.getLastArg(options::OPT_mwindows, options::OPT_mconsole);
	if (SubsysArg && SubsysArg->getOption().matches(options::OPT_mwindows)) {
	CmdArgs.push_back("--subsystem");
	CmdArgs.push_back("windows");
	} else if (SubsysArg &&
	SubsysArg->getOption().matches(options::OPT_mconsole)) {
	CmdArgs.push_back("--subsystem");
	CmdArgs.push_back("console");
	}

, so it's only when clang needs to invoke the linker that it actually consume these arguments. Your clang invocation has -c, which means no linking. -mwindows/-mconsole are flagged as TargetSpecific:

llvm-project/clang/include/clang/Driver/Options.td

Lines 4259 to 4262 in f108c6c

	let Flags = [TargetSpecific] in {
	def mconstant_cfstrings : Flag<["-"], "mconstant-cfstrings">, Group<clang_ignored_m_Group>;
	def mconsole : Joined<["-"], "mconsole">, Group<m_Group>;
	def mwindows : Joined<["-"], "mwindows">, Group<m_Group>;

, which means if they are not consumed they will cause clang to report an error:

llvm-project/clang/lib/Driver/Driver.cpp

Lines 4963 to 4964 in f108c6c

	Diag(diag::err_drv_unsupported_opt_for_target)
	<< A->getSpelling() << getTargetTriple();

We could somehow ignore these arguments when no linking is needed, but does it even make sense to pass them when not linking the final binary? Also I don't think this issue is specific to aarch64 at all.

CC @mstorsjo

[brechtsanders]

I see your point.

Ignoring these arguments would improve GCC compatibility.

The alternative is to have all projects fixed that use it in the wrong place (like NSPR), which could be several projects since GCC never gave an error.

Or is there a flag that can be used to build these projects to only treat it as a warning?

[mstorsjo]

Indeed, this is nothing aarch64 specific (even if that error message makes it sound so).

The options are, as noted, not relevant at all while compiling. I haven't run into a case so far where projects are doing this, but apparently there are some cases. Anyway, having it as a hard error is quite uncomfortable, especially as GCC doesn't warn about it at all. It's possible to silence the error with -Qunused-arguments though.

We had a similar situation a couple months ago; -mthreads used to give the warning clang: warning: argument unused during compilation: '-mthreads' [-Wunused-command-line-argument], but the flag was changed into a target-specific flag like this one, which made it a hard error. See #63282, and the fix in a79995c#diff-d20fe017578b739909633091655da0c15848a81a4b26a5091b667876d08545f4. There, @MaskRay added a mechanism for lowering the level of fatality of this option from hard error to a simple warning about the flag being unused. So we could add similar handling of -mwindows and -mconsole as we did for -mthreads.

A patch that does the same for -mwindows and -mconsole, as for -mthreads, should certainly be welcome.

[brechtsanders]

Sorry, I haven't been able to come up with a patch myself (yet).

But while building gobject-introspection-1.78.1 on Windows on ARM (aarch64) I get an error about invalid -mdll linker flag:

clang-17: error: unsupported option '-mdll' for target 'aarch64-w64-mingw32'

This seems to be a similar issue.

So if a patch is made it should address the issue for:

• -mwindows
• -mconsole
• -mdll

[Anutrix]

I get this when building Godot for Windows on x64:

clang-17: error: unsupported option '-mwindows' for target 'x86_64-w64-mingw32'

So this is not just for aarch64. I used the version from https://github.com/mstorsjo/llvm-mingw though.