Linker flag detection for lld fails when using LDFLAGS="-fuse-ld=lld"

[mhthies]

Describe the bug
When using the lld linker via Clang by setting the environment variables LD="clang" and LDFLAGS="-fuse-ld=lld", Meson fails to detect the supported lld linker flags and applies invalid flags to the linker invocation, e.g. -Wl,--allow-shlib-undefined, which is unsupported for Windows targets.

To Reproduce
Setup any Meson project on Windows for building with MinGW x86_64 CLang compiler, using the following environment variables:

CC=C:/msys64/mingw64/bin/clang.exe
LD=C:/msys64/mingw64/bin/clang.exe
CHOST=x86_64-w64-mingw64
LDFLAGS=-fuse-ld=lld
CFLAGS=-m64

A good example is the pkgconf project.

Detailed instructions for reproducing:
On a Windows machine,

• get MSYS2 install from https://www.msys2.org/ and install to C:\msys64
• in MSYS2 shell, install CLang for x86_64: pacman -S mingw-w64-x86_64-clang
• install Ninja
• clone pkgconf code from https://github.com/pkgconf/pkgconf
• apply the environment variables listed above
• run meson setup build in pkgconf repository
• run meson compile -C build in pkgconf repository

Actual behavior
meson setup succeeds, but compilation fails in linking stage with the following error:

lld: error: unknown argument: --allow-shlib-undefined
clang: error: linker command failed with exit code 1 (use -v to see invocation)

This is caused by Meson, automatically applying the linker argument -Wl,--allow-shlib-undefined, which is not supported by clang/lld on Windows.

Expected behavior
Meson should detect that the lld linker does not support the --allow-shlib-undefined flag. The build should succeed.

system parameters

• plain native build, but MinGW clang on Windows
• Windows 11
• Python 3.12
• meson 1.8.99 (latest master: 36de1c6)
• ninja 1.12.1

[bonzini]

Put both -fuse-ld=lld and -m64 in the CC environment variable (or if using a machine file, put -fuse-ld=lld in c_ld and -m64 in both c and c_ld). This is documented (but could be better) at https://mesonbuild.com/Machine-files.html#binaries.

[mhthies]

The issue is caused by the LLVMDynamicLinker.supports_flag() not using the flags from the LDFLAGS environment variable: The command invoked for checking the support for the --allow-shlib-undefined flag is: ['C:/msys64/mingw64/bin/clang.exe', '-Wl,--allow-shlib-undefined']. It returns the following output on stderr:

C:/msys64/mingw64/bin\ld: C:/msys64/mingw64\x86_64-w64-mingw32\lib/libmingw32.a(lib64_libmingw32_a-crt0_c.o): in function `main':
D:/mingwbuild/mingw-w64-crt-git/src/mingw-w64/mingw-w64-crt/crt/crt0_c.c:18: undefined reference to `WinMain'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Obviously, the wrong linker has been used and the output does not include the "unknown argument: '--allow-shlib-undefined'" string, Meson is looking for, so the incompatibility is not detected.

Instead, the following command line should be used for the check: ['C:/msys64/mingw64/bin/clang.exe', '-fuse-ld=lld', '-Wl,--allow-shlib-undefined'], which then would report

lld: error: unknown argument: --allow-shlib-undefined
clang: error: linker command failed with exit code 1 (use -v to see invocation)

which would be processed correctly by Meson.

[mhthies]

@bonzini: I've found #10654, which explicitly adds the LDFLAGS (via env.coredata.get_external_link_args()) to the command line arguments for detecting the linker. Thus, I assume that it is an oversight that these flags are not used when checking the supported linker flags.

[eli-schwartz]

Thus, I assume that it is an oversight that these flags are not used when checking the supported linker flags.

That is my feeling as well. Thanks for digging into this.