flang-new: a flag to opt-out from default runtime libraries linking on Windows

[georgthegreat]

At the time flang-new (we use flang version 20.1.8 for now) links the following libraries when targeting Windows (x86_64-pc-win32):

lld-link: error: could not open 'clang_rt.builtins.lib': No such file or directory
lld-link: error: could not open 'FortranRuntime.static.lib': No such file or directory
lld-link: error: could not open 'FortranDecimal.static.lib': No such file or directory

We run a distributed build system and thus need to have full control over every input, hence we pass flang-rt explicitly (and use our own naming schema).

I do not see an option to turn this behavior off.

Could you, please, add such flag?

At the time we pass three empty libraries to make lld-link work (and it looks like it is finally working).

[llvmbot]

@llvm/issue-subscribers-flang-driver

Author: Yuriy Chernyshov (georgthegreat)

At the time flang-new (we use `flang version 20.1.8` for now) links the following libraries when targeting windows:

lld-link: error: could not open 'clang_rt.builtins.lib': No such file or directory
lld-link: error: could not open 'FortranRuntime.static.lib': No such file or directory
lld-link: error: could not open 'FortranDecimal.static.lib': No such file or directory

We run a distributed build system and thus need to have full control over every input, hence we pass flang-runtime explicitly (and use our own naming schema).

I do not see an option to turn this behavior off.

Could you, please, add such flag?

At the time we pass three empty libraries to make lld-link work (and it looks like it is finally working).

[georgthegreat]

@eugeneepshteyn @kiranchandramohan, you have helped us with flang integration in the past.
Could you, please, take a look?

[eugeneepshteyn]

How do you configure Windows build? x86_64-pc-win32 seems to be a bit non-specific, unlike for example x86_64-pc-windows-msvc

[georgthegreat]

Will changing target triple change link command? That is quite unexpected, but I will try this trick.

[georgthegreat]

@eugeneepshteyn, changing target triple gave me no effect.
We are using custom build system (yatool) and out command to compile Fortran sources looks like:

"cmd_args": [
    "$(FLANG)/bin/flang-new",
    "--target=x86_64-pc-windows-msvc",
    "-c",
    "$(SOURCE_ROOT)/contrib/python/scipy/py3/scipy/integrate/_dop-f2pywrappers.f",
    "-o",
    "$(BUILD_ROOT)/contrib/python/scipy/py3/_/scipy/integrate/_dop-f2pywrappers.f.o"
]

It is generated from the following template:
https://github.com/yandex/yatool/blob/9760f034f7ffcd5f0cac313ab1c24ffbaa414911/build/ymake.core.conf#L3341

whereas C_FLAGS_PLATFORM is generated with:
https://github.com/yandex/yatool/blob/9760f034f7ffcd5f0cac313ab1c24ffbaa414911/build/ymake_conf.py#L505

We use lld-link directly for linking (and we use clang-cl to compile C and CPP sources in the source mixture).

[kiranchandramohan]

You can use -### to understand what the flang driver is doing to compile or link and create the executable.

Is it possible to compile the files individually with flang and then use lld to create the executable linking all the object files and required flang libraries?

BTW, you can use the name flang instead of flang-new.