[llvm-dev] Invoking lld for PE/COFF (Windows) linking

[Reid Kleckner via llvm-dev]

On Thu, Mar 30, 2017 at 6:08 AM, Edward Diener via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
>
> Are you saying that once lld is built with mingw-64/gcc on Windows it is
> impossible to tell it to handle PE/COFF files when invoking it from clang++
> using the -fuse-ld=lld parameter ? That would indeed be limiting,
> considering that on Windows I can build clang either with mingw-64/gcc or
> with Visual C++, and the -target parameter allows clang to generate the
> appropriate object files.


The logic to construct an lld-link command line is part of the MSVC
toolchain in the clang driver, so you can get clang++ to make a coff file
if you tell it you're targetting MSVC like this:
$ clang++ t.cpp  -fuse-ld=lld -o t.exe --target=x86_64-windows-msvc

Of course, that will look for libraries and headers in the wrong places if
you really want to target mingw. If I split it up like this, I get link
errors:
$ clang++ -c t.cpp -o t.o --target=x86_64-windows-gnu
$ clang++ t.o -o t.exe --target=x86_64-windows-msvc -fuse-ld=lld
C:\src\llvm-project\build_mingw\bin\lld-link.exe: warning: t.o: undefined
symbol: _ZNSt8ios_base4InitC1Ev
C:\src\llvm-project\build_mingw\bin\lld-link.exe: warning: t.o: undefined
symbol: _ZNSt8ios_base4InitD1Ev
C:\src\llvm-project\build_mingw\bin\lld-link.exe: warning: t.o: undefined
symbol: __main
C:\src\llvm-project\build_mingw\bin\lld-link.exe: warning: t.o: undefined
symbol: _ZSt4cout
C:\src\llvm-project\build_mingw\bin\lld-link.exe: warning: t.o: undefined
symbol: _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
error: link failed
clang++.exe: error: linker command failed with exit code 1 (use -v to see
invocation)

If you pile on enough link options, you can probably make it work, but at
that point you're basically reconstructing a link.exe / lld-link.exe
command line. To fix this, someone should add mingw toolchain logic to
clang's driver to support constructing lld-link command lines.
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