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

[Edward Diener via llvm-dev]

On 3/29/2017 1:38 PM, Reid Kleckner via llvm-dev wrote:
> If clang is targeting VC++, then -fuse-ld=lld should be enough to make
> it run lld-link.exe, and you won't need to set the flavor or do anything
> special to get PE/COFF files.

I build LLVM/cClang/lld with mingw-64/gcc-6.3, not with VC++. If I 
compile with clang using the target triple of "-target 
i686-pc-windows-msvc" and "-fmsc-version=1900" for VC++14, the 
compilation is successful and clang creates a VC++ compatible object 
file. If I subsequently link using clang++ with link option 
"-fuse-ld=lld" I get as output:

"lld: error: Windows targets are not supported on the ELF frontend: i386pe"

If I add the "-target i686-pc-windows-msvc" to the clang++ link command, 
the clang++ command just hangs and never completes.

>
> This example worked for me:
>
> $ cat t.cpp
> #include <iostream>
> int main() { std::cout << "hello world\n"; }
>
> $ clang++ t.cpp -fuse-ld=lld -o t.exe && ./t.exe
> hello world
>
> Unfortunately, we don't have a GNU-ld compatible command line interface
> for the COFF port of LLD, if that's what you're having trouble with.
> This is something I personally think we should add, but don't have time
> to pursue.
>
> On Wed, Mar 29, 2017 at 7:56 AM, Edward Diener via llvm-dev
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>
>     I build llvm/clang/lld from source on Windows using
>     mingw-64/gcc-6.3. I use clang++ both to test clang targeting gcc and
>     clang targeting VC++. When using clang targeting VC++ I use the
>     appropriate target triple when compiling and am trying to use lld to
>     link the object file(s) into an exe. To do that I use the clang
>     option "-fuse-ld=lld" when linking. According to the llvm doc on
>     using lld with PE/COFF on Windows at
>     https://lld.llvm.org/windows_support.html
>     <https://lld.llvm.org/windows_support.html>:
>
>     "LLD supports Windows operating system. When invoked as lld-link.exe
>     or with -flavor link, the driver for Windows operating system is
>     used to parse command line options, and it drives further linking
>     processes. LLD accepts almost all command line options that the
>     linker shipped with Microsoft Visual C++ (link.exe) supports."
>
>     Unfortunately with clang++ attempting to use "-fuse-ld=lld-link"
>     when linking outputs:
>
>     "clang++.exe: error: unsupported value 'lld_link' for -linker option"
>
>     So I must use "-fuse-ld=lld". Does this mean that I should be
>     passing "-flavor link" to the lld linker ? If so, how do I do that ?
>     If not, what do I do to have lld work with PE/COFF linking rather
>     than ELF linking ?
>
>     I have attempted to pass to the clang++ when linking:
>
>     1) "-flavor link", which outputs:
>     clang++.exe: error: unknown argument: '-flavor'
>     clang++.exe: error: no such file or directory: 'link'
>
>     2) -flavor=link", which outputs:
>     clang++.exe: error: unknown argument: '-flavor=link'
>
>     3) -Wl,-flavor,link, which outputs
>     C:\Programming\VersionControl\bninja_installed_clang\bin\lld: error:
>     unknown argument: -flavor
>
>     4) -Wl,-flavor=link, which outputs
>     C:\Programming\VersionControl\bninja_installed_clang\bin\lld: error:
>     unknown argument: -flavor=link
>
>     What is the "magic" invocation that will allow me to invoke the lld
>     linker from the clang++ command, but get the linker to work in
>     PE/COFF mode rather than in ELF mode, so it will link object files
>     created by clang targeting VC++ ?
>
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