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

[Edward Diener via llvm-dev]

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:

"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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[Reid Kleckner via llvm-dev]

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.

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.

[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.

> llvm...@lists.llvm.org <mailto:llvm...@lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>

[Edward Diener via llvm-dev]

On 3/29/2017 4:59 PM, Edward Diener via llvm-dev wrote:
> 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++.

Of course that should be:

"I build LLVM/clang/lld with mingw-64/gcc-6.3, not with VC++."

http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

[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.
>
> 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.

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.

[Reid Kleckner via llvm-dev]

On Thu, Mar 30, 2017 at 6:08 AM, Edward Diener via llvm-dev <llvm...@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.

[Edward Diener via llvm-dev]

On 3/30/2017 7:59 PM, Reid Kleckner via llvm-dev wrote:
> On Thu, Mar 30, 2017 at 6:08 AM, Edward Diener via llvm-dev
> <llvm...@lists.llvm.org <mailto:llvm...@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

Actually just doing:

clang++ -c t.cpp -o t.o --target=x86_64-windows-msvc -fmsc-version=1900

clang++ t.o -o t.exe --target=x86_64-windows-msvc -fuse-ld=lld

is not working for me. The first invocation works but the second just
hangs and never completes. Should it work ? If not, how can I invoke the
second to make it work properly ?

> 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.

[Reid Kleckner via llvm-dev]

On Thu, Mar 30, 2017 at 5:57 PM, Edward Diener via llvm-dev <llvm...@lists.llvm.org> wrote:

Actually just doing:

clang++ -c t.cpp -o t.o --target=x86_64-windows-msvc -fmsc-version=1900
clang++ t.o -o t.exe --target=x86_64-windows-msvc -fuse-ld=lld

is not working for me. The first invocation works but the second just hangs and never completes. Should it work ? If not, how can I invoke the second to make it work properly ?

Is it actually running lld-link.exe?  Does lld-link.exe exist next to clang++.exe? If you add -###, what linker command line is clang trying to run?

[Edward Diener via llvm-dev]

"C:/Programming/VersionControl/bninja_installed_clang/bin/clang++"
-o
"C:\Programming\VersionControl\modular-boost\build\boost\bin.v2\libs\preprocessor\test\config_info.test\clang-linux-5.0cl\debug\address-model-32\config_info.exe"
-Wl,--start-group
"C:\Programming\VersionControl\modular-boost\build\boost\bin.v2\libs\preprocessor\test\config_info.test\clang-linux-5.0cl\debug\address-model-32\config_info.obj"
-Wl,-Bstatic -Wl,-Bdynamic -Wl,--end-group -g
--target=i686-pc-windows-msvc -### -fuse-ld=lld

clang version 5.0.0 (trunk 298890)
Target: i686-pc-windows-msvc
Thread model: posix
InstalledDir: C:\Programming\VersionControl\bninja_installed_clang\bin

"C:\\Programming\\VersionControl\\bninja_installed_clang\\bin\\lld-link.exe"
"-out:C:\\Programming\\VersionControl\\modular-boost\\build\\boost\\bin.v2\\libs\\preprocessor\\test\\config_info.test\\clang-linux-5.0cl\\debug\\address-model-32\\config_info.exe"
"-defaultlib:libcmt" "-libpath:C:\\Program Files (x86)\\Microsoft Visual
Studio 14.0\\VC\\lib" "-libpath:C:\\Program Files (x86)\\Windows
Kits\\10\\Lib\\10.0.14393.0\\ucrt\\x86" "-libpath:C:\\Program Files
(x86)\\Windows Kits\\10\\Lib\\10.0.14393.0\\um\\x86" "-nologo" "-debug"
"--start-group"
"C:\\Programming\\VersionControl\\modular-boost\\build\\boost\\bin.v2\\libs\\preprocessor\\test\\config_info.test\\clang-linux-5.0cl\\debug\\address-model-32\\config_info.obj"
"-Bstatic" "-Bdynamic" "--end-group"

[Edward Diener via llvm-dev]

"C:/Programming/VersionControl/bninja_installed_clang/bin/clang++" -o

"C:\Programming\VersionControl\modular-boost\build\boost\bin.v2\libs\preprocessor\test\config_info.test\clang-linux-5.0cl\debug\address-model-32\config_info.exe"
-Wl,--start-group
"C:\Programming\VersionControl\modular-boost\build\boost\bin.v2\libs\preprocessor\test\config_info.test\clang-linux-5.0cl\debug\address-model-32\config_info.obj"
-Wl,-Bstatic -Wl,-Bdynamic -Wl,--end-group -g
--target=i686-pc-windows-msvc -### -fuse-ld=lld

clang version 5.0.0 (trunk 298890)
Target: i686-pc-windows-msvc
Thread model: posix
InstalledDir: C:\Programming\VersionControl\bninja_installed_clang\bin

"C:\\Programming\\VersionControl\\bninja_installed_clang\\bin\\lld-link.exe"
"-out:C:\\Programming\\VersionControl\\modular-boost\\build\\boost\\bin.v2\\libs\\preprocessor\\test\\config_info.test\\clang-linux-5.0cl\\debug\\address-model-32\\config_info.exe"
"-defaultlib:libcmt" "-libpath:C:\\Program Files (x86)\\Microsoft Visual
Studio 14.0\\VC\\lib" "-libpath:C:\\Program Files (x86)\\Windows
Kits\\10\\Lib\\10.0.14393.0\\ucrt\\x86" "-libpath:C:\\Program Files
(x86)\\Windows Kits\\10\\Lib\\10.0.14393.0\\um\\x86" "-nologo" "-debug"
"--start-group"
"C:\\Programming\\VersionControl\\modular-boost\\build\\boost\\bin.v2\\libs\\preprocessor\\test\\config_info.test\\clang-linux-5.0cl\\de

When I actually try to run the lld-link command as:

C:\Programming\VersionControl\bninja_installed_clang\bin\lld-link.exe

-out:C:\Programming\VersionControl\modular-boost\build\boost\bin.v2\libs\preprocessor\test\config_info.test\clang-linux-5.0cl\debug\address-model-32\config_info.exe
-defaultlib:libcmt "-libpath:C:\Program Files (x86)\Microsoft Visual
Studio 14.0\VC\lib" "-libpath:C:\Program Files (x86)\Windows
Kits\10\Lib\10.0.14393.0\ucrt\x86" "-libpath:C:\Program Files
(x86)\Windows Kits\10\Lib\10.0.14393.0\um\x86" -nologo -debug
--start-group
C:\Programming\VersionControl\modular-boost\build\boost\bin.v2\libs\preprocessor\test\config_info.test\clang-linux-5.0cl\debug\address-model-32\config_info.obj
-Bstatic -Bdynamic --end-group

I get for output in a Windows command window:

C:\Programming\VersionControl\bninja_installed_clang\bin\lld-link.exe:
warning:
ignoring unknown argument: --start-group
C:\Programming\VersionControl\bninja_installed_clang\bin\lld-link.exe:
warning:
ignoring unknown argument: -Bstatic
C:\Programming\VersionControl\bninja_installed_clang\bin\lld-link.exe:
warning:
ignoring unknown argument: -Bdynamic
C:\Programming\VersionControl\bninja_installed_clang\bin\lld-link.exe:
warning:
ignoring unknown argument: --end-group

and then nothing further and lld_link.exe just hangs and never
completes. So it looks to me that lld_link, in it effort to link PE/COFF
(Windows) files created by clang++ targeting VC++, has some sort of bug.

[kyra via llvm-dev]

Perhaps, you'll find these useful:
https://sourceforge.net/p/mingw-w64/bugs/597
https://github.com/lhmouse/mcfgthread/issues/21

(awson is me)

[Edward Diener via llvm-dev]

On 3/31/2017 4:53 PM, kyra via llvm-dev wrote:
> Perhaps, you'll find these useful:
> https://sourceforge.net/p/mingw-w64/bugs/597
> https://github.com/lhmouse/mcfgthread/issues/21

I appreciate the links.