[llvm-dev] Trying to cross-compile LLVM runtimes to Windows

[Martin Storsjö via llvm-dev]

On Mon, 5 Nov 2018, Petr Hosek via llvm-dev wrote:

> I've tried building libc++ for Windows on Linux by setting clang-cl as
> the compiler and lld-link as the linker, but I immediately hit
> roadblocks. The first issue seems to be the fact that CMake appears to
> ignore the value of CMAKE_RC_COMPILER variable and hardcodes "rc" as
> the resource compiler. This got resolved in upstream 12 days ago
> (https://github.com/Kitware/CMake/commit/bd9bfc644954a48b1bf7ea18fc260a1231840671)
>
> I managed to workaround this issue by symlinking llvm-rc as rc.
> However, that's not sufficient because CMake uses following invocation
> (https://github.com/Kitware/CMake/blob/master/Source/cmcmd.cxx#L1815):
>
> rc /foCMakeFiles/cmTC_2c376.dir/manifest.res
> CMakeFiles/cmTC_2c376.dir/manifest.rc
>
> llvm-rc option parses requires /fo option argument to be separate.
> I've modified the flag to support both joined and separate value which
> helped getting past that error,

Please send that patch for review (add me, zturner, amccarth and thakis as 
reviewers) - it's probably useful irrespective of the other issues you're 
running into.

> but I immediately hit another error: https://reviews.llvm.org/P8113
>
> The problem seems to be that the .rc file generated by CMake contains
> an absolute path to the resource, but llvm-rc expects the path to be
> always relative. However, I couldn't fine any documentation that would
> explain whether that's a requirement and it's CMake's fault or whether
> llvm-rc should be handling absolute paths as well. Is anyone familiar
> with Windows resource file semantics?

As the existing cmake behaviour works with rc.exe, llvm-rc should adapt to 
handle it as well.

It seems like we look in the INCLUDE environment for files included in 
resource files, but as that assert points out, doing that doesn't makse 
sense if the file name looked for actually turns out to be an absolute 
path. I'd just add an "if (!path::is_absolute(FileName))" before the 
FindInEnvPath call.

> All of these errors seem to suggest that this cross-compiling for
> Windows with Clang using CMake isn't a well explored area. Has anyone
> had more success with cross-compiling LLVM runtimes (libc++,
> compiler-rt) to Windows?

I do this, successfully - see https://github.com/mstorsjo/llvm-mingw (as 
Jean-Michaël already linked). I do this in a mingw setup though, so cmake 
doesn't use llvm-rc directly, but instead I wrap llvm-rc in a GNU windres 
like frontend, that takes care of preprocessing (in case there are 
#include etc) and option handling (wrangling options from GNU windres 
style to what llvm-rc expects).

It's definitely not a well-explored area though - as build-compiler-rt.sh 
and build-libcxx.sh in my repo shows, I go through quite a bit of manual 
fiddling to get it all to build in a bootstrap setup. There might be 
opportunities for simplifying all of it somewhat, but this setup works for 
me at least.

// Martin