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

[Petr Hosek via llvm-dev]

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, 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?

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?