Don't error on CMAKE_SYSTEM_NAME=Wasi

[matthewbauer]

Summary

When CMAKE_SYSTEM_NAME=Wasi, we are not targeting UNIX or WIN32 but Wasi:

https://wasi.dev

In this case, LLVM should recognize that this is a supported platform even though it is not UNIX or WIN32. Unlike Wasm, Wasi has a set ABI that provides some stability. The casing of "Wasi" is used instead of "WASI" to mirror that of the commonly used value of CMAKE_SYSTEM_NAME=Wasm for generic WebAssembly.

You can build libcxx and libcxxabi for Wasi given that you disable:

• threads
• exceptions
• shared linking

This patch makes it possible to avoid lying to LLVM about Wasi being "UNIX" when it isn't. HandleLLVMOptions.cmake is included by libcxxabi any time you set an LLVM_* flag like LLVM_COMPILER_CHECKED, LLVM_PATH, or LLVM_ENABLE_LIBCXX.

[sunfish]

llvm/cmake/modules/HandleLLVMOptions.cmake

129

I don't know a lot about CMake, but would be better to add a predicate for WASI so that we can change these FUCHSIA OR UNIX conditions into FUCHSIA OR UNIX OR WASI conditions?

[matthewbauer]

llvm/cmake/modules/HandleLLVMOptions.cmake

129

Maybe, although we'd have to make sure that LLVM_ON_UNIX and LLVM_HAVE_LINK_VERSION_SCRIPT are okay to set above. I think LLVM_HAVE_LINK_VERSION_SCRIPT is fine (lld should allow it) but not sure about LLVM_ON_UNIX.

We aren't really using this right now although at some point someone could port clang and llvm to run on Wasi I suppose. In the libcxxabi case LLVM_ON_UNIX might not be needed at all. We aren't actually building LLVM for Wasm, just using it for some headers.

[matthewbauer]

llvm/cmake/modules/HandleLLVMOptions.cmake

129

LLVM_ON_UNIX isn't needed at all for libcxxabi. Building LLVM itself is a different story though.

[Ericson2314]

llvm/cmake/modules/HandleLLVMOptions.cmake

129

https://github.com/Kitware/CMake/blob/master/Modules/Platform/Fuchsia.cmake this is where FUCHSIA comes from. I agree WASI should be defined upstream so it can be used here.

[sunfish]

llvm/cmake/modules/HandleLLVMOptions.cmake

129

As an updated here, I've now submitted https://gitlab.kitware.com/cmake/cmake/merge_requests/3261.

llvm/cmake/modules/HandleLLVMOptions.cmake

else(WIN32)
   if(FUCHSIA OR UNIX)
     set(LLVM_ON_WIN32 0)
     set(LLVM_ON_UNIX 1)
     if(APPLE OR ${CMAKE_SYSTEM_NAME} MATCHES "AIX")
       set(LLVM_HAVE_LINK_VERSION_SCRIPT 0)
     else()
       set(LLVM_HAVE_LINK_VERSION_SCRIPT 1)
     endif()
   else(FUCHSIA OR UNIX)
+    if(${CMAKE_SYSTEM_NAME} MATCHES "Wasi")
+    else()
       MESSAGE(SEND_ERROR "Unable to determine platform")
+    endif()
   endif(FUCHSIA OR UNIX)
 endif(WIN32)
 
 set(EXEEXT ${CMAKE_EXECUTABLE_SUFFIX})
 set(LTDL_SHLIB_EXT ${CMAKE_SHARED_LIBRARY_SUFFIX})
 
 # We use *.dylib rather than *.so on darwin.
 set(LLVM_PLUGIN_EXT ${CMAKE_SHARED_LIBRARY_SUFFIX})