llvm cmake: Fix NATIVE build when a toolchain file is already defined

[tpr]

Summary

Building on Windows with a cmake toolchain with LLVM called from an
outer cmake project ran into problems when LLVM does its inner "NATIVE"
cmake to build tablegen etc, in that some toolchain settings failed to
be conveyed into the NATIVE cmake, and it could not even run the initial
test for a working C compiler.

Fixed by passing the outer toolchain file if one was specified.

Fix from Juan Ramos, also from AMD.

Change-Id: I72d0c891d230393baaace79bfd14de953185ee21

[smeenai]

The NATIVE build is also used for cross-compilation scenarios (e.g. you're building on Windows but targeting Linux, but you still need tools like tablegen to be built targeting Windows), and for those cases, using the same CMAKE_TOOLCHAIN_FILE as the main build is completely incorrect.

What I've done to handle situations like this is set the CROSS_TOOLCHAIN_FLAGS_NATIVE cache variable to include the desired toolchain file for the NATIVE build (and any other desired CMake settings as well). Does that work for you? Something like -DCROSS_TOOLCHAIN_FLAGS_NATIVE="-DCMAKE_TOOLCHAIN_FILE=/path/to/your/toolchain/file" should do the trick.

[tpr]

This fix is wrong. I have a test build that went into an infinitely recursive loop of doing NATIVE builds.

[tpr]

In D96433#2554813, @smeenai wrote:

The NATIVE build is also used for cross-compilation scenarios (e.g. you're building on Windows but targeting Linux, but you still need tools like tablegen to be built targeting Windows), and for those cases, using the same CMAKE_TOOLCHAIN_FILE as the main build is completely incorrect.

What I've done to handle situations like this is set the CROSS_TOOLCHAIN_FLAGS_NATIVE cache variable to include the desired toolchain file for the NATIVE build (and any other desired CMake settings as well). Does that work for you? Something like -DCROSS_TOOLCHAIN_FLAGS_NATIVE="-DCMAKE_TOOLCHAIN_FILE=/path/to/your/toolchain/file" should do the trick.

Thanks; I will try that.

[tpr]

Abandoning as it is the wrong fix.

llvm/cmake/modules/CrossCompile.cmake

 include(AddLLVM)
 include(LLVMExternalProjectUtils)
 
 
 function(llvm_create_cross_target project_name target_name toolchain buildtype)
 
   if(NOT DEFINED ${project_name}_${target_name}_BUILD)
     set(${project_name}_${target_name}_BUILD
       "${CMAKE_CURRENT_BINARY_DIR}/${target_name}")
     set(${project_name}_${target_name}_BUILD
       ${${project_name}_${target_name}_BUILD} PARENT_SCOPE)
     message(STATUS "Setting native build dir to " ${${project_name}_${target_name}_BUILD})
   endif(NOT DEFINED ${project_name}_${target_name}_BUILD)
 
-  if (EXISTS ${LLVM_MAIN_SRC_DIR}/cmake/platforms/${toolchain}.cmake)
+  if (DEFINED CMAKE_TOOLCHAIN_FILE)
+    # A toolchain file has already been defined, so use it.
+    # Useful for projects consuming LLVM that define a toolchain.
+    set(CROSS_TOOLCHAIN_FLAGS_INIT "-DCMAKE_TOOLCHAIN_FILE=${CMAKE_TOOLCHAIN_FILE}")
+  elseif (EXISTS ${LLVM_MAIN_SRC_DIR}/cmake/platforms/${toolchain}.cmake)
+    # User wants a specific toolchain file.
     set(CROSS_TOOLCHAIN_FLAGS_INIT
       -DCMAKE_TOOLCHAIN_FILE=\"${LLVM_MAIN_SRC_DIR}/cmake/platforms/${toolchain}.cmake\")
   else()
+    # Else make a best guess. This should work fine for most cases.
     set(CROSS_TOOLCHAIN_FLAGS_INIT
       -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
       -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
       )
   endif()
   set(CROSS_TOOLCHAIN_FLAGS_${target_name} ${CROSS_TOOLCHAIN_FLAGS_INIT}
     CACHE STRING "Toolchain configuration for ${target_name}")