[cmake] [libcxxabi] Fix find_path() problems when cross compiling

[hintonda]

Summary

When CMAKE_SYSROOT or CMAKE_FIND_ROOT_PATH is set, cmake
recommends setting CMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY
globally which means find_path() always prepends CMAKE_SYSROOT or
CMAKE_FIND_ROOT_PATH to all paths used in the search.

However, these find_path() invocations are looking for paths in
the libcxx and libunwind projects on the host system, not the
target system, which can be done by passing
NO_CMAKE_FIND_ROOT_PATH.

[compnerd]

I think that it might be better to handle this as a single global change:

if(CMAKE_FIND_ROOT_PATH)
  set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE NEVER)
endif()

[hintonda]

In D41623#965271, @compnerd wrote:

I think that it might be better to handle this as a single global change:

if(CMAKE_FIND_ROOT_PATH)
  set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE NEVER)
endif()

When I cross compile on Darwin and target Linux, I set sysroot and only want look for headers and libs in the new sysroot, not the host system. However, in this case, I want to look for the just built headers. This change helps support this.

As for global settings, this is what I'm setting in my toolchain file:

SET(CMAKE_FIND_ROOT_PATH "${sysroot}" CACHE STRING "" FORCE)
# adjust the default behavior of the FIND_XXX() commands:
# search headers and libraries in the target environment, search
# programs in the host environment
set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER CACHE STRING "" FORCE)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY CACHE STRING "" FORCE)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY CACHE STRING "" FORCE)

[hintonda]

ping...

[hintonda]

Unless there's any further comment, I plan to commit these patches tomorrow.

[phosek]

Isn't NO_DEFAULT_PATH more appropriate here? That's what libc++ uses as well.

[hintonda]

In D41623#982189, @phosek wrote:

Isn't NO_DEFAULT_PATH more appropriate here? That's what libc++ uses as well.

Unfortunately no.

When cross compiling, it's important to look for headers in CMAKE_SYSROOT and/or CMAKE_FIND_ROOT_PATH, or you'll end up finding for headers on the host, not the target. To make sure this works as expected, cmake recommends using set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY) -- see https://cmake.org/Wiki/CMake_Cross_Compiling for details.

With CMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY and CMAKE_SYSROOT set, all find_XXX() commands will add the CMAKE_SYSROOT prefix to all the paths used in the search. So, if you are trying to look in a local source directory, you'll never find it, because find_XXX() will actually use ${CMAKE_SYSROOT}/<my local path>, which is always the wrong place.

These find_path() invocations are looking in specific places which exist on the host system, not the target system.

[hintonda]

In D41623#982262, @hintonda wrote:

In D41623#982189, @phosek wrote:

Isn't NO_DEFAULT_PATH more appropriate here? That's what libc++ uses as well.

Unfortunately no.

When cross compiling, it's important to look for headers in CMAKE_SYSROOT and/or CMAKE_FIND_ROOT_PATH, or you'll end up finding for headers on the host, not the target. To make sure this works as expected, cmake recommends using set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY) -- see https://cmake.org/Wiki/CMake_Cross_Compiling for details.

With CMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY and CMAKE_SYSROOT set, all find_XXX() commands will add the CMAKE_SYSROOT prefix to all the paths used in the search. So, if you are trying to look in a local source directory, you'll never find it, because find_XXX() will actually use ${CMAKE_SYSROOT}/<my local path>, which is always the wrong place.

These find_path() invocations are looking in specific places which exist on the host system, not the target system.

Btw, I'm not saying don't use NO_DEFAULT_PATH where appropriate, just that NO_CMAKE_FIND_ROOT_PATH is also required in this case.

CMakeLists.txt

 find_path(
   LIBCXXABI_LIBCXX_INCLUDES
   vector
   PATHS ${LIBCXXABI_LIBCXX_INCLUDES}
         ${LIBCXXABI_LIBCXX_PATH}/include
         ${CMAKE_BINARY_DIR}/${LIBCXXABI_LIBCXX_INCLUDES}
         ${LIBCXXABI_LIBCXX_INCLUDE_DIRS}
         ${LLVM_INCLUDE_DIR}/c++/v1
+  NO_CMAKE_FIND_ROOT_PATH
   )
 
 set(LIBCXXABI_LIBCXX_INCLUDES "${LIBCXXABI_LIBCXX_INCLUDES}" CACHE PATH
     "Specify path to libc++ includes." FORCE)
 
 find_path(
   LIBCXXABI_LIBCXX_PATH
   utils/libcxx/test/__init__.py
   PATHS ${LIBCXXABI_LIBCXX_PATH}
         ${LIBCXXABI_LIBCXX_INCLUDES}/../
         ${LIBCXXABI_LIBCXX_SRC_DIRS}
   NO_DEFAULT_PATH
+  NO_CMAKE_FIND_ROOT_PATH
   )
 
 if (LIBCXXABI_LIBCXX_PATH STREQUAL "LIBCXXABI_LIBCXX_PATH-NOTFOUND")
   message(WARNING "LIBCXXABI_LIBCXX_PATH was not specified and couldn't be infered.")
   set(LIBCXXABI_LIBCXX_PATH "")
 endif()
 
 set(LIBCXXABI_LIBCXX_PATH "${LIBCXXABI_LIBCXX_PATH}" CACHE PATH
find_path(
     LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL
     libunwind.h
     PATHS ${LIBCXXABI_LIBUNWIND_INCLUDES}
           ${LIBCXXABI_LIBUNWIND_PATH}/include
           ${CMAKE_BINARY_DIR}/${LIBCXXABI_LIBUNWIND_INCLUDES}
           ${LLVM_MAIN_SRC_DIR}/projects/libunwind/include
           ${LLVM_MAIN_SRC_DIR}/runtimes/libunwind/include
     NO_DEFAULT_PATH
+    NO_CMAKE_FIND_ROOT_PATH
   )
 
   if (LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL STREQUAL "LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL-NOTFOUND")
     set(LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL "")
   endif()
 
   if (NOT LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL STREQUAL "")
     include_directories("${LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL}")