Improving support for CMake-based applications

[Eric Christopher]

Hi guys,

Apologies but I've lost track of the situation. What's going on and
needs changing on the Makefile side? I'm totally down with looking at
and reviewing, just not sure where we are :)

-eric

On Mon, Jan 27, 2014 at 9:02 AM, Brad King <brad.king at kitware.com> wrote:
> Hi Takumi,
>
> Thanks for looking at the series!
>
> On 01/27/2014 11:29 AM, NAKAMURA Takumi wrote:
>> I have been playing to tweak to let Clang FindPackage-friendly.
>> FYI, my tweaks, working in progress
>>   -- https://github.com/chapuni/llvm-project/commits/cmake/sa/trunk
>
> It looks like you're trying to teach tools/clang/CMakeLists.txt to
> find_package(LLVM) when built as an outside project.  In fact I have
> changes to do that which I used as part of testing this series.  My
> approach uses those LLVM_BUILD_* variables to hook up build/source
> tree locations automatically when building Clang against the LLVM
> build tree.
>
> I plan to submit that as a follow-up series to Clang cfe-commits once
> the necessary pieces are in LLVM.  As a preview:
>
>   project(Clang)
>   cmake_minimum_required(VERSION 2.8.8)
>
>   # Allow user to optionally override LLVM build tree locations.
>   set(LLVM_TOOLS_BINARY_DIR "" CACHE PATH "Path to llvm/bin")
>   set(LLVM_LIBRARY_DIR "" CACHE PATH "Path to llvm/lib")
>   set(LLVM_MAIN_INCLUDE_DIR "" CACHE PATH "Path to llvm/include")
>   set(LLVM_BINARY_DIR "" CACHE PATH "Path to LLVM build tree")
>   set(LLVM_MAIN_SRC_DIR "" CACHE PATH "Path to LLVM source tree")
>
>   # Look for LLVM build or install tree.
>   find_package(LLVM REQUIRED)
>   list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_DIR}")
>
>   # Load default locations from LLVM build tree when using one.
>   foreach(v
>       TOOLS_BINARY_DIR
>       LIBRARY_DIR
>       MAIN_INCLUDE_DIR
>       BINARY_DIR
>       MAIN_SRC_DIR
>       )
>     if(LLVM_BUILD_${v} AND NOT LLVM_${v})
>       set(LLVM_${v} "${LLVM_BUILD_${v}}")
>     endif()
>   endforeach()
>
>   # Tell tablegen() macro to use the imported executable by logical name.
>   set(LLVM_TABLEGEN_EXE llvm-tblgen)
>
>   ...
>
>> I have a few questions.
>>
>>   - Could you introduce me how to write FindPackage-friendly
>> cmakefiles for developers?
>
> For the side providing the package, there are tutorials on our Wiki:
>
>  http://www.cmake.org/Wiki/CMake/Tutorials#CMake_Packages
>
> There is also a new "cmake-packages(7)" manual that will be part of
> the next CMake release.  It isn't published at a persistent URL
> yet but here is link to the version control viewer for the manual
> source as of today:
>
>  http://cmake.org/gitweb?p=cmake.git;a=blob;f=Help/manual/cmake-packages.7.rst;hb=d31e68a2
>
> The patch series I've proposed here along with some follow-ups I
> will send once this one is merged take care of everything covered
> there.
>
> For the side using the package (the application), just
>
>  find_package(LLVM REQUIRED)
>
> should be enough to find LLVM.  To point your local application
> build at a specific LLVM, one may set LLVM_DIR to point at the
> directory containing the LLVMConfig.cmake file:
>
>  cmake ../myapp -DLLVM_DIR=/path/to/prefix/share/llvm/cmake
>
> or just add the LLVM installation prefix or build tree to the
> list of prefixes to search:
>
>  cmake ../myapp -DCMAKE_PREFIX_PATH=/path/to/prefix
>
>>   - Would it be better to add stuff in LLVMConfig.cmake?
>>     - list(APPEND CMAKE_MODULE_PATH)
>>     - include_directories(${LLVM_INCLUDE_DIRS})
>>     - link_directories(${LLVM_LIBRAY_DIRS})
>
> No.  When an application does find_package(LLVM) the resulting
> effects should be declarative.  We don't know that the application
> actually wants to use the values immediately.  It should be up to
> the application to use LLVM_INCLUDE_DIRS, LLVM_LIBRARY_DIRS, and
> LLVM_CMAKE_DIR while invoking the above commands itself.
>
>> +set(LLVM_CONFIG_INCLUDE_DIRS
>> +  "${LLVM_MAIN_INCLUDE_DIR}"
>> +  "${LLVM_INCLUDE_DIR}"
>> +  )
>>
>> For historical reasons, we prefer to seek build tree at first in INCLUDE_DIRS.
>> See also https://github.com/chapuni/llvm-project/commit/8a09aaa4c07fc9fdd1b4a63d7accc8ccf3389032
>
> Yes, I agree.
>
> Thanks,
> -Brad
>