[llvm-dev] Upgrading LLVM's minimum required CMake version

Nikita Popov via llvm-dev llvm-dev at lists.llvm.org
Thu Mar 26 13:47:57 PDT 2020

On Thu, Mar 26, 2020 at 9:07 PM Shoaib Meenai via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> We had this discussion a few months ago and it petered out, and it’s
> recently been revived in the context of upgrading the CMake version
> specifically for libc++ (at which point people suggested upgrading the
> CMake version used by all of LLVM), so let’s try to move this forward.
> Our current required minimum version is CMake 3.4.3, which was released on
> January 25th 2016. It’s interesting to note that LLVM started requiring
> 3.4.3 on May 31st 2016, which was just 4 months after its release.
> Let’s look at the CMake versions available on various distros and
> operating systems. I’m unfamiliar with many of these, so I apologize if I
> get something wrong. (I’m using pkgs.org for most of this information.)
> * RHEL 6 (released Nov 10th 2010) : 3.6.1 (via EPEL)
> * RHEL 7 (released June 10th 2014): 3.14.7 (via EPEL)
> * RHEL 8 (released May 7th 2019): 3.11.4 (maybe pkgs.org is screwy on
> this one, because it doesn’t make sense that RHEL 7 should have a newer
> available version than RHEL 8)
> * Debian 9 (released June 17th 2017): 3.7.2
> * Debian 10 (released July 6th 2019): 3.13.4
> * Ubuntu 16.04 LTS (released April 21st 2016): 3.5.1
> * Ubuntu 18.04 LTS (released April 26th 2018): 3.10.2
> * FreeBSD 11 (released October 10th 2016): 3.15.5 (presumably upgraded in
> a point release)
> * FreeBSD 12 (released December 11th 2018): 3.15.5 (presumably upgraded in
> a point release)
> * NetBSD 8.1 (released May 31st 2019): 3.16.1
> * NetBSD 9.0 (released February 14th 2020): 3.16.1
> * OpenBSD: couldn’t find the version
> * macOS: latest version is readily available through Homebrew
> * Windows: You can install it yourself or use the one bundled with Visual
> Studio. I don't know what versions are bundled with Visual Studio; some
> searching suggests Visual Studio 2017 has CMake 3.12 and Visual Studio 2019
> has 3.15, though I have no confirmation of that.
> Note that CMake provides prebuilt binaries for Linux, macOS, and Windows,
> and it’s also straightforward to build from source (it has very
> conservative compiler requirements). One suggestion that was brought up in
> the past was for LLVM’s build system to just download a newer version of
> CMake if you attempted to build it using one that was too old, but there
> was opposition [1]. There was also a suggestion to have a script in LLVM to
> download and build CMake for you, but there were mixed opinions on this too
> [2], particularly since many developers might prefer downloading a binary
> release to building from source themselves (though of course the script
> could also download binary releases if applicable). I personally think
> downloading or building CMake yourself isn’t a high barrier for anyone
> wanting to build LLVM (and in particular it’s *much* more straightforward
> than building LLVM itself), but I can understand why people would prefer to
> stick to versions available in distros.
> Another suggestion that came up last time was to set a policy for
> upgrading CMake versions on some regular basis. The opposition to this was
> that we should upgrade CMake versions only when a newer version has a
> compelling enough feature to justify upgrading, rather than always
> upgrading. I can see arguments for both approaches, but we should
> definitely at least think about the benefits we can get from upgrading
> versions. I've gone through the CMake release notes and highlighted
> features which seemed potentially valuable for LLVM. Note that I'm only
> highlighting features for which our minimum CMake version would have to be
> bumped up in order for our build system to take advantage of. There are
> other useful features in newer CMake versions, but you can take advantage
> of them just by using a newer CMake yourself. For example, 3.9 loosens the
> dependencies of object compilation, which should result in faster Ninja
> builds.
> CMake 3.5 (released March 8th 2016):
> * install(DIRECTORY) supports generator expressions
> CMake 3.6 (released July 7th 2016):
> * install() supports EXCLUDE_FROM_ALL
> * list() supports FILTER to filter by regular expression
> * Subninja support, which could theoretically be used for much faster
> runtimes builds, although in practice we probably want to make
> ExternalProject support this directly instead of trying to layer our own
> meta-build system on top
> * CMAKE_TRY_COMPILE_TARGET_TYPE to tell try_compile to build a static
> library instead of an executable, which will greatly simplify the
> compiler-rt build
> CMake 3.7 (released November 11th 2016):
> * New if() comparison operators LESS_EQUAL, GREATER_EQUAL, STRLESS_EQUAL,
> CMake 3.8 (released April 10th 2017):
> * Compile features for C++17, which is required to build libc++ correctly
> * Support for compile features for specific C++ features instead of only
> being able to specify standard versions
> * rpath support via BUILD_RPATH target property and CMAKE_BUILD_RPATH
> variable
> * Apple framework support for static libraries
> * New swig_add_library command in the UseSWIG module
> * New generator expression $<IF:cond,true-value,false-value>
> CMake 3.9 (released July 18th 2017):
> * install(TARGETS) and install(EXPORTS) support for object libraries,
> which will simplify the compiler-rt build
> * TARGET_OBJECTS generator expression support in add_custom_command and
> file(GENERATE)
> * $<TARGET_BUNDLE_DIR:tgt> and $<TARGET_BUNDLE_CONTENT_DIR:tgt> generator
> expressions for Apple bundles
> CMake 3.10 (released November 20th 2017):
> * include_guard() command for proper guarding against double includes of
> CMake scripts
> * An interesting aside is that this is the first verion of CMake to
> require C++11 to build, which should give a good sense of how conservative
> they are about compiler requirements
> CMake 3.11 (released March 28th 2018):
> * add_library() and add_executable() can be called without sources as long
> as target_sources() is used later
> * target_compile_{definitions,features,options},
> target_include_directories(), target_sources(), and target_link_libraries()
> can set the corresponding INTERFACE_* properties on imported targets
> * COMPILE_DEFINITIONS supports generator expressions
> * COMPILE_OPTIONS source file property added
> * INCLUDE_DIRECTORIES source file property added
> * Interface libraries support custom properites
> CMake 3.12 (released July 17th 2018):
> * add_compile_definitions() added to add compile definitions for targets
> (to avoid the global pollution caused by add_definitions())
> * cmake_minimum_required() supports a version range to indicate tested
> CMake versions and set policies accordingly
> * file(TOUCH) and file(TOUCH_NOCREATE) added
> * list(JOIN), list(SUBLIST) and list(TRANSFORM) added
> * string(JOIN) added
> * SHELL: prefix support in target_compile_options to avoid errant
> deduplication
> * target_link_libraries() supports object libraries and propagates usage
> requirements
> * EXPORT_PROPERTIES target property to control the target properties
> exported by export() and install(EXPORT)
> * FindLibXml2 provides imported targets
> * New FindPython, FindPython2, and FindPython3 modules to ease location
> Python and selecting a specific version
> * Modernization of UseSWIG module
> * New generator expressions $<GENEX_EVAL:...>,
> $<TARGET_GENEX_EVAL:target,...>, $<IN_LIST:...>, $<TARGET_EXISTS:...> and
> * Compile features support for C++20
> CMake 3.13 (released November 20th 2018):
> * cmake -E create_symlink supported on Windows
> * target_link_directories() and target_link_options() commands to set link
> options instead of awkwardly having to use target_link_libraries() for this
> purpose
> * UseSWIG can manage INCLUDE_DIRECTORIES for SWIG compilation
> CMake 3.14 (released March 14th 2019):
> * file(CREATE_LINK) to create hard or symbolic links
> * if(DEFINED CACHE{VAR}) for checking if a cache variable is defined
> * $<IN_LIST:...> generator expression correctly handles empty argument
> * Fixes for object library linking propagation
> * Link options to manage position independent executables added
> automatically
> CMake 3.15 (released July 17th 2019):
> * list(PREPEND), list(POP_FRONT) and list(POP_BACK) added
> * New message() types NOTICE, VERBOSE, DEBUG and TRACE
> * string(REPEAT) added
> variable to select the runtime library type for MSVC
> and $<PLATFORM_ID:...> generator expressions support matching one value
> from a list
> * $<COMPILE_LANG_AND_ID:...> generator expression added
> * $<FILTER:list,INCLUDE|EXCLUDE,regex> generator expression added
> * $<REMOVE_DUPLICATES:list> generator expression added
> * New $<TARGET_FILE*> generator expressions added:
> * $<TARGET_OBJECTS:...> generator expression supports executables and
> static, shared, and module libraries
> CMake 3.16 (released November 26th 2019):
> * Support for generator expressions in BUILD_RPATH and INSTALL_RPATH
> CMake 3.17 (released March 20th 2020):
> * Ninja Multi-Config generator, which among other things would greatly
> * foreach(ZIP_LISTS) added to iterate multiple lists simultaneously
> * New message() keywords CHECK_START, CHECK_PASS, and CHECK_FAIL
> * INSTALL_NAME_DIR supports generator expressions
> Our build system is incredibly complex, and many of these features can be
> used to clean it up and make it much more maintainable. I would personally
> like us to at least bump up to CMake 3.12. I also do think it's worth
> establishing a policy and process around upgrading CMake versions, since
> newer versions keep on adding useful features (particularly better
> generator expression support), and we want to be able to keep taking
> advantage of them.

Thanks for the detailed analysis. Given this data, I believe the new CMake
target should be somewhere between 3.8.0 (for which there is a strong
motivation due to the libc++ requirement) and 3.10.2, which is the lower
bound of what is available on current LTS distros.

Personally, I don't think cleaning up the build system is sufficient
motivation for a version bump that would require a non-trivial fraction of
LLVM users to start using a custom CMake build. The build system already
works, so there doesn't seem to be any urgent need to improve it using new
CMake versions. If a new CMake version is needed to keep things working
going forward (as seems to be the case for libc++), that's an entirely
different matter, and a good motivator for bumping the version.

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