cmake-compile-features

Study of CMake compile features and compiler support levels for C++17, 20, 23, ...

Requirements

• GNU, Clang, Intel, or MSVC compiler
• CMake 3.8 or better
  • Needed to provide C++17 support and the cxx_std_XY features
  • 3.12 is needed for std_cxx_20 feature

Example build on Unix

$ ls
CMakeLists.txt  README.md       ccf-program.cpp
LICENSE         ccf             cmake
$ cmake -S. -B ./build
... output should report info about compiler, C++ features known to
... CMake, and C++ features supported by the compiler
$ cmake --build ./build
... Compilation commands should contain correct "-std=c++XY" line

Platforms tested by CI

GitHub Actions are used to test the platforms:

• GitHub
  • CentOS7 (Docker Image)
    • GCC 4.8.5 (native)
    • GCC 8.X (LCG_96 from sft.cern.ch CVMFS)
  • Ubuntu 18.04, 20.04
    • GCC 7, 8, 9, 10
    • Clang 8, 9, 10
  • Windows Server 2019
    • Visual Studio 2019
  • macOS 10.15 (Catalina)

For additional information, see the documentation for GitHub Hosted Runners.

Notes (In Flux, to be Updated)

Checking for C++ Support

Important to note the difference between compiler (syntax) features and standard library (implementation) features. CMake's compile features functionality deals with the former, e.g. the compiler understands the auto keyword. At link time the program is linked to the actual C++ Standard Library implementation which may be different from any supplied by the compiler vendor. For example, the Intel compiler may link to the GNU implementation of the Standard Library, Clang may link to the GNU or LLVM implementations.

Standard Library features are actual objects/functions such as

• Smart pointers
• Hashed containers
• Random numbers
• Concurrency

Checks for these are implemented using a wrapper around CMake's try_compile functionality, with success/failure being recorded in a CMake variable. That can subsequently be used to fail the configuration or provide workarounds as required. Implementation may still need some work to both fully exercise requirements on the objects and to make the tests transparent across newer standards.

This try_compile method can also perform syntax checks if required, which is the case for the simplified cxx_std_XY compile features. These only indicate that the compiler knows about C++XY not that its support for the standard is feature-complete, nor that the standard library provides a complete implementation.

Forwarding C++ Support

If we are building a library which is compiled with a given C++ standard, then clients linking to this library should compile their code against the same standard. This is the safest and most conservative approach to avoid run/deploy time issues with potential ABI differences between different standards.

If CMake is used, then compile features can help to transitively pass down the requirement. For example

add_library(foo foo.hpp foo.cpp)
target_compile_features(foo PUBLIC cxx_std_11)
# Effectively results in "g++ -std=gnu++11 foo.cpp"
...

add_executable(bar bar.cpp)
target_link_libraries(bar PRIVATE foo)
# Linking transitevely propgates compile features, so effectively
# results in "g++ -std=gnu++11 bar.cpp -lfoo"

Compile features therefore propagate over CMake targets, including via exported targets (which create imported targets that clients actually link against).

C++ Standard Library Workarounds

Illustrated by the std::make_unique case, where a useful bit of functionality didn't quite make it into a given standard (C++11) but can be implemented using the features of the older standard. The CMake script has the functionality to test for the presence and working of something in the Standard Library implementation in use, and sets boolean (CMake) variables to indicate its presence or otherwise. Here, we export this variable using configure_file to set a #cmakedefine symbol in a header. That symbol then protects a local implementation of that feature.

In this project, the exemplar std::make_unique is shown so that:

• C++11 systems can use std::make_unique
• C++14 systems that don't provide a std::make_unique implementation can use it
• There won't be a clash if the Standard Library provides an implementation

See the template file ccf/detail/ccf_stdlib_support.hpp.in in the source directory, plus the generated header ccf/detail/ccf_stdlib_support.hpp in the build directory.

Noted issues

Compiling against newer C++ Standards (14, 17, ...)

The use case here is consuming projects that require use of a newer C++ standard than currently required by this project. For example, this project only uses features of C++11, but a consuming project uses features of C++14. It's an issue because the ABI of the Standard Library may be incompatible between objects compiled against different standards. This was highlighted by early adopters of C++11 making using of the experimental and unstable ABI in GCC4.

Whilst C++'s ABI should stabilize in coming standard, allowances should be made for the same issue arising as C++17 is formalized. Forcing use of a particular standard can be done via setting the variables CMAKE_CXX_STANDARD and CMAKE_CXX_STANDARD_REQUIRED prior to target declaration. These are used to initialize the CXX_STANDARD property of targets, and this can co-exist with compile features as the newest standard required will be used. For example, a target with CXX_STANDARD set to '14' and only using C++11 compile features would result in the target's sources being compiled against C++14 (and vice versa).

Unlike compile features however, the CXX_STANDARD property of targets is not exported for use by consuming targets. This means that consuming targets would only compile against the standard required by the compile features of the library, even if this standard is older than the one actually used to compile the library against. Consequently an ABI incompatibility would occur if all objects must have been compiled/linked with the same standard.

There are several potential ways to resolve this. Whilst the CXX_STANDARD and CXX_STANDARD_REQUIRED target properties are not directly exported, it may be possible to set them manually on the imported targets created by the project's cmake config module. Initial tests suggest that this isn't passed down to consuming targets though, so would not be of great use.

Another possibility is to use the INTERFACE_COMPILE_OPTIONS target property to pass down any relevant flags to the compiler, but this requires some work with generator expressions to choose between compilers.

The final option is to artifically add a single C++14 compile feature to the required list if the compiler support C++14. This would work, but the single option would need to be supported across all compilers.

• Intel : cxx_binary_literals seems reasonable from 11-14, auto return types from 15
• MSVC : auto/decltype(auto) return types or init captures from VS14, binary literals from VS15
• GNU : binary literals from 4.9, auto return type from 4.8
• Clang : binary literals from 2.9, decltype(auto) from 3.3, and 3.3 is minimum to be C++11 complete.

So using the cxx_decltype_auto feature would probably be sufficient.

Legacy/Resolved Issues

thread_local in Xcode (AppleClang)

Older (which?) versions of Apple's Clang that comes with Xcode did not support this. A discussion about this can be found on StackOverflow. Ultimately was an upstream issue with Apple, and eventually resolved.

It's also an area where workarounds can be enabled/implemented via the compiler detection header part of CMake (see the ccf/detail/ccf_compiler_support.hpp header under the build directory). This is also the general mechanism for providing workarounds for syntax where it's possible.

Supporting newer compilers in older CMake (Intel was use case)

Appears that there isn't support for compile features or header detection on the Intel compiler. Need to find out if an extension can be provided (yes, it can, see below), or whether we have to wait for upstream CMake to support this (no, but this is the ideal case, and should submit patch!).

The list(s) of compile features can be set after a project call, and they will be recognized by targets using them as supported, e.g.

if(CMAKE_CXX_COMPILER_ID MATCHES "Intel")
  message(STATUS "CMAKE_COMPILER_ID matches 'Intel'")
  # No version checking yet, that's needed for full implementation
  set(CMAKE_CXX11_COMPILE_FEATURES cxx_auto_type cxx_range_for)
  set(CMAKE_CXX_COMPILE_FEATURES ${CMAKE_CXX11_COMPILE_FEATURES})
  set(CMAKE_CXX11_STANDARD_COMPILE_OPTION "-std=c++11")
  set(CMAKE_CXX11_EXTENSION_COMPILE_OPTION "-std=c++11")
  set(CMAKE_CXX98_STANDARD_COMPILE_OPTION  "-std=c++11")
  set(CMAKE_CXX98_EXTENSION_COMPILE_OPTION "-std=c++98")

  # This is the critical variable, there must be a default when the compiler
  # has the notion of standards
  set(CMAKE_CXX_STANDARD_DEFAULT "98")
endif()

...

add_executable(myprogram myprogram.cpp)
target_compile_features(myprogram PRIVATE cxx_auto_type)

The critical setting is the CMAKE_CXX_STANDARD_DEFAULT variable. This is required for compilers that have the notion of setting the standard.

Intel also has the issue (as with Non-Apple/BSD Clang) of Standard Library selection, though compile features should only refer to things the compiler can generate code for. Needs further investigation, e.g. to use CMake's compiler feature detection mechanism to check stdlib impl/version as part of feature checking. Or could use separate stdlib detection.