[cfe-dev] Should we stop supporting building with Visual Studio?

Aaron Ballman via cfe-dev cfe-dev at lists.llvm.org
Mon Oct 8 07:34:50 PDT 2018

On Sun, Oct 7, 2018 at 4:51 PM Zachary Turner via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> This has been on my mind for quite some time, but recently it's been popping up more and more seeing some of the issues people have run into.
> Before people get the wrong idea, let me make one thing clear.  **I am not proposing we stop supporting the CMake Visual Studio generator.  I am only proposing we stop supporting actually compiling with the generated project**.  Yes the distinction is important, and I'll elaborate more on why later.  First though, here are some of the issues with the VS generator:
> 1) Using MSBuild is slower than Ninja.
> 2) Unless you remember to pass -Thost=x64 on the command line, you won't be able to successfully build.  We can (and have) updated the documentation to indicate this, but it's not intuitive and still bites people because for some reason this is not the default.
> 3) Even if you do pass -Thost=x64 to CMake, it will apparently still fail sometimes.  See this thread for details: http://lists.llvm.org/pipermail/cfe-dev/2018-October/059609.html.  It seems the parallel build scheduler does not do a good job and can bring a machine down.  This is not the first time though, every couple of months there's a thread about how building or running tests from within VS doesn't work.
> 4) Supporting it is a continuous source of errors and mistakes when writing tests.  The VS generator outputs a project which can build Debug / Release with a single project.  This means that `CMAKE_BUILD_TYPE=Debug` is a no-op on this generator.  The reason this matters for the test suite is because `${CMAKE_CURRENT_BINARY_DIR}` isn't sufficient to identify the location of the binaries.  You need `${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR}` instead.
> There is a continuous source of problems in our CMake [1, 2, 3, 4, 5].  It also affects tests, and every time someone adds a new lit site configuration, they have to remember to add this magic block of code:
> # Support substitution of the tools_dir with user parameters. This is
> # used when we can't determine the tool dir at configuration time.
> try:
>     config.llvm_tools_dir = config.llvm_tools_dir % lit_config.params
>     config.llvm_shlib_dir = config.llvm_shlib_dir % lit_config.params
> except KeyError:
>     e = sys.exc_info()[1]
>     key, = e.args
>     lit_config.fatal("unable to find %r parameter, use '--param=%s=VALUE'" % (key,key))
> to the file (even though only about 2 people actually understand what this does), which has caused problems several times.
> 5) VSCode and Visual Studio both support opening CMake projects directly now, which bypasses MSBuild.  I don't know how well Visual Studio supports LLVM's CMake, but the last time I tried it with VSCode on Linux it worked fine.
> ----
> I mentioned earlier that the distinction between not *building* with a VS-generated project and not supporting the VS generator is important.
> I don't want to speak for everyone, but I believe that *most* people use the VS generator because they want IDE support for their projects.  They want to be able to browse code, hit F5 to debug, F9 to set breakpoints, etc.  They don't necessarily care that Ctrl+Shift+B is how the code is generated versus some other incantation.  I'm asserting that it's possible to still have all the things people actually want from the VS generator without actually building from inside of VS.  In fact, I've been doing this for several years.  The workflow is:
> 1) Run CMake twice, generating to separate output directories.  Once using -G "Visual Studio 15 2017" and once using -G Ninja, each to different directories.
> 2) Open the VS one.  You have full IDE support.
> 3) Instead of hitting Ctrl+Shift+B to build, have a command prompt window open and type ninja.  Wait for it to complete.  If you want to you can make a custom tool command in Visual Studio so that you can access this from a keyboard shortcut.
> 4) When you want to debug, set your startup project (as you normally would), right click and hit properties, go to Debugging, change Command from $(TargetPath) to <type the full path to bin/foo.exe of the program you want to debug>.
> 5) Hit F5.
> In short, with only 2 simple additional steps (run CMake an extra time, and type a path into a window), people can have the exact workflow they are used to, plus faster builds, minus all of the problems and complexities associated with building from within VS.
> And we can simplify our CMake logic and lit configuration files as well.

As someone who almost exclusively uses MSVC and not Ninja on Windows,
I'll have to try this workflow out to see how it works for me in
practice. Unfortunately, I won't really be able to test it for a few

However, I'm concerned by the idea that we want to drop support for
building with the standard workflow in a toolchain that's as popular
as MSVC is. "Here's a simple five-step process to make this
plausible", while technically feasible, does not sound like a better
user experience for people trying to get into LLVM development on
Windows. It's also hard for me to foresee what pain points the
research teams at my company will have from such a transition (they
have out-of-tree workflows that have worked fine for them for years
and this transition will incur expenses to update scripts, etc).

Based on your statements about simplifying cmake, I'm reading "stop
supporting" as "prevent entirely; do not accept patches retaining that
support". Is that accurate? If so, what timeline do you envision and
what kind of transition story is there, especially for out-of-tree


More information about the cfe-dev mailing list