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

Greg Bedwell via lldb-dev lldb-dev at lists.llvm.org
Mon Oct 8 07:42:29 PDT 2018

Thanks for raising this.

This is a topic I've been interested in for a while too, as I've had to do
a few of those lite.site.cfg fix-ups that you mention (in fact I have one
sitting unreviewed at https://reviews.llvm.org/D40522 although I've not
pinged it in a long time so I'll need to double check that it's still an
issue).  There are also other issues.  For
example LLVM_ENABLE_ABI_BREAKING_CHECKS is implemented in such a way that
by default the value is defined at CMake time based on the value of
LLVM_ENABLE_ASSERTIONS which gets confusing with the Visual Studio
generator where the value of LLVM_ENABLE_ASSERTIONS does not necessarily
correspond to whether assertions are enabled or not.

As I understand it, what you're proposing is to not support building for
any configs that return true for GENERATOR_IS_MULTI_CONFIG.  This includes
all of the Visual Studio generators, but also the Xcode generator.  I'm not
an Xcode user. Does anyone make use of that generator or is it entirely
replaced in practice by single-config generators, i.e. Ninja?

We're still using the Visual Studio generators in production at Sony at
the moment.  This is largely because until recently they were actually
faster than Ninja for us due to the availability of distributed builds on
our network.  We've recently patched in support for our system into our
private branch of Ninja now so in theory it should be faster/on-par again
but we've not yet pulled the trigger on making them the default.  If
there's consensus that this is the way forward, then we'll definitely need
some time to make the change internally.  I'm only speaking personally in
this reply as I'll need to discuss with the rest of the team before we can
reach a position, but basically I wouldn't want the conclusion of this
thread to be "No dissenting voices, so here's an immediate patch to remove

I've not tried the workflow you describe.  I'll try it out in the coming
days to see how it works for me.  My main concerns are:

* How far will it raise the barrier of entry to new developers?  My
impression is that a lot of students coming to LLVM for the first time,
first build out of the box with Visual Studio before later discovering this
magical thing called Ninja that will speed things up.  Potentially this
could be mitigated with good enough documentation in the getting started
guide I expect.

* LLVM's CMake is super-slow on Windows, and we'd need to run it twice
whenever there are project changes.  This could be a significant drawback
in the proposed workflow but I'll need to try it before I can say that for

* My muscle memory causing repeated Ctrl+Shift+B :-).  I wonder if we could
add a PRE_BUILD custom target conditional on GENERATOR_IS_MULTI_CONFIG to
automatically fail any builds with a useful help message.

If the decision is that we continue supporting these generators, then at
the very least we should look into adding a buildbot configured to use one
of the Visual Studio Generators rather than ninja so that issues get
spotted on commit.


On Sun, 7 Oct 2018 at 21:51, 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.
> ----
> [1] - https://reviews.llvm.org/D43096
> [2] - https://reviews.llvm.org/D46642
> [3] - https://reviews.llvm.org/D45918
> [4] - https://reviews.llvm.org/D45333
> [5] - https://reviews.llvm.org/D46334
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