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

[Zachary Turner via llvm-dev]

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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