[cfe-dev] RFC: Removing clang-cl's /fallback option

[Zachary Turner via cfe-dev]

Although we don't use fallback mode, we do use /ZW for WinRT extensions,
and this isn't supported by clang-cl (the corollary of this is that we
don't use clang-cl).  I'm mostly just posting this as a way to say that /ZW
is important for many users of cl, and without /fallback I think those
users will be prevented from using clang-cl.

What currently happens when you use clang for Windows support that comes
with recent versions of Visual Studio?  Does it silently append /fallback
on the user's behalf?  Or does it just give you an error if you try to use
an unsupported option?

On Wed, Jan 20, 2021 at 6:53 AM Aaron Ballman via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On Mon, Jan 18, 2021 at 5:32 AM Hans Wennborg via cfe-dev
> <cfe-dev at lists.llvm.org> wrote:
> >
> > clang-cl's /fallback option was added back in 2013 [1] as part of
> > bringing up clang's native Windows support.
> >
> > Since then, the Windows support has become quite stable, and I'm not
> > aware of anyone using /fallback anymore. In Chromium we stopped using
> > it in 2015 [2].
> >
> > While /fallback doesn't require a lot of code, it does add a bit of
> > complexity to the driver (for example it added a special type of Job),
> > and I worry about the code rotting since it's not getting updated to
> > handle new flags.
> >
> > So, are there any objections to removing the /fallback mode? My
> > proposal is to put a note in the upcoming Clang 12 release, and then
> > delete the code after the branch point, meaning it would be gone in
> > Clang 13.
> >
> > Please let me know what you think.
>
> No concerns from me, thank you for looking into it!
>
> ~Aaron
>
> >
> > Thanks,
> > Hans
> >
> > 1.
> https://github.com/llvm/llvm-project/commit/87cfa71071d73c702a833f19c9fe035f31c1275d
> > 2. http://crrev.com/340533
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