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

[Hans Wennborg via cfe-dev]

I just checked, and Microsoft's Visual Studio integration doesn't use
the /fallback flag.

Probably users who build with /ZW would apply that to the whole
project, which wouldn't make clang-cl /fallback any different from
using MSVC in the first place.

On Wed, Jan 20, 2021 at 8:22 PM Zachary Turner <zturner at roblox.com> wrote:
>
> 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
>> > _______________________________________________
>> > cfe-dev mailing list
>> > cfe-dev at lists.llvm.org
>> > https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev