[llvm-dev] RFC Enable -Wimplicit-fallthrough for clang as well as GCC

[Justin Bogner via llvm-dev]

+1. I tried to do this a couple of years ago (not knowing about the
proposal in 2012) but there was too much to annotate at the time. It
seems like this is easy to do now.

Reid Kleckner via llvm-dev <llvm-dev at lists.llvm.org> writes:
> Alex Kornienko proposed enabling this warning back in 2012 here:
> http://lists.llvm.org/pipermail/llvm-dev/2012-July/051386.html
>
> At the time, Chris Lattner said he didn't feel it was worth annotating
> all of LLVM and Clang with a new macro to enable this warning.
>
> However, GCC 7 added this warning as part of -Wextra, and we've slowly
> annotated most of LLVM and Clang with LLVM_FALLTHROUGH.
>
> At this point, I think we should re-evaluate this decision and enable
> this warning for both clang and GCC.
>
> Since our codebase is already annotated, it will help developers (like
> me), who use Clang locally, to find unintended fallthrough bugs in their
> code. For example, I committed r345676, which had to be reverted because
> of an unintended fallthrough. This warning would've helped.
>
> There is also marginal benefit to aligning warnings between GCC and
> Clang. While there will always be divergence in warnings between GCC and
> Clang, when possible, it saves time when clang can diagnose things that
> would later become a -Werror warning on some GCC 7 buildbot.
>
> This is a summary of differences in the behavior of
> -Wimplicit-fallthrough between clang and GCC:
> 1. GCC recognizes comments that say "fall through" as annotations, clang
>    doesn't
> 2. GCC doesn't warn on "case N: foo(); default: break;", clang does
> 3. GCC doesn't warn when the case contains a switch, but falls through
>    the outer case. See the AArch64ISelLowering.cpp change for an
>    instance where this almost caused a bug, but a redundant check saved
>    us. I've removed the redundant check.
> 4. Clang warns on LLVM_FALLTHROUGH after llvm_unreachable. GCC doesn't,
>    so I removed the one instance of this that I found.
>
> Changing Clang's behavior in light of these differences is out of scope
> for me. I want developers who compile with any of the last 4 years of
> clang releases to be able to use this warning, and those releases have
> the behavior described above. If you want to discuss changing Clang to
> be more like GCC here, please file a bug or start a thread on cfe-dev.
>
> I posted a patch with the this RFC as the commit message here so you can
> see what this looks like now:
> https://reviews.llvm.org/D53950
>
> To summarize, this warning is already enabled for GCC and
> we've already annotated most of our codebase for it, so let's enable the
> warning for clang.
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