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

[Reid Kleckner via llvm-dev]

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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