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Re: [Qemu-devel] Handling of fall through code (was: [PATCH v8 04/87] ta

From: Peter Maydell
Subject: Re: [Qemu-devel] Handling of fall through code (was: [PATCH v8 04/87] target/mips: Mark switch fallthroughs with interpretable comments
Date: Mon, 8 Jul 2019 09:42:31 +0100

On Sun, 7 Jul 2019 at 21:26, Stefan Weil <address@hidden> wrote:
> This is a general problem all over the QEMU code. I usually compile with
> nearly all warnings enabled and get now lots of errors with the latest
> code and after updating to gcc-8.3.0 (Debian buster). It should be
> reproducible by enabling -Werror=implicit-fallthrough.

Coverity warns about implicit fallthroughs, and we are
currently warning-free in that department, so I think
our remaining problems are largely down to perhaps
using idioms which the compiler doesn't spot.
Being able to enable gcc implicit-fallthrough errors would
definitely be better than spotting them only after the
fact with Coverity.

> I suggest to enable -Werror=implicit-fallthrough by default and add a
> new macro to mark all fallthrough locations which are correct, but not
> accepted by the compiler.

I'm not sure why we need a macro. Our standard way to
mark fallthrough is /* fall through */, which has hundreds
of uses in the codebase. -Wimplicit-fallthrough=2 will match this,
so it seems simpler to just use that rather than to rework
how we mark fallthroughs.

Since vixl is 3rd-party code it might be easier to just
add -Wno-implicit-fallthrough to the cflags that
disas/libvixl/Makefile.objs sets up for building those files.
(We should check also for newer libvixl and/or suggest
something upstream that works with gcc.)

-- PMM

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