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Subject: Re: [PATCH][next] ACPICA: Use fallthrough pseudo-keyword
From:   Joe Perches <joe@perches.com>
To:     "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        "Moore, Robert" <robert.moore@intel.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc:     "Kaneda, Erik" <erik.kaneda@intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@intel.com>,
        Len Brown <lenb@kernel.org>,
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        "open list:ACPI COMPONENT ARCHITECTURE (ACPICA)" <devel@acpica.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Date:   Mon, 27 Jul 2020 19:22:18 -0700
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On Mon, 2020-07-27 at 17:23 -0500, Gustavo A. R. Silva wrote:
> Hi,
> 
> 
> This is a macro pseudo-keyword, which expands to /* fallthrough */
> for compilers that don't support the attribute __fallthrough__. See:

Not really.

It expands to
	do {} while (0)
for compilers that do not support the __fallthrough__ attribute.

The /* fallthrough */ after that is for the human reader
and is stripped before compilation.

> include/linux/compiler_attributes.h:213:
> 213 #if __has_attribute(__fallthrough__)
> 214 # define fallthrough                    __attribute__((__fallthrough__))
> 215 #else
> 216 # define fallthrough                    do {} while (0)  /* fallthrough */
> 217 #endif
> 
> So, any compiler (older or new) will be fine with it.

But old compilers should not emit warnings for these uses.