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From: Peter Maydell <peter.maydell@linaro.org>
Date: Tue, 31 Jan 2017 16:54:50 +0000
Message-ID: <CAFEAcA-PLbA80Hftk+M1DbfCv4vpnSsyiGCynspO9eiyBbsLiw@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Subject: Re: [Qemu-devel] [PATCH] QEMU_BUILD_BUG_ON: use __COUNTER__
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To: Eric Blake <eblake@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>, QEMU Developers <qemu-devel@nongnu.org>, Paolo Bonzini <pbonzini@redhat.com>

On 31 January 2017 at 16:37, Eric Blake <eblake@redhat.com> wrote:
> On 01/31/2017 10:07 AM, Markus Armbruster wrote:
>>> I tried that first thing.
>>> This generates lots of warnings if you have multiple users
>>> within a function: gcc is unhappy about the redundant extern
>>> declarations.
>>
>> #define QEMU_BUILD_BUG_ON(x) \
>>     _Pragma("GCC diagnostic push") \
>>     _Pragma("GCC diagnostic ignored \"-Wredundant-decls\"") \
>>     extern char qemu_build_bug_on_[QEMU_BUILD_BUG_ON_ZERO((x))] __attribute__((unused)); \
>>     _Pragma("GCC diagnostic pop")
>
> Do we know if pragma GCC diagnostic push is supported in our minimum
> compiler requirements (gcc 4.1, and presumably clang just ignores the
> pragma)?

Nope; that's gcc 4.6 or newer. They're also a bit flaky between
gcc and clang because they rely on the warning being the same
in both cases. Using them in QEMU_BUILD_BUG_ON would be
massively overkill, especially since gcc 4.6 and on support
_Static_assert() which will get us better error messages
than this stuff.

thanks
-- PMM