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From: Markus Armbruster <armbru@redhat.com>
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Date: Thu, 19 Jan 2017 14:33:40 +0100
In-Reply-To: <8721518c-5063-bd5b-60a9-3a3752894e74@redhat.com> (Paolo
	Bonzini's message of "Thu, 19 Jan 2017 11:01:11 +0100")
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Subject: Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of
 QEMU_BUILD_BUG_ON
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>, Peter Maydell <peter.maydell@linaro.org>, qemu-devel@nongnu.org, Richard Henderson <rth@twiddle.net>

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 19/01/2017 09:12, Markus Armbruster wrote:
>> "Michael S. Tsirkin" <mst@redhat.com> writes:
>> 
>>> QEMU_BUILD_BUG_ON uses a typedef in order to be safe
>>> to use outside functions, but sometimes it's useful
>>> to have a version that can be used within an expression.
>>> Following what Linux does, introduce QEMU_BUILD_BUG_ON_ZERO
>>> that return zero after checking condition at build time.
>> 
>> Following Linux's example makes sense, but I can't help but wonder
>> whether we need both QEMU_BUILD_BUG_ON_ZERO() and QEMU_BUILD_BUG_ON().
>
> I think so, most notably QEMU_BUILD_BUG_ON was added to C11 as
> _Static_assert but QEMU_BUILD_BUG_ON_ZERO wasn't.

Okay.

> But we can indeed redefine QEMU_BUILD_BUG_ON to
> (void)QEMU_BUILD_BUG_ON_ZERO(x) like Linux does, until we add optional
> support for _Static_assert.

Yes, please.

>>> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>>> ---
>>>  include/qemu/compiler.h | 2 ++
>>>  1 file changed, 2 insertions(+)
>>>
>>> diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
>>> index 2882470..f4cf13b 100644
>>> --- a/include/qemu/compiler.h
>>> +++ b/include/qemu/compiler.h
>>> @@ -89,6 +89,8 @@
>>>      typedef char glue(qemu_build_bug_on__,__LINE__)[(x) ? -1 : 1] \
>>>          __attribute__((unused))
>>>  
>>> +#define QEMU_BUILD_BUG_ON_ZERO(x) (sizeof(int[(x) ? -1 : 1]) - sizeof(int))
>
> Linux here uses:
>
> #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
>
> and the issue is that sizeof(int[(x) ? -1 : 1]) could be
> runtime-evaluated (the type is a variable-length array).

Let's copy both macros from Linux.