Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON

[Paolo Bonzini <pbonzini@redhat.com>]

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.

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.


>> 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).

Paolo

>>  #if defined __GNUC__
>>  # if !QEMU_GNUC_PREREQ(4, 4)
>>     /* gcc versions before 4.4.x don't support gnu_printf, so use printf. */
> 
> More so since your QEMU_BUILD_BUG_ON_ZERO seems easier to understand: no
> token pasting.
> 
> Anyway,
> Reviewed-by: Markus Armbruster <armbru@redhat.com>
>