Re: [Qemu-devel] [RFC PATCH 0/3] tweaks for QEMU's C standard

[Alex Bennée]

Daniel P. Berrangé <address@hidden> writes:

> On Mon, Aug 13, 2018 at 10:07:05AM +0100, Daniel P. Berrangé wrote:
>> On Fri, Aug 10, 2018 at 06:10:58PM +0100, Alex Bennée wrote:
>> > Hi,
>> >
>> > While I was reviewing Richard's SVE series I found Travis choking on
>> > some perfectly valid c99. It turns out that Travis default image is
>> > old enough that gcc defaults to -std=gnu89 hence the problem. However
>> > switching to c99 isn't enough as we use GNUisms and even gnu99 still
>> > trips up on qemu-secomp.
>> >
>> > Of course we could just jump to C11 already?
>>
>> We've always required GCC or a compatible compiler (CLang is only viable
>> alternative option really). We use a number of GCC extensions to the C
>> standard and I don't see a compelling reason to stop using them.
>>
>> From that POV I think we do *NOT* need to care about official C standards
>> (c89, c99, c11, etc), only the GNU C standards (gnu89, gnu99, gnu11, etc).
>>
>> > This is an RFC because this could descend into a C standards
>> > bike-shedding exercise but I thought I'd at least put it out there on
>> > a Friday afternoon ;-)
>>
>> I did some archeology to inform our plans...
>>
>> The default GCC C standards across various versions are:
>>
>>   8.2.1: gnu17
>>   7.3.1: gnu11
>>   6.4.1: gnu11
>>   5.3.1: gnu11
>>   4.9.1: gnu89
>>   4.4.7: gnu89
>>
>> Interesting to note that no version of GCC ever defaulted to gnu99. It was
>> not fully implemented initially and by the time the standard was fully
>> implemented, gnu11 was already good enough to be the default. So GCC jumped
>> straight from gnu89 as default to gnu11 as default.
>>
>> Across the various distros we aim to support we have:
>>
>>       RHEL-7: 4.8.5
>>       Debian (Stretch): 6.3.0
>>       Debian (Jessie): 4.9.2
>>       OpenBSD (Ports): 4.9.4
>>       FreeBSD (Ports): 8.2.0
>>       OpenSUSE Leap 15: 7.3.1
>>       SLE12-SP2:
>>       Ubuntu (Xenial): 5.4.0
>>       macOS (Homebrew): 8.2.0
>>
>> IOW plenty of our plaforms are still on 4.x which defaults to gnu89.
>>
>> In GCC 4.x, gnu99 is said to be incomplete (but usable) and gnu11
>> are said to be incomplete and experimental (ie don't use it).
>>
>> The lowest common denominator supported by all our platforms is thus
>> gnu89.
>>
>> If we don't mind that gnu99 is not fully complete in 4.x, we could use
>> that standard.

Thanks for the digging. I wonder what is missing in 4.x?

As far as I can tell we make heavy use of typeof() but I don't know how
to audit for other GNUisms? Everything should be in a compiler.h right?

I just noticed we have a C11-like generics macro in there ;-)

>>
>> We definitely can't use gnu11 any time soon.
>>
>> Given that many modern platforms default to gnu11, I think we should
>> set an explicit -std=gnu89, or -std=gnu99, because otherwise we risk
>> accidentally introducing code that relies on gnu11 features.
>
> Also, we should ensure the min required GCC version via biuld time
> check of some kind. eg something like
>
> #if !(__GNUC_PREREQ(4, 4) || defined(__clang__))
> # error "QEMU requires GCC >= 4.4, or CLang"
> #endif
>
>
> We can even check the C standard at build time if desired. eg I see
> these symbols defined for various -std=xxx args:
>
>  gnu89: #undef __STDC_VERSION__
>  gnu99: #define __STDC_VERSION__ 199901
>  gnu11: #define __STDC_VERSION__ 201112L
>  gnu17: #define __STDC_VERSION__ 201710L
>
>
> (See  "gcc -std=XXX  -dM -E - < /dev/null")
>
> Regards,
> Daniel


--
Alex Bennée