Daniel P. Berrangé <berra...@redhat.com> 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
