On Thu, Mar 9, 2017 at 4:26 PM, Måns Rullgård <mans@xxxxxxxxx> wrote: > Tomas Winkler <tomasw@xxxxxxxxx> writes: > >> On Thu, Mar 9, 2017 at 4:16 PM, Måns Rullgård <mans@xxxxxxxxx> wrote: >>> Tomas Winkler <tomasw@xxxxxxxxx> writes: >>> >>>> On Thu, Mar 9, 2017 at 3:02 PM, Måns Rullgård <mans@xxxxxxxxx> wrote: >>>>> Tomas Winkler <tomasw@xxxxxxxxx> writes: >>>>> >>>>>> On Mon, Mar 6, 2017 at 2:31 AM, Måns Rullgård <mans@xxxxxxxxx> wrote: >>>>>>> Henrique de Moraes Holschuh <hmh@xxxxxxxxxx> writes: >>>>>>> >>>>>>>> On Sun, 05 Mar 2017, Måns Rullgård wrote: >>>>>>>>> Tomas Winkler <tomasw@xxxxxxxxx> writes: >>>>>>>>> > Sparse complains for arrays declared with variable length >>>>>>>>> > >>>>>>>>> > 'warning: Variable length array is used' >>>>>>>>> > >>>>>>>>> > Prior to c99 this was not allowed but lgcc (c99) doesn't have problem >>>>>>>>> > with that https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html. >>>>>>>>> > And also Linux kernel compilation with W=1 doesn't complain. >>>>>>>>> > >>>>>>>>> > Since sparse is used extensively would like to ask what is the correct >>>>>>>>> > usage of arrays of variable length >>>>>>>>> > within Linux Kernel. >>>>>>>>> >>>>>>>>> Variable-length arrays are a very bad idea. Don't use them, ever. >>>>>>>>> If the size has a sane upper bound, just use that value statically. >>>>>>>>> Otherwise, you have a stack overflow waiting to happen and should be >>>>>>>>> using some kind of dynamic allocation instead. >>>>>>>>> >>>>>>>>> Furthermore, use of VLAs generally results in less efficient code. For >>>>>>>>> instance, it forces gcc to waste a register for the frame pointer, and >>>>>>>>> it often prevents inlining. >>>>>>>> >>>>>>>> Well, if we're going to forbid VLAs in the kernel, IMHO the kernel build >>>>>>>> system should call gcc with -Werror=vla to get that point across early, >>>>>>>> and flush out any offenders. >>>>>>> >>>>>>> If it were up to me, that's exactly what I'd do. >>>>>> >>>>>>> >>>>>> Some parts of the kernel depends on VLA such as ___ON_STACK macros in >>>>>> include/crypto/hash.h >>>>>> It's actually pretty neat implementation, maybe it's too harsh to >>>>>> disable VLA completely. >>>>> >>>>> And what happens if the requested size is insane? >>>> >>>> One option is to add '-Wvla-larger-than=n' >>> >>> If you know the upper bound, why use VLAs in the first place? >> >> This is a water mark and not actual usage, but maybe I didn't >> understand your comment. > > If there is an upper bound known at compile time, why not simply use > that size statically? If there is no upper bound, well, then you have a > problem. If the compiler can do the job, why not to use this flexibility ? Tomas -- To unsubscribe from this list: send the line "unsubscribe linux-sparse" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
