On Thu, 2018-10-11 at 15:24 +0100, John Garry wrote: > > > +#define BUILD_BUG_ON_RET_ZERO(cond) (sizeof(char[1 - 2*!!(cond)]) - 1) > > +#define BUILD_BUG_ON_NOT_POW2_RET_ZERO(n) BUILD_BUG_ON_RET_ZERO(((n) & ((n) - 1)) != 0) > Seems reasonable. However I did try this and was getting compiler > warnings about VLA, from a non-constant being fed into > BUILD_BUG_ON_RET_ZERO(), related to sizeof char[]: > drivers/iio/adc/meson_saradc.c:375:2: warning: ISO C90 forbids variable > length array [-Wvla] > regval = FIELD_PREP(MESON_SAR_ADC_CHAN_LIST_ENTRY_MASK(0), Hmm, what's the code there? I don't see why the compiler should think it's a variable length? > Surely __NLA_ENSURE is getting a similar issue as it uses a similar > principle, no? I see that this is in -next now, but could not this macro > or derivatives being referenced. Yeah, I have a patch now to reference it, but I don't see anything from -Wvla with gcc 8.1? See https://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git/commit/?id=3d7af878357acd9e37fc156928106f1a969c8942 and its parent. Do you see -Wvla warnings there? Any idea how I could reproduce them? johannes
