Re: Question on FIELD_PREP() for static array

[Johannes Berg <johannes@sipsolutions.net>]

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