Messages in this thread | | | Subject | Re: Question on FIELD_PREP() for static array | From | John Garry <> | Date | Thu, 11 Oct 2018 17:16:41 +0100 |
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On 11/10/2018 16:23, Johannes Berg wrote:
Hi
> 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?
Nothing special, it was just a sample. Here'e the code: regval = FIELD_PREP(MESON_SAR_ADC_CHAN_LIST_ENTRY_MASK(0), chan->address);
So val is a variable, and I find if remove both of the BUILD_BUG_ON_RET_ZERO()' which use __bf_shf() then it goes away.
> > 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?
I'm using a 7.3.1-based toolchain
> > 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?
I'll try it, thanks
John
> > johannes > > . >
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