linux-kernel - Re: [PATCH] linux/bits.h: Add compile time sanity check of GENMASK inputs

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Date:   Tue, 06 Aug 2019 14:15:54 -0700
From:   Joe Perches <joe@...ches.com>
To:     Rikard Falkeborn <rikard.falkeborn@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Berg <johannes@...solutions.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] linux/bits.h: Add compile time sanity check of GENMASK
 inputs

On Tue, 2019-08-06 at 21:27 +0200, Rikard Falkeborn wrote:
> On Wed, Aug 07, 2019 at 12:19:36AM +0900, Masahiro Yamada wrote:
> > How about this?
> > #define GENMASK_INPUT_CHECK(high, low) \
> >        BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> >               __builtin_constant_p((low) > (high)), (low) > (high), 0))
> Thanks for the feedback, your version looks much cleaner than mine. I
> *think* I had a reason for using __is_constexpr() instead of
> __builtin_constant_p but I'll try a full rebuild to see if something
> comes up.

Perhaps a statement expression so high and low aren't possibly
evaluated multiple times?

#define GENMASK_INPUT_CHECK(high, low)				\
({								\
	typeof(high) _high = high;				\
	typeof(low) _low = low;					\
	BUILD_BUG_ON_ZERO(__builtin_constant_p(_low > _high,	\
					       _low > _high,	\
					       0))		\
})