linux-kernel - Re: [PATCH] bitfield: use BUILD_BUG_ON

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Message-ID: <18f6349cacfb6dc6fe8f1d12685ff6368c1cfbe0.camel@sipsolutions.net>
Date:   Fri, 12 Oct 2018 21:45:03 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     linux-wireless@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, John Garry <john.garry@...wei.com>,
        nbd@....name
Subject: Re: [PATCH] bitfield: use BUILD_BUG_ON_ZERO()

Umm, I really should've tested this :-)

As John points out, it can't possibly compile, e.g. because

> +#define __BF_FIELD_CHECK(_mask, _reg, _val)				\
> +	BUILD_BUG_ON_ZERO(!__builtin_constant_p(_mask)) +		\

this isn't actually a constant expression.

This would be harmless, since e.g.

> +	BUILD_BUG_ON_ZERO((_mask) == 0) +				\

this already forces _mask to be a constant, but

> +	BUILD_BUG_ON_ZERO(__builtin_constant_p(_val) ?			\
> +				~((_mask) >> __bf_shf(_mask)) & (_val) : 0) + \

we can't actually replicate this check.

We could just remove this as well, at the expense of not getting any
warnings if you write something like

	FIELD_PREP(0xf, 32);

Realistically, FIELD_PREP() is the only one that makes sense in a pure
constant context though, so we could just add __FIELD_PREP for that,
*with* all the checks. I'll send out a patch in a second that has this
and also adds __{u,le,be}{16,32,64}encode_bits().

johannes