linux-kernel - Re: [RFC][PATCH] new byteorder primitives

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Message-ID: <20171213003659.GA21978@ZenIV.linux.org.uk>
Date:   Wed, 13 Dec 2017 00:36:59 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Jakub Kicinski <kubakici@...pl>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] new byteorder primitives - ..._{replace,get}_bits()

On Tue, Dec 12, 2017 at 03:59:33PM -0800, Jakub Kicinski wrote:
> > +static __always_inline __##type type##_replace_bits(__##type old,	\
> > +					base val, base mask)		\
> > +{									\
> > +	__##type m = to(mask);						\
> > +        if (__builtin_constant_p(val) &&				\
> 
> Is the lack of a __builtin_constant_p(mask) test intentional?  Sometimes
> the bitfield is a packed array and people may have a helper to which
> only the mask is passed as non-constant and the value is implied by the
> helper, thus constant.

If the mask in non-constant, we probably shouldn't be using that at all;
could you show a real-world example where that would be the case?