linux-kernel - RE: [PATCH 0/6] Don't use BIT() macro in UAPI headers

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Date:   Mon, 24 May 2021 16:34:39 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Mark Rutland' <mark.rutland@....com>,
        Christoph Hellwig <hch@...radead.org>
CC:     Joe Richey <joerichey94@...il.com>,
        "trivial@...nel.org" <trivial@...nel.org>,
        Joe Richey <joerichey@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Zhangfei Gao <zhangfei.gao@...aro.org>,
        Zhou Wang <wangzhou1@...ilicon.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "linux-accelerators@...ts.ozlabs.org" 
        <linux-accelerators@...ts.ozlabs.org>
Subject: RE: [PATCH 0/6] Don't use BIT() macro in UAPI headers

From: Mark Rutland
> Sent: 24 May 2021 13:29
> 
> On Mon, May 24, 2021 at 12:46:26PM +0100, Christoph Hellwig wrote:
> > On Thu, May 20, 2021 at 03:43:37AM -0700, Joe Richey wrote:
> > > This patch series changes all UAPI uses of BIT() to just be open-coded.
> > > However, there really should be a check for this in checkpatch.pl
> > > Currently, the script actually _encourages_ users to use the BIT macro
> > > even if adding things to UAPI.
> >
> > Yes.  In fact it should warn about BIT() in general.  It is totally
> > pointless obsfucation that doesn't even save any typing at all.
> 
> That's not quite true; the point is that if you use BIT() consistently,
> then even when you refer to bits 32 to 63 you won't accidentally
> introduce shifts of more than the width of int, and the definition will
> work equally well for assembly and C, which isn't true if you use `1UL`
> in the definition.
> 
> With that in mind it's shorter and clearer than its functional
> equivalent:
> 
>   BIT(x)
>   (UL(1) << (x))
> 
> So IMO it's preferable to use BIT() generally, or _BITUL() in uapi
> headers.

And then, suddenly the compiler warns about truncation of the
high bits when ~BIT(x) is used to mask a 32bit value on 64bit systems.

Once the C standard committee had decided to change from K&R's
'sign preserving' integer promotions to 'value preserving'
you always lose somewhere.

Personally I prefer hex constants - I can't count bits at all.

	David

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