linux-kernel - Re: [PATCH 1/4] media: si2168: use bits instead of bool for flags

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Date:   Thu, 10 Oct 2019 08:42:20 -0300
From:   Mauro Carvalho Chehab <mchehab@...radead.org>
To:     Gon Solo <gonsolo@...il.com>
Cc:     Linux Media Mailing List <linux-media@...r.kernel.org>,
        JP <jp@...w.nl>, crope@....fi, Sean Young <sean@...s.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] media: si2168: use bits instead of bool for flags

Em Thu, 10 Oct 2019 08:34:23 -0300
Mauro Carvalho Chehab <mchehab+samsung@...nel.org> escreveu:

> Em Thu, 10 Oct 2019 12:55:44 +0200
> Gon Solo <gonsolo@...il.com> escreveu:
> 
> > On Fri, Oct 04, 2019 at 10:15:22AM -0300, Mauro Carvalho Chehab wrote:  
> > > Using bool on struct is not recommended, as it wastes lots of
> > > space. So, instead, let's use bits.    
> > 
> > Wouldn't "bool b:1;" even be better? I performed a little test:

> > Result:
> > 
> > bit_uints: 4
> > bit_bools: 1

> > I know with different types within the struct it looks different, but
> > still.  
> 
> No. In practice, the compiler will add 3 bytes of pad after bit_bools
> (on 32-bit archs), due to performance reasons.

Btw, if you want to test, just add something after the bits, and you'll
see that it will now report the PAD bytes too:

struct bit_uints {
	unsigned int a0:1;
	unsigned int a1:1;
	unsigned int a2:1;
	unsigned int a3:1;
	unsigned int a4:1;
	unsigned int a5:1;
	unsigned int a6:1;
	unsigned int a7:1;

	int i;
};

struct bit_bools {
	bool a0:1;
	bool a1:1;
	bool a2:1;
	bool a3:1;
	bool a4:1;
	bool a5:1;
	bool a6:1;
	bool a7:1;

	int i;
};

bit_uints: 8
bit_bools: 8

Thanks,
Mauro