linux-kernel - Re: [PATCH v3 6/6] iio: adc: ad7173: Reduce device info struct size

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Date: Thu, 30 May 2024 08:17:47 +0200
From: Nuno Sá <noname.nuno@...il.com>
To: David Lechner <dlechner@...libre.com>, dumitru.ceclan@...log.com
Cc: Lars-Peter Clausen <lars@...afoo.de>, Michael Hennerich
 <Michael.Hennerich@...log.com>, Jonathan Cameron <jic23@...nel.org>, Rob
 Herring <robh@...nel.org>, Krzysztof Kozlowski
 <krzysztof.kozlowski+dt@...aro.org>, Conor Dooley <conor+dt@...nel.org>,
 linux-iio@...r.kernel.org, devicetree@...r.kernel.org, 
 linux-kernel@...r.kernel.org, Dumitru Ceclan <mitrutzceclan@...il.com>
Subject: Re: [PATCH v3 6/6] iio: adc: ad7173: Reduce device info struct size

On Wed, 2024-05-29 at 15:32 -0500, David Lechner wrote:
> On 5/29/24 7:23 AM, Nuno Sá wrote:
> > On Mon, 2024-05-27 at 20:02 +0300, Dumitru Ceclan via B4 Relay wrote:
> > > From: Dumitru Ceclan <dumitru.ceclan@...log.com>
> > > 
> > > Reduce the size used by the device info struct by packing the bool
> > >  fields within the same byte. This reduces the struct size from 52 bytes
> > >  to 44 bytes.
> > > 
> > > Signed-off-by: Dumitru Ceclan <dumitru.ceclan@...log.com>
> > > ---
> > >  drivers/iio/adc/ad7173.c | 16 ++++++++--------
> > >  1 file changed, 8 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/drivers/iio/adc/ad7173.c b/drivers/iio/adc/ad7173.c
> > > index 328685ce25e0..e8357a21d513 100644
> > > --- a/drivers/iio/adc/ad7173.c
> > > +++ b/drivers/iio/adc/ad7173.c
> > > @@ -179,15 +179,15 @@ struct ad7173_device_info {
> > >  	unsigned int clock;
> > >  	unsigned int id;
> > >  	char *name;
> > > -	bool has_current_inputs;
> > > -	bool has_vcom_input;
> > > -	bool has_temp;
> > > +	bool has_current_inputs		:1;
> > > +	bool has_vcom_input		:1;
> > > +	bool has_temp			:1;
> > >  	/* ((AVDD1 − AVSS)/5) */
> > > -	bool has_common_input;
> > > -	bool has_input_buf;
> > > -	bool has_int_ref;
> > > -	bool has_ref2;
> > > -	bool higher_gpio_bits;
> > > +	bool has_common_input		:1;
> > > +	bool has_input_buf		:1;
> > > +	bool has_int_ref		:1;
> > > +	bool has_ref2			:1;
> > > +	bool higher_gpio_bits		:1;
> > >  	u8 num_gpios;
> > >  };
> > >  
> > > 
> > 
> > This is really a very micro optimization... I would drop it tbh but no strong
> > feelings about it.
> > 
> > - Nuno Sá
> 
> This only considers RAM size and not code size too. At least on ARM arch
> every time we read or write to one of these fields, the code is now
> implicitly `((field & 0x1) >> bits)` so two extra assembly instructions
> for each read and write. This could be bigger than the size saved in
> the structs.
> 
> 

very good point...

- Nuno Sá