From: Himanshu Jha <himanshujha199640@gmail.com>
To: Matt Ranostay <matt.ranostay@konsulko.com>
Cc: Amir Mahdi Ghorbanian <indigoomega021@gmail.com>,
lars@metafoo.de, Michael.Hennerich@analog.com, jic23@kernel.org,
knaack.h@gmx.de, pmeerw@pmeerw.net, gregkh@linuxfoundation.org,
linux-iio@vger.kernel.org, devel@driverdev.osuosl.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Staging: iio: ad7192: replaced bool in struct
Date: Mon, 7 Jan 2019 23:27:48 +0530 [thread overview]
Message-ID: <20190107175748.GA29220@himanshu-Vostro-3559> (raw)
In-Reply-To: <8E720BF2-7AFC-4FFA-8FA2-5A403FD5DD1D@konsulko.com>
On Wed, Jan 02, 2019 at 01:25:27PM -0800, Matt Ranostay wrote:
>
> > On Dec 24, 2018, at 01:58, Himanshu Jha <himanshujha199640@gmail.com> wrote:
> >
> >> On Fri, Dec 21, 2018 at 03:26:26PM -0800, Amir Mahdi Ghorbanian wrote:
> >> Replaced bool in struct with unsigned int bitfield to conserve space and
> >> more clearly define size of varibales
>
> Important thing to note is depending on padding, alignment, and position of field it probably won’t save any space.
Well, then what do you say about the following results ;-)
Before:
------
himanshu@himanshu-Vostro-3559:~/gsoc/linux$ pahole -C ad7192_platform_data drivers/staging/iio/adc/ad7192.o
struct ad7192_platform_data {
u16 vref_mv; /* 0 2 */
u8 clock_source_sel; /* 2 1 */
/* XXX 1 byte hole, try to pack */
u32 ext_clk_hz; /* 4 4 */
bool refin2_en; /* 8 1 */
bool rej60_en; /* 9 1 */
bool sinc3_en; /* 10 1 */
bool chop_en; /* 11 1 */
bool buf_en; /* 12 1 */
bool unipolar_en; /* 13 1 */
bool burnout_curr_en; /* 14 1 */
/* size: 16, cachelines: 1, members: 10 */
/* sum members: 14, holes: 1, sum holes: 1 */
/* padding: 1 */
/* last cacheline: 16 bytes */
};
After:
-----
himanshu@himanshu-Vostro-3559:~/gsoc/linux$ pahole -C ad7192_platform_data drivers/staging/iio/adc/ad7192.o
struct ad7192_platform_data {
u16 vref_mv; /* 0 2 */
u8 clock_source_sel; /* 2 1 */
/* XXX 1 byte hole, try to pack */
u32 ext_clk_hz; /* 4 4 */
unsigned int refin2_en:1; /* 8:31 4 */
unsigned int rej60_en:1; /* 8:30 4 */
unsigned int sinc3_en:1; /* 8:29 4 */
unsigned int chop_en:1; /* 8:28 4 */
unsigned int buf_en:1; /* 8:27 4 */
unsigned int unipolar_en:1; /* 8:26 4 */
unsigned int burnout_curr_en:1; /* 8:25 4 */
/* size: 12, cachelines: 1, members: 10 */
/* sum members: 11, holes: 1, sum holes: 1 */
/* bit_padding: 25 bits */
/* last cacheline: 12 bytes */
};
> Also for internal unpacked structures it really makes little sense to save a few bytes of data. Don’t read into that packed structures are good.. they usually aren’t :)
Yes, agreed, but I often see patches to save few bytes by marking
array as static.
There is some effort in this direction:
https://lore.kernel.org/lkml/20181221235230.GC13168@ziepe.ca/
Very likely to get more patches if the above patch gets merged.
--
Himanshu Jha
Undergraduate Student
Department of Electronics & Communication
Guru Tegh Bahadur Institute of Technology
next prev parent reply other threads:[~2019-01-07 17:57 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-21 23:26 [PATCH] Staging: iio: ad7192: replaced bool in struct Amir Mahdi Ghorbanian
2018-12-22 17:28 ` Jonathan Cameron
2018-12-24 9:58 ` Himanshu Jha
2019-01-02 21:25 ` Matt Ranostay
2019-01-07 17:57 ` Himanshu Jha [this message]
2019-01-12 16:59 ` Jonathan Cameron
2019-01-02 11:33 ` Dan Carpenter
[not found] <CAMW2b-maU7+WrwYnqqr7eUVw+S_MC9_do27ms-or8wWDapbqZA@mail.gmail.com>
2019-01-05 18:22 ` Jonathan Cameron
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