linux-kernel - Re: [PATCH] video: limit stack usage of ir-kbd-i2c.c

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Date:	Thu, 28 Feb 2008 19:29:24 +0100
From:	Jean Delvare <khali@...ux-fr.org>
To:	Marcin Slusarz <marcin.slusarz@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Mauro Carvalho Chehab <mchehab@...radead.org>,
	i2c@...sensors.org, video4linux-list@...hat.com
Subject: Re: [PATCH] video: limit stack usage of ir-kbd-i2c.c

On Wed, 27 Feb 2008 11:23:26 +0100, Marcin Slusarz wrote:
> On Tue, Feb 26, 2008 at 11:23:20PM +0100, Jean Delvare wrote:
> > * You can move the handling of some ioctls to dedicated functions, just
> > like I did in i2c-dev:
> > http://lists.lm-sensors.org/pipermail/i2c/2008-February/003010.html
> > However there is a risk that gcc will inline these functions (that's
> > what happened to me...) Not sure how to prevent gcc from inlining.
>
> There's "noinline" attribute in linux/compiler.h (compiler-gcc.h actually)
> for these situations.

I didn't know about noinline, thanks for the tip. It works as expected,
however in the case of i2cdev_ioctl I'm not sure if it's worth it.
Without it, I have:

0x02ae i2cdev_ioctl [i2c-dev]:                          192

With noinline, I have:

0x02ac i2cdev_ioctl_smbus [i2c-dev]:                    112
0x05bc i2cdev_ioctl_rdrw [i2c-dev]:                     112
0x087e i2cdev_ioctl [i2c-dev]:                          32

32 + 112 = 144, 192 - 144 = only 48 bytes saved on the stack, at the
price of some performance loss. OTOH this makes the binary slightly
smaller (not sure why), which is always nice:

add/remove: 2/0 grow/shrink: 0/1 up/down: 1121/-1182 (-61)
function                                     old     new   delta
i2cdev_ioctl_rdrw                              -     696    +696
i2cdev_ioctl_smbus                             -     425    +425
i2cdev_ioctl                                1482     300   -1182

-- 
Jean Delvare
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