linux-kernel - RE: [PATCH 08/15] drivers: net: Drop unlikely before IS_ERR(_OR

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Date:	Fri, 31 Jul 2015 15:39:56 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Murali Karicheri' <m-karicheri2@...com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
CC:	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"TI NETCP ETHERNET DRIVER" <netdev@...r.kernel.org>,
	Wingman Kwok <w-kwok2@...com>
Subject: RE: [PATCH 08/15] drivers: net: Drop unlikely before
 IS_ERR(_OR_NULL)

From: Murali Karicheri
> Sent: 31 July 2015 16:04
> On 07/31/2015 04:38 AM, Viresh Kumar wrote:
> > IS_ERR(_OR_NULL) already contain an 'unlikely' compiler flag and there
> > is no need to do that again from its callers. Drop it.
> >
> 
> IS_ERR_OR_NULL() is defined as
> 
> static inline bool __must_check IS_ERR_OR_NULL(__force const void *ptr)
> {
>          return !ptr || IS_ERR_VALUE((unsigned long)ptr);
> }
> 
> So the unlikely() applies only to second part. Wouldn't that be a
> problem for optimization?

Ugg...

The unlikely() in IS_ERR_VALUE() is likely to stop the compiler
doing a single 'window' comparison for the range [-MAX_ERROR .. 0].
So you are likely to end up with 2 comparisons.
I suspect that:

	return IS_ERR_VALUE((unsigned long)ptr - 1);

would be a much better test.
(Ignoring the off-by-one for the highest error.)

	David

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