linux-kernel - Re: [PATCH] [media] zl10353: use div_u64 instead of do

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Message-ID: <CAKv+Gu9YWEkArjssR9Urh0_MOR3duNOo2UNiV=tXoQNgFtDngQ@mail.gmail.com>
Date:	Sun, 14 Feb 2016 08:57:36 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Arnd Bergmann <arnd@...db.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	linux-media@...r.kernel.org,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>
Subject: Re: [PATCH] [media] zl10353: use div_u64 instead of do_div

On 13 February 2016 at 22:57, Nicolas Pitre <nicolas.pitre@...aro.org> wrote:
> On Sat, 13 Feb 2016, Ard Biesheuvel wrote:
>
>> On 12 February 2016 at 22:01, Arnd Bergmann <arnd@...db.de> wrote:
>> > However, I did stumble over an older patch I did now, which I could
>> > not remember what it was good for. It does fix the problem, and
>> > it seems to be a better solution.
>> >
>> >         Arnd
>> >
>> > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
>> > index b5acbb404854..b5ff9881bef8 100644
>> > --- a/include/linux/compiler.h
>> > +++ b/include/linux/compiler.h
>> > @@ -148,7 +148,7 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
>> >   */
>> >  #define if(cond, ...) __trace_if( (cond , ## __VA_ARGS__) )
>> >  #define __trace_if(cond) \
>> > -       if (__builtin_constant_p((cond)) ? !!(cond) :                   \
>> > +       if (__builtin_constant_p(!!(cond)) ? !!(cond) :                 \
>> >         ({                                                              \
>> >                 int ______r;                                            \
>> >                 static struct ftrace_branch_data                        \
>> >
>>
>> I remember seeing this patch, but I don't remember the exact context.
>> But when you think about it, !!cond can be a build time constant even
>> if cond is not, as long as you can prove statically that cond != 0. So
>
> You're right.  I just tested it and to my surprise gcc is smart enough
> to figure that case out.
>
>> I think this change is obviously correct, and an improvement since it
>> will remove the profiling overhead of branches that are not true
>> branches in the first place.
>
> Indeed.
>

... and perhaps we should not evaluate cond twice either?