linux-kernel - Re: + kernelh-make-abs-work-with-64-bit-types.patch added to -mm tree

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Date:	Thu, 24 Sep 2015 17:26:59 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Michal Nazarewicz <mina86@...a86.com>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	John Stultz <john.stultz@...aro.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: + kernelh-make-abs-work-with-64-bit-types.patch added to -mm tree

On Thu, Sep 24, 2015 at 4:32 PM, Michal Nazarewicz <mina86@...a86.com> wrote:
> On Wed, Sep 23 2015, Alexey Dobriyan wrote:
>> I've sent kabs() before which didn't go in because it didn't work for
>> INT_MAX et al
>> (don't worry, this abs() doens't as well) but it is nicer that this
>> version in other aspects
>> (hopefully).
>>
>> [PATCH v2] Add kabs()
>> http://marc.info/?l=linux-kernel&m=133518745522740&w=4
>
> Perhaps:
>
> +       (void)(_x)))))));                                               \
>
> instead of:
>
> +       _x))))));                                                       \
>
> at the end.  Since kabs makes no sense for unsigned types it’s best to
> fail with compile-time error than to let user think that the call is
> actually doing something.

I thought so, but the amount of uses like

  unsigned int = abs(unsigned int - unsigned int)

is non trivial. With a few exceptions it is about 170 on
x86_64 allmodconfig. Most of them are correct because abs()
casts to signed first. So if you want to expose such uses,
then you are forced to write

  unsigned int = kabs((int)(unsigned int - unsigned int));

Don't know if it's a good thing.

> Also, you don’t need ({ … }) around the ‘_x < 0 ? -_x : _x’ expression,
> do you?

Seems so, thank you for noticing.
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