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

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Date:	Wed, 23 Sep 2015 16:44:29 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Michal Nazarewicz <mina86@...a86.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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 Fri, Sep 18, 2015 at 12:36 AM,  <akpm@...ux-foundation.org> wrote:


> -#define abs(x) ({                                              \
> -               long ret;                                       \
> -               if (sizeof(x) == sizeof(long)) {                \
> -                       long __x = (x);                         \
> -                       ret = (__x < 0) ? -__x : __x;           \
> -               } else {                                        \
> -                       int __x = (x);                          \
> -                       ret = (__x < 0) ? -__x : __x;           \
> -               }                                               \
> -               ret;                                            \
> -       })
> +#define abs(x) __builtin_choose_expr(sizeof(x) == sizeof(s64), ({      \
> +               s64 __x = (x);                                          \
> +               (__x < 0) ? -__x : __x;                                 \
> +       }), ({                                                          \
> +               long ret;                                               \
> +               if (sizeof(x) == sizeof(long)) {                        \
> +                       long __x = (x);                                 \
> +                       ret = (__x < 0) ? -__x : __x;                   \
> +               } else {                                                \
> +                       int __x = (x);                                  \
> +                       ret = (__x < 0) ? -__x : __x;                   \
> +               }                                                       \
> +               ret;                                                    \
> +       }))

1. "char" should be banned, because it's signedness is not well defined.
2. there is unnecessary expansion to long.

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

As for INT_MIN, it pretty impossible to make compiler to not generate a branch
despite generating correct result without a branch.
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