Re: abs() vs. abs64() (was: Re: [PATCH] fbdev: fix nearest mode search)

Geert Uytterhoeven
2010-11-20T08:56:28Z

On Sat, Nov 20, 2010 at 00:19, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Fri, 19 Nov 2010 15:04:11 -0800
> Andrew Morton <akpm@linux-foundation.org> wrote:
>
>> > Looks good to me.  I posted essentially the same thing some 3 months ago
>> > (http://marc.info/?l=linux-kernel&m=128033094822201&w=2) but it then
>> > failed to get any traction.  At any rate, I like your version better as
>> > it seems more readable.
>> >
>>
>> I spose we should document it.   Does this look complete and accurate?
>>
>> --- a/include/linux/kernel.h~include-linux-kernelh-abs-fix-handling-of-32-bit-unsigneds-on-64-bit-fix
>> +++ a/include/linux/kernel.h
>> @@ -143,6 +143,13 @@ extern int _cond_resched(void);
>>
>>  #define might_sleep_if(cond) do { if (cond) might_sleep(); } while (0)
>>
>> +/*
>> + * abs() handles unsigned and signed longs, ints, shorts and chars.  For long
>> + * types it returns a signed long.  For int, short and char types it returns a
>> + * signed int.
>> + * abs() should not be used for 64-bit types (s64, u64, long long) - use abs64()
>> + * for those.
>> + */
>>  #define abs(x) ({                                            \
>>               long ret;                                       \
>>               if (sizeof(x) == sizeof(long)) {                \
>
> Well that was a load of bollocks.  2nd attempt:

Yeah, I was just gonna complain "but long _is_ 64-bit on 64-bit platforms"...

> --- a/include/linux/kernel.h~include-linux-kernelh-abs-fix-handling-of-32-bit-unsigneds-on-64-bit-fix
> +++ a/include/linux/kernel.h
> @@ -143,6 +143,12 @@ extern int _cond_resched(void);
>
>  #define might_sleep_if(cond) do { if (cond) might_sleep(); } while (0)
>
> +/*
> + * abs() handles unsigned and signed longs, ints, shorts and chars.  For all
> + * input types abs() returns a signed long.
> + * abs() should not be used for 64-bit types (s64, u64, long long) - use abs64()
> + * for those.
> + */
>  #define abs(x) ({                                              \
>                long ret;                                       \
>                if (sizeof(x) == sizeof(long)) {                \

After some second thinking, I think this is OK...

In the first patch:
> of similarity between video modes. The arguments of abs() are sometimes
> unsigned numbers. This worked fine until commit a49c59c0 ("Make sure the

The "sometimes" is when the parameter is the difference of 2 numbers, which is a
highly likely use case. Unlike most people's intuitive mathematical
feelings, a difference
is always unsigned in C (that was incorrect in the comment of Michal's
first version).
So I think it's worth mentioning that explicitly.

... and please llet me do the third thinking during the rest of the day ;-)

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds