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

[Geert Uytterhoeven]

On Sat, Nov 20, 2010 at 00:19, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, 19 Nov 2010 15:04:11 -0800
> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
>> > Looks good to me. ÂI 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. ÂAt any rate, I like your version better as
>> > it seems more readable.
>> >
>>
>> I spose we should document it. Â 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);
>>
>> Â#define might_sleep_if(cond) do { if (cond) might_sleep(); } while (0)
>>
>> +/*
>> + * abs() handles unsigned and signed longs, ints, shorts and chars. ÂFor long
>> + * types it returns a signed long. ÂFor 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.
>> + */
>> Â#define abs(x) ({ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â\
>> Â Â Â Â Â Â Â long ret; Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â \
>> Â Â Â Â Â Â Â if (sizeof(x) == sizeof(long)) { Â Â Â Â Â Â Â Â\
>
> Well that was a load of bollocks. Â2nd 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);
>
> Â#define might_sleep_if(cond) do { if (cond) might_sleep(); } while (0)
>
> +/*
> + * abs() handles unsigned and signed longs, ints, shorts and chars. ÂFor 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.
> + */
> Â#define abs(x) ({ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â\
> Â Â Â Â Â Â Â Âlong ret; Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â \
> Â Â Â Â Â Â Â Âif (sizeof(x) == sizeof(long)) { Â Â Â Â Â Â Â Â\

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@xxxxxxxxxxxxxx

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.
             Â Â -- Linus Torvalds
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