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

From: Linus Torvalds
Date: Thu Sep 24 2015 - 14:03:14 EST

On Wed, Sep 23, 2015 at 6:44 AM, Alexey Dobriyan <adobriyan@xxxxxxxxx> wrote:
> I've sent kabs() before which didn't go in because it didn't work for
> INT_MAX et al
> [PATCH v2] Add kabs()

Yeah, no, that's bad.

Testing against char/short is pointless. They should get upgraded to "int".

And guys, stop the idiotic "shouldn't work on unsigned values" crap.
It damn well should work on unsigned values, and the semantics we want
is that we treat it as a signed value.

Why? That's how "abs()" works. Really. Stop fighting it. If you were
to use a real "abs()", it takes an "int" argument, and if you pass it
an unsigned value, it gets converted to "int".

End of story. Stop with the broken "but but but unsigned" crap already.

So the end result is that

(a) we should look at the *size* of the argument type, not the
signedness, because the signedness is immaterial

(b) we should *not* cast the thing to unsigned, because we
traditionally haven't.

(c) we should definitely not use smaller than "int" as a minimum
size, no crazy games with char/short. And considering our legacy, I
think we should probably skip "int" and stay with "long" as the
minimum size.

So I *much* prefer Michal's "abs()" definition that doesn't radically
change the meaning (it keeps the old behavior *except* if you pass in
a bigger size than "long", in which case it will auto-widen to "s64").
That's the maximally compatible model given the "we handle bigger
types automatically" extension.

Because "maximally compatible" is a strong argument. The signedness
stuff and the "let's try to use the exact same type" is just bogus.

One thing that *is* interesting is "what if 'long' and 's64' are the
same size?" In particular, it means that right now Michal's patch
*always* returns "long" on a 64-bit architecture, but will return
"long" or "s64" on a 32-bit one. The reason that is somewhat
interesting is that while the sizes and values are the same, and the
resulting C type expansions are "equivalent" types, i people *print*
things, you have to use different modifiers for the two cases. So you
might get warnings on 32-bit architectures and not get them on 64-bit,
or vice versa.

However, I don't see a good solution for that. And assuming we don't
use "abs()" in an expression to printk(), I guess it doesn't much
matter either.

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