Re: [PATCH] 2.4.22pre10: {,un}likely_p() macros for pointers

[Chip Salzenberg <chip@pobox.com>]

According to Jamie Lokier:
> Chip Salzenberg wrote:
> > According to Willy Tarreau:
> > >   likely => __builtin_expect(!(x), 0)
> > > unlikely => __builtin_expect((x), 0)
> > 
> > Well, I'm not sure about the polarity, but that unlikely() macro isn't
> > good -- it the same old problem that first prompted my message, namely
> > that it's nonportable when (x) has a pointer type.
> 
> It's portable as long as the compiler is GCC :)

No; wrong; please pay attention.

Both parameters of __builtin_expect() are long ints.  On an
architecture where there's a pointer type larger than long[1],
__builtin_expect() won't just warn, it'll *fail*.  Also, on an
architecture where a conversion of a null pointer to long results in
a non-zero value[2], it'll *fail*.  That makes it non-portable twice
over.  Wouldn't you agree?

Allow me to quote gcc's documentation:

   Since you are limited to integral expressions for exp, you should use constructions such as

        if (__builtin_expect (ptr != NULL, 1))
          error ();

   when testing pointer or floating-point values.

Could you please believe the docs?

[1] Yes, they exit.
[2] I don't know if they exist, but they're allowed by ANSI C.
-- 
Chip Salzenberg               - a.k.a. -               <chip@pobox.com>
"I wanted to play hopscotch with the impenetrable mystery of existence,
    but he stepped in a wormhole and had to go in early."  // MST3K