[lkml]   [2003]   [Aug]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH] 2.4.22pre10: {,un}likely_p() macros for pointers
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. - <>
"I wanted to play hopscotch with the impenetrable mystery of existence,
but he stepped in a wormhole and had to go in early." // MST3K
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2005-03-22 13:47    [W:0.065 / U:0.220 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site