Date: Mon, 11 Aug 2003 05:30:56 +0100
From: Jamie Lokier <jamie@shareable.org>
To: Chip Salzenberg <chip@pobox.com>
Cc: Willy Tarreau <willy@w.ods.org>,
       Albert Cahalan <albert@users.sourceforge.net>,
       linux-kernel mailing list <linux-kernel@vger.kernel.org>,
       davem@redhat.com
Subject: Re: [PATCH] 2.4.22pre10: {,un}likely_p() macros for pointers
Message-ID: <20030811043056.GG10446@mail.jlokier.co.uk>
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Chip Salzenberg wrote:
> > It's portable as long as the compiler is GCC :)
> No; wrong; please pay attention.

I was being facetious :)

> Both parameters of __builtin_expect() are long ints.

So it is broken if passed a "long long"?  The documentation says "you
are limited to integral expressions".

...You're right.  The documentation is wrong.  It's strictly takes
"long int" arguments and returns a long.

> On an architecture where there's a pointer type larger than long[1],
> __builtin_expect() won't just warn, it'll *fail*.

A pointer really should fail on all architectures.
Fortunately you do get a warning.

> 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?

[2] - I don't believe such architectures are supported by GCC, hence
the facetious comment.

>    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.

I think we all agree with this.

-- Jamie
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