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

From: Willy Tarreau <willy@w.ods.org>
To: Albert Cahalan <albert@users.sourceforge.net>
Cc: linux-kernel mailing list <linux-kernel@vger.kernel.org>,
	davem@redhat.com, jamie@shareable.org, chip@pobox.com
Subject: Re: [PATCH] 2.4.22pre10: {,un}likely_p() macros for pointers
Date: Sun, 10 Aug 2003 09:29:45 +0200	[thread overview]
Message-ID: <20030810072945.GA14038@alpha.home.local> (raw)
In-Reply-To: <1060488233.780.65.camel@cube>

Hi Albert,

On Sun, Aug 10, 2003 at 12:03:54AM -0400, Albert Cahalan wrote:
> I looked at the assembly (ppc, gcc 3.2.3) and didn't
> see any overhead.

same here on x86, gcc-2.95.3 and gcc-3.3.1. The compiler is smart enough not
to add several intermediate tests for !!(x).

> The !!x gives you a 0 or 1 while converting the type.
> So a (char*)0xfda9c80300000000ull will become a 1 of
> an acceptable type, allowing the macro to work as desired.

I agree (I didn't think about pointers, BTW). But what I meant is that we
don't need the result to be precisely 1, but we need it to be something the
compiler interpretes as different from zero, to match the condition. So it
should be cleaner to always check against 0 which is also OK for pointers,
whatever their type (according to Chip's link) :

  likely => __builtin_expect(!(x), 0)
unlikely => __builtin_expect((x), 0)

Cheers,
Willy