linux-kernel - Re: [Cbe-oss-dev] [PATCH 1/3] Fix Unlikely(x) == y

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Sun, 17 Feb 2008 01:45:23 -0800
From:	"Andrew Pinski" <pinskia@...il.com>
To:	"Willy Tarreau" <w@....eu>
Cc:	"Arjan van de Ven" <arjan@...radead.org>, linuxppc-dev@...abs.org,
	"Roel Kluin" <12o3l@...cali.nl>, cbe-oss-dev@...abs.org,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [Cbe-oss-dev] [PATCH 1/3] Fix Unlikely(x) == y

On Feb 16, 2008 9:58 AM, Willy Tarreau <w@....eu> wrote:
> Last but not least, gcc 4 tends to emit stupid checks, to the point that I
> have replaced unlikely(x) with (x) in my code when gcc >= 4 is detected. What
> I observe is that the following code :
>
>     if (unlikely(p == NULL)) ...
>
> often gets coded like this :
>
>     reg1 = (p == NULL)
>     if (reg1 != 0) ...
>
> ... which clobbers reg1 for nothing and performs a double test.

This really only can happen in GCC 4.0.x and 4.1.x and cannot happen
for 4.2 or 4.3 really because of the way __builtin_expect is handled
for those two.

-- Pinski
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/