linux-kernel - Re: [PATCH 2/8] i386: bitops: Rectify bogus "Ir" constraints

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 for Android: free password hash cracker in your pocket

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

Date:	Mon, 23 Jul 2007 11:28:53 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <ak@...e.de>
CC:	Satyam Sharma <ssatyam@....iitk.ac.in>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>, jh@...e.cz
Subject: Re: [PATCH 2/8] i386: bitops: Rectify bogus "Ir" constraints

H. Peter Anvin wrote:
> 
> "I" is correct.  The Intel documentation on this is highly confusing
> (and has bugs in it), but it does unambiguously state:
> 
> "Some assemblers support immediate bit offsets larger than 31 by using
> the immediate bit offset field in combination with the displacement
> field in the memory operand ... The processor will ignore the high-order
> bits if they are not zero."  AMD processors might be different for all I
> know.
> 
> So unless gas is capable of doing this transformation (and it's not as
> of binutils-2.17.50.0.6) "I" is what's needed here.
> 

Just tested it on a K8 machine; AMD behaves the same way.  So "I" is
correct, and changing it to "N" would introduce a bug.

The only way to optimize this is by using __builtin_constant_p() and
adjust the offset appropriately.

	-hpa
-
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/