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Message-ID: <46A4F365.50708@zytor.com>
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
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