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

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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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