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

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Date:	Mon, 23 Jul 2007 21:51:53 +0530 (IST)
From:	Satyam Sharma <ssatyam@....iitk.ac.in>
To:	Andi Kleen <ak@...e.de>
cc:	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>
Subject: Re: [PATCH 2/8] i386: bitops: Rectify bogus "Ir" constraints

Hi Andi,


On Mon, 23 Jul 2007, Andi Kleen wrote:

> On Monday 23 July 2007 18:05:38 Satyam Sharma wrote:
> > From: Satyam Sharma <ssatyam@....iitk.ac.in>
> > 
> > [2/8] i386: bitops: Rectify bogus "Ir" constraints
> > 
> > The "I" constraint (on the i386 platform) is used to restrict constants to
> > the 0..31 range, for use with instructions that must deal with bit numbers.
> 
> It means I or r, not I modified by r. This means either a immediate constant
> 0..31 or a register, which is correct.
> 
> % cat t18.c 
> 
> f()
> {
>         asm("xxx %0" :: "rI" (10));
>         asm("yyy %0" :: "rI" (100));
> }
> % gcc -O2 -S t18.c
> % cat t18.s
> ...
> f:
> .LFB2:
> #APP
>         xxx $10
> #NO_APP
>         movl    $100, %eax
> #APP
>         yyy %eax
> #NO_APP
>         ret
> .LFE2:
> ...


Whoa, thanks for explaining that to me -- I didn't know, obviously. I had
just written a test program that used "Ir" with an automatic variable
defined in the inline function (as is the case with these bitops) and
observed that even when I gave > 32 values, it would still work -- hence
my conclusion.

However, the patch still stands, does it not? [ I will modify the
changelog, obviously. ] The thing is that we don't want to limit
@nr to <= 31 in the first place, or am I wrong again? :-)

Thanks,
Satyam
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