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

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Date:	Mon, 23 Jul 2007 18:36:49 +0200
From:	Jan Hubicka <jh@...e.cz>
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

> 
> > 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? :-)
> 
> These bit operations only allow 8 bit immediates, so 0..255 would
> be correct. N might work from the 4.1 docs, but I don't know if it works 
> in all old supported gccs (3.2+)

'N' constraint was there pretty much forever, originally intended for
in/out operands.  It is in 2.95 docs
http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_16.html#SEC175

Honza
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