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

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Date:	Tue, 24 Jul 2007 00:22:30 +0530 (IST)
From:	Satyam Sharma <ssatyam@....iitk.ac.in>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/8] i386: bitops: Rectify bogus "Ir" constraints

On Mon, 23 Jul 2007, H. Peter Anvin wrote:

> Linus Torvalds wrote:
> > 
> > On Mon, 23 Jul 2007, Satyam Sharma wrote:
> >> * The "I" constraint modifier is applicable only to immediate-value operands,
> >>   and combining it with "r" is bogus.
> > 
> > This is wrong too.
> > 
> > The whole point of a "Ir" modifier is to say that the instruction takes 
> > *either* an "I" or an "r".
> > 
> > Andrew - the ones I've looked at were all wrong. Please don't take this 
> > series.
> > 
> 
> Incidentally, I just noticed the x86-64 bitops have "dIr" as their
> constraint set.  "d" would normally be redundant with "r", and as far as
> I know, gcc doesn't prefer one over the other without having "?" or "!"
> as part of the constraint, so is is "d" a stray or is there some meaning
> behind it?


Yup, I had noticed that myself. We would need to use "J" if we want
to limit the offsets to 0..63, but "d" sounds weird / stray indeed,
unless there's yet another undocumented/wrongly-documented mystery
behind this one too ... (I'd like to know even if that's the case,
obviously).

Satyam
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