linux-kernel - Re: [PATCH 7/8] i386: bitops: Kill needless usage of __asm__ __volatile_

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Date:	Mon, 23 Jul 2007 22:13:01 +0530 (IST)
From:	Satyam Sharma <ssatyam@....iitk.ac.in>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
cc:	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>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 7/8] i386: bitops: Kill needless usage of __asm__
 __volatile__

Hi Jeremy,

On Mon, 23 Jul 2007, Jeremy Fitzhardinge wrote:

> Satyam Sharma wrote:
> > From: Satyam Sharma <ssatyam@....iitk.ac.in>
> >
> > [7/8] i386: bitops: Kill needless usage of __asm__ __volatile__
> >
> > Another oddity I noticed in this file. The semantics of __volatile__
> > when used to qualify inline __asm__ are that the compiler will not
> > (1) elid, or, (2) reorder, or, (3) intersperse, our inline asm with
> > the rest of the generated code.
> >   
> 
> "asm volatile" does not mean that at all.  It only guarantees (1),

Actually, you're probably right about (2), but (3)?

>From the gcc manual:

<quote>

Similarly, you can't expect a sequence of volatile asm instructions to
remain perfectly consecutive. If you want consecutive output, use a
single asm. Also GCC will perform some optimizations across a volatile
asm instruction, GCC does not "forget everything" when it encounters a
volatile asm instruction the way some other compilers do.

</quote>

I'm reading "Similarly, you can't expect a sequence of volatile asm
instructions to remain perfectly consecutive" to mean they're talking
about something like:

asm volatile(...);
asm volatile(...);
asm volatile(...);

But "If you want consecutive output, use a single asm" probably means:

asm volatile(... multiple instructions here ...);

would actually ensure the code written in there would not be
interspersed ... at least that's how I read it.

[ BTW "Also GCC will perform some optimizations across a volatile
asm instruction ..." is exactly the kind of optimizations we must
allow the compiler to do, but that's not related to point at hand. ]

> and
> only then if the asm is ever reachable.

Yup, of course.

Satyam
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