Subject: Re: [PATCH] make atomic_t volatile on all architectures

From: Martin Schwidefsky <[email-masked]>
Date: Thu, 09 Aug 2007 20:20:46 +0200

On Thu, 2007-08-09 at 10:55 -0700, Linus Torvalds wrote:
> > You can use this forget() macro to make the compiler reread a variable:
> > 
> > #define forget(var) asm volatile ("" : "=m"(var))
> 
> No. That will also make the compiler "forget" any previous writes to it, 
> so it changes behaviour.
> 
> You'd have to use "+m".

Yes, though I would use "=m" on the output list and "m" on the input
list. The reason is that I've seen gcc fall on its face with an ICE on
s390 due to "+m". The explanation I've got from our compiler people was
quite esoteric, as far as I remember gcc splits "+m" to an input operand
and an output operand. Now it can happen that the compiler chooses two
different registers to access the same memory location. "+m" requires
that the two memory references are identical which causes the ICE if
they are not. I do not know if the current compilers still do this. Has
anyone else seen this happen ?

-- 
blue skies,
  Martin.

"Reality continues to ruin my life." - Calvin.