Re: [PATCH] get rid of "+m" constraint in i386 rwsems

[Russell King <rmk+lkml@arm.linux.org.uk>]

On Thu, May 06, 2004 at 01:58:16PM +0100, David Howells wrote:
> > Can you explain the need for the change?
> 
> gcc-3.4 generates warnings about it:
> 
> include/asm/rwsem.h: In function `avc_audit':
> include/asm/rwsem.h:126: warning: read-write constraint does not allow a register
> include/asm/rwsem.h:126: warning: read-write constraint does not allow a register
> 
> The gcc people (or at least one of them) seem to think that these warnings are
> correct on a "+m" constraint. See:
> 
> 	http://marc.theaimsgroup.com/?l=linux-kernel&m=107475162200773&w=2
> 
> I understood "+m" to be a shorthand way of specifying "=m" and "m" on the same
> bit of memory, but apparently it that's not what it means.

After reading Richard's post, I wonder if, in the case of:

	"=m" (x) : "m" (x)

whether assembly should assume that %0 is the same as %1.  Do they
just happen to be the same thing?  I'm thinking of the case where
there may be two different ways GCC may reference the same memory
location.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 PCMCIA      - http://pcmcia.arm.linux.org.uk/
                 2.6 Serial core