Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Aug 17, 2007 at 09:13:35PM -0700, Linus Torvalds wrote:
> 
> 
> On Sat, 18 Aug 2007, Satyam Sharma wrote:
> > 
> > No code does (or would do, or should do):
> > 
> > 	x.counter++;
> > 
> > on an "atomic_t x;" anyway.
> 
> That's just an example of a general problem.
> 
> No, you don't use "x.counter++". But you *do* use
> 
> 	if (atomic_read(&x) <= 1)
> 
> and loading into a register is stupid and pointless, when you could just 
> do it as a regular memory-operand to the cmp instruction.
> 
> And as far as the compiler is concerned, the problem is the 100% same: 
> combining operations with the volatile memop.
> 
> The fact is, a compiler that thinks that
> 
> 	movl mem,reg
> 	cmpl $val,reg
> 
> is any better than
> 
> 	cmpl $val,mem
> 
> is just not a very good compiler. But when talking about "volatile", 
> that's exactly what ytou always get (and always have gotten - this is 
> not a regression, and I doubt gcc is alone in this).

One of the gcc guys claimed that he thought that the two-instruction
sequence would be faster on some x86 machines.  I pointed out that
there might be a concern about code size.  I chose not to point out
that people might also care about the other x86 machines.  ;-)

							Thanx, Paul
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/