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

[Linus Torvalds]

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).

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