On Fri, 17 Aug 2007, Nick Piggin wrote:

I'm surprised too. Numbers were from the "...use asm() like the other
atomic operations already do" thread. According to them,

  text    data     bss     dec     hex filename
3434150  249176  176128 3859454  3ae3fe atomic_normal/vmlinux
3436203  249176  176128 3861507  3aec03 atomic_volatile/vmlinux

The first one is a stock kenel, the second is with atomic_read/set
cast to volatile. gcc-4.1 -- maybe if you have an earlier gcc it
won't optimise as much?

No, see my earlier reply. "volatile" really *is* an incredible piece of 
crap.

Just try it yourself:

	volatile int i;
	int j;

	int testme(void)
	{
	        return i <= 1;
	}

	int testme2(void)
	{
	        return j <= 1;
	}

and compile with all the optimizations you can.

I get:

	testme:
	        movl    i(%rip), %eax
	        subl    $1, %eax
	        setle   %al
	        movzbl  %al, %eax
	        ret

vs

	testme2:
	        xorl    %eax, %eax
	        cmpl    $1, j(%rip)
	        setle   %al
	        ret

(now, whether that "xorl + setle" is better than "setle + movzbl", I don't 
really know - maybe it is. But that's not thepoint. The point is the 
difference between

                movl    i(%rip), %eax
                subl    $1, %eax

and

                cmpl    $1, j(%rip)

and imagine this being done for *every* single volatile access.

Just do a 

	git grep atomic_read

to see how atomics are actually used. A lot of them are exactly the above 
kind of "compare against a value".

			Linus