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

On Wed, 15 Aug 2007, Segher Boessenkool wrote:

[...]

quoted

BTW:

#define atomic_read(a)	(*(volatile int *)&(a))
#define atomic_set(a,i)	(*(volatile int *)&(a) = (i))

int a;

void func(void)
{
	int b;

	b = atomic_read(a);
	atomic_set(a, 20);
	b = atomic_read(a);
}

gives:

func:
	pushl	%ebp
	movl	a, %eax
	movl	%esp, %ebp
	movl	$20, a
	movl	a, %eax
	popl	%ebp
	ret

so the first atomic_read() wasn't optimized away.

Of course.  It is executed by the abstract machine, so
it will be executed by the actual machine.  On the other
hand, try

	b = 0;
	if (b)
		b = atomic_read(a);

or similar.

Yup, obviously. Volatile accesses (or any access to volatile objects),
or even "__volatile__ asms" (which gcc normally promises never to elid)
can always be optimized for cases such as these where the compiler can
trivially determine that the code in question is not reachable.