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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 




On Wed, 15 Aug 2007, Segher Boessenkool wrote:

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

[Index of Archives]     [Kernel Newbies]     [Netfilter]     [Bugtraq]     [Photo]     [Stuff]     [Gimp]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Video 4 Linux]     [Linux for the blind]     [Linux Resources]
  Powered by Linux