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

[Segher Boessenkool]

> In a reasonable world, gcc should just make that be (on x86)
>
> 	addl $1,i(%rip)
>
> on x86-64, which is indeed what it does without the volatile. But with 
> the
> volatile, the compiler gets really nervous, and doesn't dare do it in 
> one
> instruction, and thus generates crap like
>
>         movl    i(%rip), %eax
>         addl    $1, %eax
>         movl    %eax, i(%rip)
>
> instead. For no good reason, except that "volatile" just doesn't have 
> any
> good/clear semantics for the compiler, so most compilers will just 
> make it
> be "I will not touch this access in any way, shape, or form". Including
> even trivially correct instruction optimization/combination.

It's just a (target-specific, perhaps) missed-optimisation kind
of bug in GCC.  Care to file a bug report?

> but is
> (again) something that gcc doesn't dare do, since "i" is volatile.

Just nobody taught it it can do this; perhaps no one wanted to
add optimisations like that, maybe with a reasoning like "people
who hit the go-slow-in-unspecified-ways button should get what
they deserve" ;-)


Segher

-
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/