Re: [RFC/PATCH] doc: volatile considered evil

Randy Dunlap
2007-05-08T21:25:49Z

Jeremy Fitzhardinge wrote:
> David Rientjes wrote:
>> Since 'volatile' has two different semantics depending on the context in
>> which it is used, this warning should be appended to include the fact that
>> it is legitimate to use for inline assembly.
>>
>
>
> It's probably worth noting that "asm volatile (...)" doesn't mean what
> many people think it means: specifically, it *does not* prevent the asm
> from being reordered with respect to the surrounding code. It may not
> even prevent it from being reordered with respect to other asm
> volatiles. *All* it means is that the asm code will be emitted even if
> the compiler doesn't think its results will be used. Note that an
> "asm()" with no outputs is implicitly "asm volatile()" - on the grounds
> that it would be otherwise useless as far as gcc can tell.
>
> If you need to guarantee ordering of asm statements, you must do it
> explicitly, with either a "memory" clobber, or some finer-grain
> serialization variable (like the _proxy_pda stuff). It would be useful
> if you could tell gcc "I'm passing this variable to the asm for
> serialization purposes, but there's no need to generate any explicit
> references to it", but as far as I know there's no support for that.
>
> J

The doc. should just be talking about "volatile" in C mostly.
Any asm volatile comments are "extra".

--
~Randy
*** Remember to use Documentation/SubmitChecklist when testing your code ***