On Tue, 8 May 2007 14:27:33 -0700 (PDT) David Rientjes wrote: > On Tue, 8 May 2007, Jeremy Fitzhardinge wrote: > > > 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. > >
Well, the document is really about "volatile" in C, not in gcc asm extensions. But if you want to add paragraphs(s) to the file, that's OK too. > Ok, so let's take your second paragraph and my email of an hour ago: > > In an asm construct, if all your input operands are modified and > specified as output operands as well, volatile must be added so > that the entire construct is not optimized away. Additionally, > it must be added if your construct modifies memory that is neither > listed in inputs nor outputs to the construct so that it is known > to have at least one side-effect. Then, the compiler cannot > delete your construct if it is reachable because it may produce > such side-effects. > > and add it to any proposed change to CodingStyle that suggests against the > 'volatile' keyword since there exists a distinct difference in behavior > between using the keyword as a type qualifier for an object and as a > qualifier for an asm construct. --- ~Randy *** Remember to use Documentation/SubmitChecklist when testing your code *** - 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/
