On Thu, Feb 20, 2014 at 09:34:57AM -0800, Linus Torvalds wrote:
> On Thu, Feb 20, 2014 at 9:14 AM, Torvald Riegel <triegel@xxxxxxxxxx> wrote:
> >>
> >> So the clarification is basically to the statement that the "if
> >> (consume(p)) a" version *would* have an ordering guarantee between the
> >> read of "p" and "a", but the "consume(p) ? a : b" would *not* have
> >> such an ordering guarantee. Yes?
> >
> > Not as I understand it. If my reply above wasn't clear, let me know and
> > I'll try to rephrase it into something that is.
>
> Yeah, so you and Paul agree. And as I mentioned in the email that
> crossed with yours, I think that means that the standard is overly
> complex, hard to understand, fragile, and all *pointlessly* so.
>
> Btw, there are many ways that "use a consume as an input to a
> conditional" can happen. In particular, even if the result is actually
> *used* like a pointer as far as the programmer is concerned, tricks
> like pointer compression etc can well mean that the "pointer" is
> actually at least partly implemented using conditionals, so that some
> paths end up being only dependent through a comparison of the pointer
> value.
>
> So I very much did *not* want to complicate the "litmus test" code
> snippet when Paul tried to make it more complex, but I do think that
> there are cases where code that "looks" like pure pointer chasing
> actually is not for some cases, and then can become basically that
> litmus test for some path.
>
> Just to give you an example: in simple list handling it is not at all
> unusual to have a special node that is discovered by comparing the
> address, not by just loading from the pointer and following the list
> itself. Examples of that would be a HEAD node in a doubly linked list
> (Linux uses this concept quite widely, it's our default list
> implementation), or it could be a place-marker ("cursor" entry) in the
> list for safe traversal in the presence of concurrent deletes etc.
Yep, good example. And also an example where the comparison does not
need to create any particular ordering.
> And obviously there is the already much earlier mentioned
> compiler-induced compare, due to value speculation, that can basically
> create such sequences even wherethey did not originally exist in the
> source code itself.
>
> So even if you work with "pointer dereferences", and thus match that
> particular consume pattern, I really don't see why anybody would think
> that "hey, we can ignore any control dependencies" is a good idea.
> It's a *TERRIBLE* idea.
>
> And as mentioned, it's a terrible idea with no upsides. It doesn't
> help compiler optimizations for the case it's *intended* to help,
> since those optimizations can still be done without the horribly
> broken semantics. It doesn't help compiler writers, it just confuses
> them.
>
> And it sure as hell doesn't help users.
Yep. And in 2014 we know a lot more about the hardware, so could make a
reasonable proposal. In contrast, back in 2007 and 2008 memory ordering
was much more of a dark art, and proposals for control dependencies
therefore didn't get very far.
Thanx, Paul
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