On Sun, Jun 06, 2021 at 06:36:51AM -0500, Segher Boessenkool wrote: > On Fri, Jun 04, 2021 at 01:40:42PM -0700, Paul E. McKenney wrote: > > On Fri, Jun 04, 2021 at 02:53:01PM -0500, Segher Boessenkool wrote: > > > On Fri, Jun 04, 2021 at 11:55:26AM -0700, Paul E. McKenney wrote: > > > > On Fri, Jun 04, 2021 at 11:40:47AM -0500, Segher Boessenkool wrote: > > > > > My point is that you ask compiler developers to paint themselves into a > > > > > corner if you ask them to change such fundamental C syntax. > > > > > > > > Once we have some experience with a language extension, the official > > > > syntax for a standardized version of that extension can be bikeshedded. > > > > Committees being what they are, what we use in the meantime will > > > > definitely not be what is chosen, so there is not a whole lot of point > > > > in worrying about the exact syntax in the meantime. ;-) > > > > > > I am only saying that it is unlikely any compiler that is used in > > > production will want to experiment with "volatile if". > > > > That unfortunately matches my experience over quite a few years. But if > > something can be implemented using existing extensions, the conversations > > often get easier. Especially given many more people are now familiar > > with concurrency. > > This was about the syntax "volatile if", not about the concept, let's > call that "volatile_if". And no, it was not me who brought this up :-) I agree that it is likely that the syntax "volatile if" would be at best a very reluctantly acquired taste among most of the committee. But some might point to the evolving semantics of "auto" as a counter-example, to say nothing of the celebrated spaceship operator. Me, I am not all that worried about the exact syntax. > > > > Which is exactly why these conversations are often difficult. There is > > > > a tension between pushing the as-if rule as far as possible within the > > > > compiler on the one hand and allowing developers to write code that does > > > > what is needed on the other. ;-) > > > > > > There is a tension between what users expect from the compiler and what > > > actually is promised. The compiler is not pushing the as-if rule any > > > further than it always has: it just becomes better at optimising over > > > time. The as-if rule is and always has been absolute. > > > > Heh! The fact that the compiler has become better at optimizing > > over time is exactly what has been pushing the as-if rule further. > > > > The underlying problem is that it is often impossible to write large > > applications (such as the Linux kernel) completely within the confines of > > the standard. Thus, most large applications, and especially concurrent > > applications, are vulnerable to either the compiler becoming better > > at optimizing or compilers pushing the as-if rule, however you want to > > say it. > > Oh definitely. But there is nothing the compiler can do about most > cases of undefined behaviour: it cannot detect it, and there is no way > it *can* be handled sanely. Take for example dereferencing a pointer > that does not point to an object. Almost. The compiler's use of provenance allows detection in some cases. For a stupid example, please see https://godbolt.org/z/z9cWvqdhE. Less stupidly, this sort of thing can be quite annoying to people trying to use ABA-tolerant concurrent algorithms. See for example P1726R4 [1] (update in progress) and for an even more controversial proposal, P2188R1 [2]. The Lifo Singly Linked Push algorithm described beginning on page 14 of [1] is a simple example of an ABA-tolerant algorithm that was already in use when I first programmed a computer. ;-) Thanx, Paul [1] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1726r4.pdf [2] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p2188r1.html
