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 :-) > > > 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. Segher
