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". > > I would love to see something that meshes well with the rest of C. But > > there is no 1-1 translation from C code to machine code (not in either > > direction), so anything that more or less depends on that will always > > be awkward. If you can actually express the dependency in your source > > code that will get us 95% to where we want to be. ^^^ > > > Data dependencies, control dependencies and address dependencies, C > > > doesn't really like them, we rely on them. It would be awesome if we can > > > fix this. > > > > Yes. The problem is that C is a high-level language. All C semantics > > are expressed on a an "as-if" level, never as "do this, then that" -- > > well, of course that *is* what it says, it's an imperative language just > > like most, but that is just how you *think* about things on a conceptual > > level, there is nothing that says the machine code has to do the same > > thing in the same order as you wrote! > > 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. What is needed to get any progress is for user expectations to be feasible and not contradict existing requirements. See "^^^" above. Segher
