On Fri, Jun 04, 2021 at 11:40:47AM -0500, Segher Boessenkool wrote: > On Fri, Jun 04, 2021 at 06:10:55PM +0200, Peter Zijlstra wrote: > > On Fri, Jun 04, 2021 at 10:35:18AM -0500, Segher Boessenkool wrote: > > > On Fri, Jun 04, 2021 at 01:44:37PM +0200, Peter Zijlstra wrote: > > > > On naming (sorry Paul for forgetting that in the initial mail); while I > > > > think using the volatile qualifier for the language feature (can we haz > > > > plz, kthxbai) makes perfect sense, Paul felt that we might use a > > > > 'better' name for the kernel use, ctrl_dep_if() was proposed. > > > > > > In standard C statements do not have qualifiers. Unless you can > > > convince the ISO C committee to have them on "if", you will have a very > > > hard time convincing any serious compiler to do this. > > > > While some people like talking to the Committee, I would much rather > > explore language extensions with the compiler communities. Such > > extensions can then make their way into the Committee once they show > > their usefulness. > > 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. ;-) > > If you have another proposal on how to express this; one you'd rather > > see implemented, I'm all ears. > > 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. ;-) Thanx, Paul
