On Mon, Jan 31, 2022 at 12:45:20PM -0800, Nick Desaulniers wrote: > On Tue, Jan 25, 2022 at 3:34 PM Segher Boessenkool > <segher@xxxxxxxxxxxxxxxxxxx> wrote: > > > > Hi! > > > > On Mon, Jan 24, 2022 at 03:26:36PM -0800, Nick Desaulniers wrote: > > > I'm more confident that we can remove the `volatile` keyword (I was > > > thinking about adding a new diagnostic to clang to warn that volatile > > > is redundate+implied for asm goto or inline asm that doesn't have > > > outputs) though that's not the problem here and will probably generate > > > some kernel wide cleanup before we could enable such a flag. > > > > Its main value is that it would discourage users from thinking volatile > > is magic. Seriously worth some pain! > > https://reviews.llvm.org/D118297 > PTAL "" Really the volatile asm-qualifier exists only to signal that an asm statement should not be DCE'd (when it has outputs but they are unused), CSE'd, or LICM'd. It is not a general compiler barrier. It means that the asm has a side effect (one unknown to the compiler), so it must be executed in the real machine just where it would be in the abstract machine. It *can* be CSEd, it *can* be DCEd, it can even be optimised by LICM in certain cases: but it has to be executed as often (and in the same order etc.) in the resulting machine code as it would be if you single-stepped through the source code by hand. Those are fine examples if you add "in most cases" (and that they are just examples, it's not an exhaustive list). Thanks, Segher
