On Tue, Jun 30, 2020 at 01:30:16PM -0700, Paul E. McKenney wrote: > On Tue, Jun 30, 2020 at 10:12:43PM +0200, Peter Zijlstra wrote: > > I'm not convinced C11 memory_order_consume would actually work for us, > > even if it would work. That is, given: > > > > https://lore.kernel.org/lkml/20150520005510.GA23559@xxxxxxxxxxxxxxxxxx/ > > > > only pointers can have consume, but like I pointed out, we have code > > that relies on dependent loads from integers. > > I agree that C11 memory_order_consume is not normally what we want, > given that it is universally promoted to memory_order_acquire. > > However, dependent loads from integers are, if anything, more difficult > to defend from the compiler than are control dependencies. This applies > doubly to integers that are used to index two-element arrays, in which > case you are just asking the compiler to destroy your dependent loads > by converting them into control dependencies. Yes, I'm aware. However, as you might know, I'm firmly in the 'C is a glorified assembler' camp (as I expect most actual OS people are, out of necessity if nothing else) and if I wanted a control dependency I would've bloody well written one. I think an optimizing compiler is awesome, but only in so far as that optimization is actually helpful -- and yes, I just stepped into a giant twilight zone there. That is, any optimization that has _any_ controversy should be controllable (like -fno-strict-overflow -fno-strict-aliasing) and I'd very much like the same here. In a larger context, I still think that eliminating speculative stores is both necessary and sufficient to avoid out-of-thin-air. So I'd also love to get some control on that.
