On Mon, Jun 07, 2021 at 05:40:37PM -0500, Segher Boessenkool wrote: > On Mon, Jun 07, 2021 at 01:16:33PM -0700, Paul E. McKenney wrote: > > On Mon, Jun 07, 2021 at 03:51:44PM -0400, Alan Stern wrote: > > > On Mon, Jun 07, 2021 at 01:23:35PM -0500, Segher Boessenkool wrote: > > > > On Mon, Jun 07, 2021 at 08:27:12AM -0700, Paul E. McKenney wrote: > > > > > > > > > The barrier() thing can work - all we need to do is to simply make it > > > > > > > > > impossible for gcc to validly create anything but a conditional > > > > > > > > > branch. > > > > > > > > > > > What would you suggest as a way of instructing the compiler to emit the > > > > > > > conditional branch that we are looking for? > > > > > > > > > > > > You write it in the assembler code. > > > > > > > > > > > > Yes, it sucks. But it is the only way to get a branch if you really > > > > > > want one. Now, you do not really need one here anyway, so there may be > > > > > > some other way to satisfy the actual requirements. > > > > > > > > > > Hmmm... What do you see Peter asking for that is different than what > > > > > I am asking for? ;-) > > > > > > > > I don't know what you are referring to, sorry? > > > > > > > > I know what you asked for: literally some way to tell the compiler to > > > > emit a conditional branch. If that is what you want, the only way to > > > > make sure that is what you get is by writing exactly that in assembler. > > > > > > That's not necessarily it. > > > > > > People would be happy to have an easy way of telling the compiler that > > > all writes in the "if" branch of an if statement must be ordered after > > > any reads that the condition depends on. Or maybe all writes in either > > > the "if" branch or the "else" branch. And maybe not all reads that the > > > condition depends on, but just the reads appearing syntactically in the > > > condition. Or maybe even just the volatile reads appearing in the > > > condition. Nobody has said exactly. > > > > > > The exact method used for doing this doesn't matter. It could be > > > accomplished by treating those reads as load-acquires. Or it could be > > > done by ensuring that the object code contains a dependency (control or > > > data) from the reads to the writes. Or it could be done by treating > > > the writes as store-releases. But we do want the execution-time > > > penalty to be small. > > > > > > In short, we want to guarantee somehow that the conditional writes are > > > not re-ordered before the reads in the condition. (But note that > > > "conditional writes" includes identical writes occurring in both > > > branches.) > > > > What Alan said! ;-) > > Okay, I'll think about that. > > But you wrote: > > > > > > > > What would you suggest as a way of instructing the compiler to emit the > > > > > > > conditional branch that we are looking for? > > ... and that is what I answered. I am sorry if you do not like being > taken literally, but that is how I read technical remarks: as literally > what they say. If you say you want a branch, I take it you want a > branch! :-) When it is the cheapest means of providing the needed ordering, I really do want a branch. ;-) And a branch would implement Alan's "control dependency" above. Thanx, Paul
