On Wed, Jul 01, 2020 at 01:40:27PM +0200, Peter Zijlstra wrote: > On Wed, Jul 01, 2020 at 11:41:17AM +0200, Marco Elver wrote: > > On Tue, 30 Jun 2020 at 22:30, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > On Tue, Jun 30, 2020 at 10:12:43PM +0200, Peter Zijlstra wrote: > > > > On Tue, Jun 30, 2020 at 09:19:31PM +0200, Marco Elver wrote: > > > > > > Thoughts? > > > > > > > > How hard would it be to creates something that analyzes a build and > > > > looks for all 'dependent load -> control dependency' transformations > > > > headed by a volatile (and/or from asm) load and issues a warning for > > > > them? > > > > I was thinking about this, but in the context of the "auto-promote to > > acquire" which you didn't like. Issuing a warning should certainly be > > simpler. > > > > I think there is no one place where we know these transformations > > happen, but rather, need to analyze the IR before transformations, > > take note of all the dependent loads headed by volatile+asm, and then > > run an analysis after optimizations checking the dependencies are > > still there. > > Urgh, that sounds nasty. The thing is, as I've hinted at in my other > reply, I would really like a compiler switch to disable this > optimization entirely -- knowing how relevant the trnaformation is, is > simply a first step towards that. > > In order to control the tranformation, you have to actually know where > in the optimization passes it happens. > > Also, if (big if in my book) we find the optimization is actually > beneficial, we can invert the warning when using the switch and warn > about lost optimization possibilities and manually re-write the code to > use control deps. There are lots of optimization passes and any of them might decide to destroy dependencies. :-( > > > > This would give us an indication of how valuable this transformation is > > > > for the kernel. I'm hoping/expecting it's vanishingly rare, but what do > > > > I know. > > > > > > This could be quite useful! > > > > We might then even be able to say, "if you get this warning, turn on > > CONFIG_ACQUIRE_READ_DEPENDENCIES" (or however the option will be > > named). > > I was going to suggest: if this happens, employ -fno-wreck-dependencies > :-) The current state in the C++ committee is that marking variables carrying dependencies is the way forward. This is of course not what the Linux kernel community does, but it should not be hard to have a -fall-variables-dependent or some such that causes all variables to be treated as if they were marked. Though I was hoping for only pointers. Are they -sure- that they -absolutely- need to carry dependencies through integers??? Anyway, the next step is to provide this functionality in one of the major compilers. Akshat Garg started this in GCC as a GSoC project by duplicating "volatile" functionality with a _Dependent_ptr keyword. Next steps would include removing "volatile" functionality not required for dependencies. Here is a random posting, which if I remember correctly raised some doubts as to whether "volatile" was really carried through everywhere that it needs to for things like LTO: https://gcc.gnu.org/legacy-ml/gcc/2019-07/msg00139.html What happened to this effort? Akshat graduated and got an unrelated job, you know, the usual. ;-) Thanx, Paul
