On Mon, Feb 4, 2019 at 6:06 AM Martin Jambor <mjam...@suse.cz> wrote: > > I have received the following idea for a Google Summer of Code project, > see the quotation from Paul McKenney below (I do not know myself where > exactly it is from). I consider memory consistency models a very tough > topic and so am doubly reluctant to just post it to wiki without having > a mentor for it. On the other hand, with the right mentors it > definitely can be quite a remarkable project with a big potential. > > Paul, this may come as a surprise for you, but would you be willing to > (co-)mentor such a project if there is a student brave enough to > undertake it? > > C++ front-end guys, would you please consider co-mentoring this project > if Paul was willing to do so?
I wouldn't expect this project to touch the C++ front-end at all; any compiler work would all be in the middle/back-end. There's some previous discussion of these issues at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59448 So I'd suggest pinging Andrew Macleod. Jason > Anybody else interested in getting involved? > > Any other suggestions/comments? > > Thank you very much in advance, > > Martin > > > -------------------- Start of forwarded message -------------------- > > Hi Martin, > > I don't think I have a mentor for this yet though I wonder if Paul McKenney > could be persuaded for this from the memory model side and someone familiar > with the C++ frontend on the GCC side ? > > <quote Paul> > -- > > One could argue that compilers in fact implement the C and C++ > memory_order_consume facility. However, all known compilers do so by > promoting it to memory_order_acquire, which on weakly ordered systems > can result in unnecessary memory-barrier instructions on your fastpaths, > which might not be what you want. The reason for the promotion to > memory_order_acquire is the difficutlies faced by compiler writers when > attempting to trace dependencies at the C/C++ source-code level. In fact, > there is a proposal to temporarily deprecate memory_order_consume [1]. > > So what is to be done? One proposal [2] restricts dependency chains > to cases where it is difficult for the compiler to break them, and > further requires that pointer variables carrying dependencies be marked. > (This proposal also includes prototype wording for the C++ standard, > a number of litmus tests, and some discussion.) Such marking might not > go down well with the Linux kernel community, which has been carrying > dependencies in unmarked variables for more than 15 years, so there is > further informal proposal asking C and C++ implementations to provide a > command-line option forcing the compiler to treat any pointer variable > as if it had been marked. (Why informal? Because command-line options > are outside of the scope of the standard.) > > There is a prototype implementation that obtains the functionality of > memory_order_consume without actually using memory_order_consume, which > is briefly described in a recent C++ working paper [3]. However, the > committee was not all that happy with this approach, preferring marking > of a single pointer variable to maintaining a separate variable to carry > the dependency. > > It would therefore be quite desirable to have an implementation that > allowed pointers to be marked as carrying dependencies, that avoided > the specified dependency-breaking optimizations on such pointers, and > that provided a command-line switch that caused the compiler to treat > all pointers as if they were to marked [2]. > > > [1] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0371r0.html > [2] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0190r4.pdf > [3] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0750r1.html > > --- > > Ramana > -------------------- End of forwarded message --------------------
