Messages in this thread | | | Date | Wed, 7 Oct 2020 14:20:06 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Control Dependencies vs C Compilers |
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On Wed, Oct 07, 2020 at 11:07:17PM +0200, Peter Zijlstra wrote: > On Wed, Oct 07, 2020 at 10:11:07AM -0700, Paul E. McKenney wrote: > > > Challenges include: > > > > o Unmarked accesses. Compilers are quite aggressive about > > moving normal code. > > Which is why this thread exists :-) We wants to dis-allow lifting the > stores over our volatile-if.
Of course. But you should expect this point to be a continual source of shock and surprise to compiler folks. ;-)
> > o Separately compiled code. For example, does the compiler have > > unfortunatel optimization opportunities when "volatile if" > > appears in one translation unit and the dependent stores in > > some other translation unit? > > It can hardly lift anything outside a TU (barring the next point). So I > don't see how it can go wrong here. This is in fact the case with the > perf ringbuffer. The ctrl-dep lives in a different TU from the > stores.
I don't see how it could either, but I have been surprised before.
> > o LTO, as has already been mentioned in this thread. > > So I would probably advocate the volatile-if to be a full sync point, > and LTO would have to preserve that.
Completely agreed! And probably not the only place that LTO needs to be reined in a bit.
Thanx, Paul
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