Re: Control Dependencies vs C Compilers

Peter Zijlstra
2020-10-07T21:07:34Z

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.

> 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.

> 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.