Re: C aggregate passing (Rust kernel policy)

Paul E. McKenney
2025-02-27T17:35:11Z

On Thu, Feb 27, 2025 at 09:29:49AM -0500, Steven Rostedt wrote:
> On Thu, 27 Feb 2025 07:56:47 +0100
> Martin Uecker <uecker@tugraz.at> wrote:
>
> > Observable is I/O and volatile accesses. These are things considered
> > observable from the outside of a process and the only things an
> > optimizer has to preserve.
> >
> > Visibility is related to when stores are visible to other threads of
> > the same process. But this is just an internal concept to give
> > evaluation of expressions semantics in a multi-threaded
> > program when objects are accessed from different threads. But
> > the compiler is free to change any aspect of it, as  long as the
> > observable behavior stays the same.
> >
> > In practice the difference is not so big for a traditional
> > optimizer that only has a limited local view and where
> > "another thread" is basically part of the "outside world".
>
> So basically you are saying that if the compiler has access to the entire
> program (sees the use cases for variables in all threads) that it can
> determine what is visible to other threads and what is not, and optimize
> accordingly?
>
> Like LTO in the kernel?

LTO is a small step in that direction. In the most extreme case, the
compiler simply takes a quick glance at the code and the input data and
oracularly generates the output.

Which is why my arguments against duplicating atomic loads have been
based on examples where doing so breaks basic arithmetic. :-/

Thanx, Paul