Re: [WIP 0/3] Memory model and atomic API in Rust

Paul E. McKenney
2024-04-09T04:47:30Z

On Mon, Apr 08, 2024 at 08:58:18PM -0400, Kent Overstreet wrote:
> On Mon, Apr 08, 2024 at 06:03:11PM +0100, Matthew Wilcox wrote:
> > On Mon, Apr 08, 2024 at 09:55:23AM -0700, Paul E. McKenney wrote:
> > > On Mon, Apr 08, 2024 at 05:02:37PM +0100, Matthew Wilcox wrote:
> > > > In my ideal world, the compiler would turn this into:
> > > >
> > > > newfolio->flags |= folio->flags & MIGRATE_MASK;
> > >
> > > Why not accumulate the changes in a mask, and then apply the mask the
> > > one time? (In situations where __folio_set_foo() need not apply.)
> >
> > But it irks me that we can't tell the compiler this is a safe
> > transformation for it to make. There are a number of places where
> > similar things happen.
>
> Same thing comes up with bignum code - you really want to be able to
> tell the compiler "you can apply x/y/z optimizations for these
> functions", e.g. replace add(mul(a, b), c) with fma(a, b, c).
>
> Compiler optimizations are just algebraic transformations, we just need
> a way to tell the compiler what the algebraic properties of our
> functions are.

That might indeed be more straightforward than doing this on a per-type
basis. But the C++ guys would likely just start shouting "template
metaprogramming!!!" ;-)

Thanx, Paul