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

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Date: Mon, 8 Apr 2024 21:47:04 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Matthew Wilcox <willy@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Philipp Stanner <pstanner@...hat.com>,
	Boqun Feng <boqun.feng@...il.com>, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	llvm@...ts.linux.dev, Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Andrea Parri <parri.andrea@...il.com>,
	Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nicholas Piggin <npiggin@...il.com>,
	David Howells <dhowells@...hat.com>,
	Jade Alglave <j.alglave@....ac.uk>,
	Luc Maranget <luc.maranget@...ia.fr>,
	Akira Yokosawa <akiyks@...il.com>,
	Daniel Lustig <dlustig@...dia.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	kent.overstreet@...il.com,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, elver@...gle.com,
	Mark Rutland <mark.rutland@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-fsdevel@...r.kernel.org
Subject: Re: [WIP 0/3] Memory model and atomic API in Rust

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