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

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Date: Mon, 8 Apr 2024 20:58:18 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Matthew Wilcox <willy@...radead.org>
Cc: "Paul E. McKenney" <paulmck@...nel.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 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.