linux-kernel - Re: [RFC] LKMM: Add volatile

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Message-ID: <20210607141658.GE18427@gate.crashing.org>
Date:   Mon, 7 Jun 2021 09:16:58 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Will Deacon <will@...nel.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nick 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>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-toolchains@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC] LKMM: Add volatile_if()

On Mon, Jun 07, 2021 at 12:52:15PM +0200, Peter Zijlstra wrote:
> On Sun, Jun 06, 2021 at 03:26:16PM -0500, Segher Boessenkool wrote:
> > I am saying that if you depend on that some C code you write will result
> > in some particular machine code, without actually *forcing* the compiler
> > to output that exact machine code, then you will be disappointed.  Maybe
> > not today, and maybe it will take years, if you are lucky.
> > 
> > (s/forcing/instructing/ of course, compilers have feelings too!)
> 
> And hence the request for a language extension. Both compilers have a
> vast array of language extensions that are outside of the C spec (thank
> you!), so can we please get one more?

I don't see why not?  It will need to be well-defined, so that it *can*
be implemented.  And ideally it will be useful for other applications as
well.  Finally, it should "play nice" with other extensions and language
features.


Segher