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

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Date:   Sun, 6 Jun 2021 12:15:29 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Jakub Jelinek <jakub@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Peter Zijlstra <peterz@...radead.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 Sun, Jun 06, 2021 at 08:59:22PM +0200, Jakub Jelinek wrote:
> On Sat, Jun 05, 2021 at 08:41:00PM -0700, Linus Torvalds wrote:
> > Something like this *does* seem to work:
> > 
> >    #define ____barrier(id) __asm__ __volatile__("#" #id: : :"memory")
> >    #define __barrier(id) ____barrier(id)
> >    #define barrier() __barrier(__COUNTER__)
> > 
> > which is "interesting" or "disgusting" depending on how you happen to feel.
> 
> I think just
> #define barrier() __asm__ __volatile__("" : : "i" (__COUNTER__) : "memory")
> should be enough (or "X" instead of "i" if some arch uses -fpic and will not
> accept small constants in PIC code), for CSE gcc compares that the asm template
> string and all arguments are the same.

This does seem to do the trick: https://godbolt.org/z/K5j3bYqGT

So thank you for that!

							Thanx, Paul

> As for volatile, that is implicit on asm without any output operands and
> it is about whether the inline asm can be DCEd, not whether it can be CSEd.
> 
> 	Jakub
>