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Message-ID: <YLpWwm1lDwBaUven@hirez.programming.kicks-ass.net>
Date: Fri, 4 Jun 2021 18:37:22 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Will Deacon <will@...nel.org>,
"Paul E. McKenney" <paulmck@...nel.org>,
Alan Stern <stern@...land.harvard.edu>,
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 Fri, Jun 04, 2021 at 09:30:01AM -0700, Linus Torvalds wrote:
> On Fri, Jun 4, 2021 at 3:12 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > I've converted most architectures we care about, and the rest will get
> > an extra smp_mb() by means of the 'generic' fallback implementation (for
> > now).
>
> Why is "volatile_if()" not just
>
> #define barier_true() ({ barrier(); 1; })
>
> #define volatile_if(x) if ((x) && barrier_true())
>
> because that should essentially cause the same thing - the compiler
> should be *forced* to create one conditional branch (because "barrier"
> is an asm that can't be done on the false side, so it can't do it with
> arithmetic or other games), and after that we're done.
>
> No need for per-architecture "asm goto" games. No new memory barriers.
> No actual new code generation (except for the empty asm volatile that
> is a barrier).
Because we weren't sure compilers weren't still allowed to optimize the
branch away. If compiler folks can guarantee us your thing (along with
maybe the BUILD_BUG_ON(__builtin_constant_p(cond)) thing) always shall
generate a conditional branch instruction, then Yay!
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