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

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Date:   Sun, 6 Jun 2021 09:45:32 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.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 06:53:36AM -0500, Segher Boessenkool wrote:
> On Sat, Jun 05, 2021 at 09:29:03PM -0400, Alan Stern wrote:
> > Interesting.  And changing one of the branches from barrier() to __asm__ 
> > __volatile__("nop": : :"memory") also causes a branch to be emitted.  So 
> > even though the compiler doesn't "look inside" assembly code, it does 
> > compare two pieces at least textually and apparently assumes if they are 
> > identical then they do the same thing.
> 
> And that is a simple fact, since the same assembler code (at the same
> spot in the program) will do the same thing no matter how that ended up
> there.

Sure.  But the same assembler code at two different spots in the program 
might not do the same thing.  (Think of code that stores the current EIP 
register's value into a variable.)

So while de-duplicating such code may be allowed, it will give rise to 
observable results at execution time.

Alan

> And the compiler always is allowed to duplicate, join, delete, you name
> it, inline assembler code.  The only thing that it cares about is
> semantics of the code, just like for any other code.
> 
> 
> Segher